Maria, a community gardener, wants to find out whether the amount of fertilizer affects how many tomatoes her plants produce. She sets up 20 tomato plants of the same variety in identical pots filled with the same soil. She divides the plants into four groups of 5 plants each.
Each week, Group 1 gets no fertilizer, Group 2 gets 5 g, Group 3 gets 10 g, and Group 4 gets 15 g. All plants get 500 mL of water each day, and all are placed in the same sunny spot. After 10 weeks, Maria weighs all the tomatoes from each plant and calculates the average mass of tomatoes per plant for each group.
Group
Fertilizer per week (g)
Average mass of tomatoes per plant (kg)
1
0
1.2
2
5
1.8
3
10
2.3
4
15
2.4
What is the dependent variable in Maria's investigation?
Hint
The dependent variable is the result that is measured at the end. It depends on what the experimenter changed.
Step-by-step solution
The dependent variable is what the experimenter measures to see the effect. The passage says that after 10 weeks, Maria "weighs all the tomatoes from each plant and calculates the average mass of tomatoes per plant." The table's last column, "Average mass of tomatoes per plant (kg)", holds those results.
Step by step:
What did Maria change on purpose? The fertilizer amount (0, 5, 10, 15 g). That is the independent variable, so A is wrong.
What did she measure as the outcome? The mass of tomatoes. That is the dependent variable.
Water (500 mL) and the number of plants per group (5) were kept the same, so they are controlled variables.
The correct answer is B.
Tip: Ask "What did they change?" (independent) and "What did they measure?" (dependent). The dependent variable is usually the last column of a results table.
Question 2 of 36
Fertilizer and Tomato Plants
Maria, a community gardener, wants to find out whether the amount of fertilizer affects how many tomatoes her plants produce. She sets up 20 tomato plants of the same variety in identical pots filled with the same soil. She divides the plants into four groups of 5 plants each.
Each week, Group 1 gets no fertilizer, Group 2 gets 5 g, Group 3 gets 10 g, and Group 4 gets 15 g. All plants get 500 mL of water each day, and all are placed in the same sunny spot. After 10 weeks, Maria weighs all the tomatoes from each plant and calculates the average mass of tomatoes per plant for each group.
Group
Fertilizer per week (g)
Average mass of tomatoes per plant (kg)
1
0
1.2
2
5
1.8
3
10
2.3
4
15
2.4
Which group serves as the control group in Maria's investigation?
Hint
A control group does not receive the treatment being tested.
Step-by-step solution
A control group gets none of the treatment being tested, so it shows what happens without it. Maria is testing fertilizer, and the passage says "Group 1 gets no fertilizer." The table confirms Group 1 received 0 g per week.
Step by step:
The treatment being tested is fertilizer.
Find the group with none: Group 1 (0 g).
Groups 2, 3 and 4 all received fertilizer, so they are experimental groups.
Group 4 had the largest result (2.4 kg), but the best result is not what makes a group the control.
The correct answer is A, Group 1.
Tip: The control group is the "normal" or "nothing added" group used for comparison. Without it, Maria could not tell whether fertilizer helped at all.
Question 3 of 36
Select allFertilizer and Tomato Plants
Maria, a community gardener, wants to find out whether the amount of fertilizer affects how many tomatoes her plants produce. She sets up 20 tomato plants of the same variety in identical pots filled with the same soil. She divides the plants into four groups of 5 plants each.
Each week, Group 1 gets no fertilizer, Group 2 gets 5 g, Group 3 gets 10 g, and Group 4 gets 15 g. All plants get 500 mL of water each day, and all are placed in the same sunny spot. After 10 weeks, Maria weighs all the tomatoes from each plant and calculates the average mass of tomatoes per plant for each group.
Group
Fertilizer per week (g)
Average mass of tomatoes per plant (kg)
1
0
1.2
2
5
1.8
3
10
2.3
4
15
2.4
Which factors did Maria keep the same for all plants (controlled variables)? Select all that apply.
Select all that apply.
Hint
Look for every sentence in the passage that says something was identical, the same or given to all plants.
Step-by-step solution
Controlled variables are the conditions kept the same so that only the fertilizer differs. The passage names them:
"20 tomato plants of the same variety" → plant variety is controlled.
"All plants get 500 mL of water each day" → water is controlled.
"all are placed in the same sunny spot" → location and sunlight are controlled.
Step by step:
The fertilizer amount was changed on purpose (0 to 15 g), so it is the independent variable, not controlled.
The tomato mass was measured as the result, so it is the dependent variable.
The remaining three choices were all kept the same.
The correct answers are A, C and E.
Tip: Keeping other variables the same makes the test fair: if tomato mass changes, the fertilizer is the only explanation left.
Question 4 of 36
Ramp Height and a Rolling Toy Car
Jamal wants to know how the height of a ramp affects how far a toy car rolls after it leaves the ramp. He props one end of a 1-meter wooden board on a stack of books so that the top of the ramp is 10, 20, 30 or 40 cm above the floor.
For every run, Jamal uses the same toy car and lets it go from the top of the ramp without pushing it. The car rolls off the ramp onto the same smooth tile floor. He measures the distance from the bottom of the ramp to the spot where the car stops. He does three runs at each height.
Ramp height (cm)
Run 1 (cm)
Run 2 (cm)
Run 3 (cm)
10
62
58
60
20
118
124
121
30
175
169
172
40
230
236
233
Jamal changes one factor on purpose from one set of runs to the next. Which factor is the independent variable?
Hint
The independent variable is the one factor the investigator changes on purpose.
Step-by-step solution
The independent variable is the factor the scientist deliberately changes to see what effect it has. The passage says Jamal props the board 'so that the top of the ramp is 10, 20, 30 or 40 cm above the floor.' The ramp height is the only thing he changes on purpose.
Step by step:
Ask: what is different from one set of runs to the next? Only the ramp height.
Ask: what does Jamal measure? The distance the car rolls; that is the dependent variable.
Ask: what stays the same? The car, the release method and the floor; those are controlled variables.
The correct answer is A, the height of the ramp.
Check: Each row of the table starts with a ramp height, which is a clue: tables usually list the independent variable in the first column.
Tip: Use the sentence pattern 'Does changing ___ affect ___?' The first blank is the independent variable; the second is the dependent variable.
Question 5 of 36
Ramp Height and a Rolling Toy Car
Jamal wants to know how the height of a ramp affects how far a toy car rolls after it leaves the ramp. He props one end of a 1-meter wooden board on a stack of books so that the top of the ramp is 10, 20, 30 or 40 cm above the floor.
For every run, Jamal uses the same toy car and lets it go from the top of the ramp without pushing it. The car rolls off the ramp onto the same smooth tile floor. He measures the distance from the bottom of the ramp to the spot where the car stops. He does three runs at each height.
Ramp height (cm)
Run 1 (cm)
Run 2 (cm)
Run 3 (cm)
10
62
58
60
20
118
124
121
30
175
169
172
40
230
236
233
The numbers under Run 1, Run 2 and Run 3 in Jamal's table record his dependent variable. What does that variable describe?
Hint
The dependent variable is what is measured to see the result of the change.
Step-by-step solution
The dependent variable is the outcome the scientist measures. The passage says Jamal 'measures the distance from the bottom of the ramp to the spot where the car stops.' That distance depends on the ramp height, which is what he changes.
Step by step:
Find the measurement in the passage: distance from the bottom of the ramp to where the car stops.
Confirm it in the table: the run columns are distances in centimeters.
It changes in response to the ramp height (about 60 cm at 10 cm high, about 233 cm at 40 cm high).
The correct answer is C, how far the car rolls after leaving the ramp.
Check: Ramp height is what Jamal changes (independent). The number of runs is part of the method. Jamal never times the car, so speed in seconds was not measured.
Tip: Look for verbs such as measures, counts, records or times. Whatever follows them is usually the dependent variable.
Question 6 of 36
Select allRamp Height and a Rolling Toy Car
Jamal wants to know how the height of a ramp affects how far a toy car rolls after it leaves the ramp. He props one end of a 1-meter wooden board on a stack of books so that the top of the ramp is 10, 20, 30 or 40 cm above the floor.
For every run, Jamal uses the same toy car and lets it go from the top of the ramp without pushing it. The car rolls off the ramp onto the same smooth tile floor. He measures the distance from the bottom of the ramp to the spot where the car stops. He does three runs at each height.
Ramp height (cm)
Run 1 (cm)
Run 2 (cm)
Run 3 (cm)
10
62
58
60
20
118
124
121
30
175
169
172
40
230
236
233
Jamal wants only the ramp height to affect how far the car goes. Which factors did he hold constant in every run? Select all that apply.
Select all that apply.
Hint
Look for words such as 'same' and 'every run' in the passage.
Step-by-step solution
Controlled variables are the factors kept the same so that only the independent variable can affect the results. The passage names them directly.
Step by step:
A: controlled. 'For every run, Jamal uses the same toy car.'
B: not controlled. The ramp height changes (10, 20, 30, 40 cm); it is the independent variable.
C: controlled. He 'lets it go from the top of the ramp without pushing it' every time.
D: controlled. The car rolls 'onto the same smooth tile floor.'
E: not controlled. The distance changes from run to run; it is the dependent variable that Jamal measures.
The correct answers are A, C and D.
Check: If Jamal had pushed the car on some runs, or used a carpet for some heights, those differences could change the distance and the test would not be fair.
Tip: A factor cannot be both controlled and measured. Rule out the independent and dependent variables first, then check that each remaining factor really stayed the same.
Question 7 of 36
Yeast, Sugar and Balloons
Yeast is a single-celled fungus. When yeast breaks down sugar, it releases carbon dioxide gas. Priya wants to find out how the amount of sugar affects how much gas yeast produces.
She fills five identical bottles with 250 mL of warm water (35 °C) and 7 g of dry yeast each. She then adds 0 g, 5 g, 10 g, 15 g or 20 g of sugar, one amount per bottle, and swirls each bottle the same way. She stretches an identical balloon over the mouth of each bottle. After 30 minutes, she measures the distance around the widest part of each balloon (its circumference).
Bottle
Sugar added (g)
Balloon circumference after 30 min (cm)
1
0
10
2
5
18
3
10
26
4
15
31
5
20
33
Priya compares the other bottles with one bottle to see what happens without the factor she is testing. Which bottle is this control?
Hint
A control is set up the same way as the others but does not receive the factor being tested.
Step-by-step solution
A control is the setup that does not get the treatment being tested, so the other results can be compared with it. Priya is testing sugar, so the control is the bottle with no sugar.
Step by step:
Identify the factor being tested: the amount of sugar.
Find the bottle that gets none of it: Bottle 1, with 0 g of sugar.
Bottle 1 still has the same water, temperature, yeast and balloon as the others, so it shows what happens without sugar. Its balloon (10 cm) shows the baseline.
The correct answer is A, Bottle 1 (0 g of sugar).
Check: Bottle 5 had the biggest effect, but having the most treatment does not make it the control. Bottle 3 is simply the middle amount. 'All five bottles' confuses controlled variables (kept the same) with the control group.
Tip: The control is the 'nothing added' or 'normal conditions' group. It answers the question: 'What would happen without the treatment?'
Question 8 of 36
Salt and Freezing Water
Dev wants to test an idea about salt and the freezing of water. He fills four identical plastic cups with 200 mL of tap water each. He stirs 0 g, 10 g, 20 g or 30 g of table salt into the cups, one amount per cup, until the salt dissolves. He places all four cups on the same shelf of his freezer with a thermometer in each cup. He watches the cups and records the temperature at which ice first begins to form in each one. He tests each cup only one time.
Salt added (g)
Temperature when ice first formed (°C)
0
0.0
10
−2.9
20
−5.8
30
−8.9
Which hypothesis is Dev's investigation designed to test?
Hint
A testable hypothesis links the variable Dev changed to the variable he measured.
Step-by-step solution
A hypothesis should connect the independent variable (what Dev changed: the amount of salt, 0 to 30 g) to the dependent variable (what he measured: "the temperature at which ice first begins to form").
Step by step:
Choice C names both: more salt → lower freezing temperature. It is written as a testable "If…, then…" statement, and Dev's design can test it directly.
Choice A says nothing about salt, so his salt amounts would not test it.
Choice B is about how fast water freezes. Dev recorded temperatures, not times, so his data cannot test speed.
Choice D is about roads and safety, which his cups of water did not measure.
The correct answer is C.
Tip: Match the hypothesis to the design. The right hypothesis mentions what was changed and what was measured, and nothing the experiment did not test.
Question 9 of 36
Salt and Freezing Water
Dev wants to test an idea about salt and the freezing of water. He fills four identical plastic cups with 200 mL of tap water each. He stirs 0 g, 10 g, 20 g or 30 g of table salt into the cups, one amount per cup, until the salt dissolves. He places all four cups on the same shelf of his freezer with a thermometer in each cup. He watches the cups and records the temperature at which ice first begins to form in each one. He tests each cup only one time.
Salt added (g)
Temperature when ice first formed (°C)
0
0.0
10
−2.9
20
−5.8
30
−8.9
Which change would best improve the reliability of Dev's results?
Hint
Reread the last sentence of the passage. What did Dev do only once?
Step-by-step solution
The passage ends: "He tests each cup only one time." A single trial can be thrown off by a misread thermometer or a cup placed near the freezer door. Repeating each salt amount several times and averaging the results makes the data more reliable (more likely to give the same result again).
Step by step:
Identify the weakness: one trial per salt amount.
Choice B fixes that weakness: several cups for each salt amount, then average.
Choice A changes a controlled variable (water volume) between cups, making the test unfair.
Choice C adds a new independent variable (sugar) instead of improving reliability.
Choice D removes the control (0 g salt), which is needed for comparison.
The correct answer is B.
Tip: On the GED, "reliable" means repeatable. The fix is almost always "repeat the trials" or "use a larger sample", then average.
Question 10 of 36
Drag and dropSalt and Freezing Water
Dev wants to test an idea about salt and the freezing of water. He fills four identical plastic cups with 200 mL of tap water each. He stirs 0 g, 10 g, 20 g or 30 g of table salt into the cups, one amount per cup, until the salt dissolves. He places all four cups on the same shelf of his freezer with a thermometer in each cup. He watches the cups and records the temperature at which ice first begins to form in each one. He tests each cup only one time.
Salt added (g)
Temperature when ice first formed (°C)
0
0.0
10
−2.9
20
−5.8
30
−8.9
Drag each factor in Dev's investigation into the correct box.
Independent variable
Dependent variable
Controlled variables
Tap a tile, then tap a box to place it.
Hint
Ask of each factor: Did Dev change it on purpose, measure it, or keep it the same?
Step-by-step solution
Sort each factor by its role in the passage.
Step by step:
Amount of salt → Dev "stirs 0 g, 10 g, 20 g or 30 g of table salt into the cups." He chose and changed it, so it is the independent variable.
Temperature when ice first formed → he "records the temperature at which ice first begins to form". It is the measured outcome, the dependent variable.
Volume of water → every cup held "200 mL of tap water", so it is controlled.
Freezer shelf → all cups were on "the same shelf", so it is controlled.
Type of cup → "four identical plastic cups", so it is controlled.
Independent: amount of salt. Dependent: temperature when ice first formed. Controlled: volume of water, freezer shelf, type of cup.
Tip: There is usually one independent and one dependent variable, but there can be many controlled variables. Look for the words same, identical and each.
Question 11 of 36
A Sports Drink Study
A beverage company wanted to show that its new sports drink, called Drink Z here, helps people run faster. The volunteers chose which Saturday they wanted to take part.
Saturday 1, 8:00 a.m. (55 °F): 10 volunteers drank one bottle of Drink Z and then ran a 5-kilometer course.
Saturday 2, 3:00 p.m. (88 °F): 10 different volunteers drank one bottle of water and then ran the same course.
Group
Number of runners
Average time (minutes)
Drink Z
10
26
Water
10
29
Based on these results, the company concluded that Drink Z makes runners faster.
Which is the most serious flaw in the company's study?
Hint
Besides the drink, what else was different between the two groups?
Step-by-step solution
A fair test changes only one variable. Here the drink was supposed to be the only difference, but the passage shows other big differences: the Drink Z group ran at "8:00 a.m. (55 °F)" while the water group ran at "3:00 p.m. (88 °F)." Running in 88 °F heat is much harder than running in cool morning air, so the heat alone could explain the 3-minute difference. This is a confounding variable.
Step by step:
List what differed between groups: the drink, the day, the time of day, the temperature, and the people.
Temperature is a known, powerful influence on running times, so it could cause the result by itself.
Choices A and C describe parts of the design that are reasonable. Comparing averages is normal, and a 5-km course is a fine test of running speed.
Choice D is not a flaw: drinking a liquid before running is exactly what both groups were supposed to do.
The correct answer is B.
Tip: When a study's conclusion seems too good, look for a second variable that changed along with the one being tested.
Question 12 of 36
A Sports Drink Study
A beverage company wanted to show that its new sports drink, called Drink Z here, helps people run faster. The volunteers chose which Saturday they wanted to take part.
Saturday 1, 8:00 a.m. (55 °F): 10 volunteers drank one bottle of Drink Z and then ran a 5-kilometer course.
Saturday 2, 3:00 p.m. (88 °F): 10 different volunteers drank one bottle of water and then ran the same course.
Group
Number of runners
Average time (minutes)
Drink Z
10
26
Water
10
29
Based on these results, the company concluded that Drink Z makes runners faster.
Which change would make this study a fair test of whether Drink Z makes runners faster?
Hint
A fair test keeps everything the same except the drink, and makes the two groups as alike as possible.
Step-by-step solution
To isolate the effect of the drink, both groups must run under the same conditions, and the groups should be formed by random assignment so that faster runners are not more likely to end up in one group. In the original study "the volunteers chose which Saturday they wanted to take part" and the two runs happened at different temperatures.
Step by step:
Choice C removes the time and temperature difference (same day, same time) and uses random assignment. Only the drink differs.
Choice A keeps the hot-versus-cool problem; it just repeats it.
Choice B has no comparison group at all, so there is nothing to compare to.
Choice D lets people choose, which can create groups that differ from the start (for example, serious runners might prefer the sports drink).
The correct answer is C.
Tip: The gold-standard design is: random assignment into groups + same conditions + only one difference (the treatment).
Question 13 of 36
Drop-downA Sports Drink Study
A beverage company wanted to show that its new sports drink, called Drink Z here, helps people run faster. The volunteers chose which Saturday they wanted to take part.
Saturday 1, 8:00 a.m. (55 °F): 10 volunteers drank one bottle of Drink Z and then ran a 5-kilometer course.
Saturday 2, 3:00 p.m. (88 °F): 10 different volunteers drank one bottle of water and then ran the same course.
Group
Number of runners
Average time (minutes)
Drink Z
10
26
Water
10
29
Based on these results, the company concluded that Drink Z makes runners faster.
Complete the sentence about improving the study.
To reduce the chance that the results are caused by a few unusually fast or slow runners, the company should the number of volunteers. To make the two groups as similar as possible at the start, volunteers should be placed in groups .
Choose an answer from each menu.
Hint
Think about how a bigger sample and a coin-flip style assignment each make a study fairer.
Step-by-step solution
The passage shows only "10" runners per group, and that "the volunteers chose which Saturday they wanted to take part."
Step by step:
Sample size. With only 10 runners per group, one or two very fast runners could pull a group's average down a lot. A larger sample makes the average more stable, so the company should increase the number of volunteers.
Assignment. When people choose their own group, the groups may differ in fitness from the start. Random assignment (like flipping a coin for each person) spreads fast and slow runners evenly across both groups.
Assigning by last week's speed would deliberately make the groups different, which is the opposite of what is needed.
The answers are b1 = increase and b2 = at random.
Tip: Small samples and self-chosen groups are two of the most common flaws the GED asks about. The fixes are "larger sample" and "random assignment".
Question 14 of 36
Yeast, Sugar and Balloons
Yeast is a single-celled fungus. When yeast breaks down sugar, it releases carbon dioxide gas. Priya wants to find out how the amount of sugar affects how much gas yeast produces.
She fills five identical bottles with 250 mL of warm water (35 °C) and 7 g of dry yeast each. She then adds 0 g, 5 g, 10 g, 15 g or 20 g of sugar, one amount per bottle, and swirls each bottle the same way. She stretches an identical balloon over the mouth of each bottle. After 30 minutes, she measures the distance around the widest part of each balloon (its circumference).
Bottle
Sugar added (g)
Balloon circumference after 30 min (cm)
1
0
10
2
5
18
3
10
26
4
15
31
5
20
33
Priya's results will support or reject one testable prediction. Which hypothesis was her setup built to test?
Hint
A good hypothesis names the factor Priya changed and the result she measured.
Step-by-step solution
A testable hypothesis links the independent variable to the dependent variable, usually as an if–then statement. Priya changes the amount of sugar and measures balloon size, which shows how much carbon dioxide gas was made.
Step by step:
Independent variable: sugar added (0–20 g).
Dependent variable: balloon circumference after 30 minutes, a measure of gas produced.
The matching hypothesis: if more sugar is added, the yeast will produce more gas, so the balloon will get bigger.
The correct answer is C.
Check: A is about water temperature, which Priya kept at 35 °C for all bottles. B is about balloon type, which was identical. D is about how long the yeast lives, which Priya never measured.
Tip: To check a hypothesis choice, ask two questions: Is the factor in it the one that was changed? Is the outcome in it the one that was measured? Both must be yes.
Question 15 of 36
Drag and dropYeast, Sugar and Balloons
Yeast is a single-celled fungus. When yeast breaks down sugar, it releases carbon dioxide gas. Priya wants to find out how the amount of sugar affects how much gas yeast produces.
She fills five identical bottles with 250 mL of warm water (35 °C) and 7 g of dry yeast each. She then adds 0 g, 5 g, 10 g, 15 g or 20 g of sugar, one amount per bottle, and swirls each bottle the same way. She stretches an identical balloon over the mouth of each bottle. After 30 minutes, she measures the distance around the widest part of each balloon (its circumference).
Bottle
Sugar added (g)
Balloon circumference after 30 min (cm)
1
0
10
2
5
18
3
10
26
4
15
31
5
20
33
Sort the factors from Priya's yeast experiment by dragging each one into the box that names its role.
Independent variable
Dependent variable
Controlled variables
Tap a tile, then tap a box to place it.
Hint
One factor was changed on purpose, one was measured and the rest were kept the same.
Step-by-step solution
Sort each factor by asking what Priya did with it.
Step by step:
Changed on purpose → independent variable: the amount of sugar (0, 5, 10, 15 or 20 g).
Measured → dependent variable: the balloon circumference after 30 minutes.
Kept the same → controlled variables: the water temperature (35 °C), the amount of yeast (7 g) and the volume of water (250 mL). The passage says she used 'five identical bottles with 250 mL of warm water (35 °C) and 7 g of dry yeast each.'
The correct sorting is sugar → independent; balloon circumference → dependent; water temperature, amount of yeast and volume of water → controlled.
Check: If the amount of yeast had been different in each bottle, a bigger balloon could be caused by more yeast instead of more sugar. That is why it must be controlled.
Tip: There is usually only one independent and one dependent variable in a GED investigation; everything else that is 'the same' or 'identical' is controlled.
Question 16 of 36
Comparing How Soils Drain
Keisha wants to compare how well three types of soil drain water. She lines three identical funnels with coffee filters. She puts 300 g of sand into the first funnel, 200 g of clay into the second and 250 g of loam into the third. She sets each funnel over a plastic cup.
Keisha pours 200 mL of water into each funnel. After 5 minutes, she estimates how much water has dripped into each cup. The cups have marks only at 50, 100, 150 and 200 mL. She tests each soil once.
Soil
Water collected after 5 minutes (estimated)
Sand
about 150 mL
Loam
about 100 mL
Clay
about 50 mL
Keisha concludes that sand drains water better than the other two soils.
Besides the type of soil, which factor differed among the funnels and could have affected Keisha's results?
Hint
Read the first paragraph carefully. Was everything except the soil type really the same?
Step-by-step solution
In a fair test, only the independent variable (here, soil type) should differ. The passage says Keisha used '300 g of sand,' '200 g of clay' and '250 g of loam.' The mass of soil was different in every funnel.
Step by step:
List what was the same: identical funnels, coffee filters, 200 mL of water, 5 minutes.
Find what else changed: the amount of soil (300 g, 200 g, 250 g).
A different amount of soil could hold back a different amount of water, so the results might reflect the soil amount instead of the soil type. This is a confounding variable.
The correct answer is D, the mass of soil in each funnel.
Check: The water volume, the funnels and the 5-minute time were all the same for every soil, so they cannot explain the differences.
Tip: To find a design flaw, list every factor and mark it 'same' or 'different.' Any 'different' factor other than the independent variable is a confound.
Question 17 of 36
Comparing How Soils Drain
Keisha wants to compare how well three types of soil drain water. She lines three identical funnels with coffee filters. She puts 300 g of sand into the first funnel, 200 g of clay into the second and 250 g of loam into the third. She sets each funnel over a plastic cup.
Keisha pours 200 mL of water into each funnel. After 5 minutes, she estimates how much water has dripped into each cup. The cups have marks only at 50, 100, 150 and 200 mL. She tests each soil once.
Soil
Water collected after 5 minutes (estimated)
Sand
about 150 mL
Loam
about 100 mL
Clay
about 50 mL
Keisha concludes that sand drains water better than the other two soils.
Which change would best make Keisha's investigation a fair test of how well each soil drains?
Hint
A fair test changes only one variable. Which choice removes the extra difference and still compares all three soils?
Step-by-step solution
Keisha's main problem is that the soils had different masses (300 g, 200 g, 250 g), and she tested each soil only once. The best redesign fixes the confound and keeps the comparison.
Step by step:
Make the soil mass the same for every soil, for example 250 g each, so soil type is the only difference.
Keep everything else the same: funnels, filters, 200 mL of water, 5 minutes.
Repeat each test several times so one odd result does not decide the conclusion.
The correct answer is D.
Check: A adds a second difference (water amount) instead of removing one. B stops comparing soils, so it cannot answer the question. C adds the time of day as a new variable.
Tip: The best 'fix the design' answer removes the confound without adding a new variable and keeps the comparison the question is about.
Question 18 of 36
Comparing How Soils Drain
Keisha wants to compare how well three types of soil drain water. She lines three identical funnels with coffee filters. She puts 300 g of sand into the first funnel, 200 g of clay into the second and 250 g of loam into the third. She sets each funnel over a plastic cup.
Keisha pours 200 mL of water into each funnel. After 5 minutes, she estimates how much water has dripped into each cup. The cups have marks only at 50, 100, 150 and 200 mL. She tests each soil once.
Soil
Water collected after 5 minutes (estimated)
Sand
about 150 mL
Loam
about 100 mL
Clay
about 50 mL
Keisha concludes that sand drains water better than the other two soils.
Keisha's measurements of the collected water are only estimates. Which change would most improve the precision of these measurements?
Hint
Look at how the cups are marked. What tool would let her read the volume more exactly?
Step-by-step solution
The passage says the cups 'have marks only at 50, 100, 150 and 200 mL' and that Keisha 'estimates' the water collected. With marks 50 mL apart, a reading of 'about 100 mL' could really be anywhere from roughly 75 to 125 mL. The measuring tool is a source of measurement error.
Step by step:
Identify the weak measurement: estimating volume in a cup marked every 50 mL.
Choose a tool with finer markings: a graduated cylinder marked every 1 mL.
Pouring each cup's water into the cylinder gives a far more precise reading.
The correct answer is A.
Check: B and C change the amount of soil or water, which does not make the reading more precise. D changes the soil type tested and does nothing about the measuring tool.
Tip: Precision depends on the smallest marking of a tool. If the question mentions estimating or rough marks, look for an answer that uses a tool with finer markings.
Question 19 of 36
Testing a Hand Sanitizer
A school nurse wants to know whether a new hand sanitizer reduces the number of bacteria on people's hands. Twelve volunteers each press their fingertips onto a nutrient agar plate, a dish of gel on which bacteria can grow into visible spots called colonies.
Next, six volunteers rub 2 mL of the sanitizer over their hands for 20 seconds. The other six volunteers rub 2 mL of plain water over their hands for 20 seconds. One minute later, every volunteer presses their fingertips onto a second plate. All 24 plates are covered and kept in the same incubator at 30 °C for 48 hours. Then the nurse counts the colonies on each plate.
Group
Average colonies before rubbing
Average colonies after rubbing
Sanitizer
84
9
Plain water
81
66
To judge the sanitizer, the nurse needs a comparison group that does everything except use the sanitizer. Which group is that control group?
Hint
Which group went through every step except receiving the sanitizer?
Step-by-step solution
The control group goes through the same procedure as the experimental group but does not receive the treatment being tested. Here the treatment is the sanitizer.
Step by step:
The experimental group rubbed '2 mL of the sanitizer over their hands for 20 seconds.'
The other group rubbed '2 mL of plain water over their hands for 20 seconds,' the same action without the sanitizer.
Comparing the two groups shows how much of the drop in bacteria is due to the sanitizer and how much is due to simply rubbing the hands. (Water alone lowered the average from 81 to 66; sanitizer lowered it from 84 to 9.)
The correct answer is C, the six volunteers who rubbed plain water on their hands.
Check: The 'before' plates are a baseline for each person, but the group that tells you what happens without sanitizer is the plain-water group. The incubator is a controlled condition, not a group.
Tip: A good control matches the treatment group in every step (here, same volume, same rubbing time) except the one factor being tested.
Question 20 of 36
Drop-downTesting a Hand Sanitizer
A school nurse wants to know whether a new hand sanitizer reduces the number of bacteria on people's hands. Twelve volunteers each press their fingertips onto a nutrient agar plate, a dish of gel on which bacteria can grow into visible spots called colonies.
Next, six volunteers rub 2 mL of the sanitizer over their hands for 20 seconds. The other six volunteers rub 2 mL of plain water over their hands for 20 seconds. One minute later, every volunteer presses their fingertips onto a second plate. All 24 plates are covered and kept in the same incubator at 30 °C for 48 hours. Then the nurse counts the colonies on each plate.
Group
Average colonies before rubbing
Average colonies after rubbing
Sanitizer
84
9
Plain water
81
66
Complete the sentences about how the nurse kept the test fair.
Both groups rubbed their hands for . After pressing, all of the plates were kept .
Choose an answer from each menu.
Hint
Find the details in the passage that are described as the same for everyone.
Step-by-step solution
Controlled variables are kept the same so that the only difference between groups is the sanitizer. The passage gives these details.
Step by step:
Rubbing time: six volunteers rubbed sanitizer 'for 20 seconds' and the other six rubbed water 'for 20 seconds.' So both groups rubbed for the same 20 seconds.
Plate storage: 'All 24 plates are covered and kept in the same incubator at 30 °C for 48 hours.' So all plates were kept at the same temperature for the same time.
The correct answers are the same 20 seconds and in the same incubator at 30 °C for 48 hours.
Check: If the sanitizer group had rubbed longer, or if their plates were kept cooler (slowing bacterial growth), those differences could explain the lower colony counts instead of the sanitizer.
Tip: On fair-test questions, look for every condition that could affect the measured result (time, temperature, amount) and check that it is identical for all groups.
Question 21 of 36
Select allTesting a Hand Sanitizer
A school nurse wants to know whether a new hand sanitizer reduces the number of bacteria on people's hands. Twelve volunteers each press their fingertips onto a nutrient agar plate, a dish of gel on which bacteria can grow into visible spots called colonies.
Next, six volunteers rub 2 mL of the sanitizer over their hands for 20 seconds. The other six volunteers rub 2 mL of plain water over their hands for 20 seconds. One minute later, every volunteer presses their fingertips onto a second plate. All 24 plates are covered and kept in the same incubator at 30 °C for 48 hours. Then the nurse counts the colonies on each plate.
Group
Average colonies before rubbing
Average colonies after rubbing
Sanitizer
84
9
Plain water
81
66
Which of the following could be sources of error in the nurse's colony counts? Select all that apply.
Select all that apply.
Hint
A source of error is something that could make a measurement inaccurate or uneven. A condition that was kept the same for everyone is not an error.
Step-by-step solution
A source of error is anything that could make the measured colony counts less accurate or less comparable. Conditions that the nurse deliberately kept the same are controls, not errors.
Step by step:
A: possible error. If volunteers touched a doorknob or phone during the minute between rubbing and pressing, they could pick up new bacteria.
B: possible error. Colonies that grow into each other may be counted as one, giving a count that is too low.
C: possible error. Pressing harder on one plate than another transfers more bacteria, so counts are not comparable.
D: not an error. Keeping every plate at 30 °C for 48 hours is a controlled variable that makes the test fairer.
E: not an error. Using 2 mL for both groups is also a control.
The correct answers are A, B and C.
Tip: When asked for sources of error, look for steps where a human or a tool could vary from trial to trial. Steps that were carefully kept identical reduce error.
Question 22 of 36
Glow Sticks and Temperature
A glow stick gives off light because of a chemical reaction that starts when the stick is bent. Omar wants to find out how temperature affects how brightly glow sticks glow.
He places identical, unbent glow sticks in water baths at 5 °C, 20 °C, 35 °C and 50 °C for 5 minutes. He then bends each stick and, in a dark room, measures its brightness with a light meter 2 minutes later. Brightness is measured in lux. He tests three glow sticks at each temperature.
Water temperature (°C)
Stick 1 (lux)
Stick 2 (lux)
Stick 3 (lux)
5
12
14
13
20
31
29
33
35
52
55
49
50
74
70
72
Before he started, Omar wrote an if-then prediction that his procedure could check. Which hypothesis fits his investigation?
Hint
Omar changed one thing and measured one thing. The hypothesis must connect those two.
Step-by-step solution
A hypothesis for this investigation must connect what Omar changed (the water temperature) with what he measured (brightness in lux, 2 minutes after bending).
Step by step:
Independent variable: temperature of the water bath (5, 20, 35, 50 °C).
Dependent variable: brightness measured with a light meter.
Matching hypothesis: if glow sticks are warmer, then they will glow more brightly, because the chemical reaction goes faster at higher temperatures.
The correct answer is B.
Check: A is about how long the sticks glow, but Omar measured brightness only once, at 2 minutes. C is about color, which was not varied (all sticks were identical). D is about a dark room, but every measurement was made in the same dark room.
Tip: Eliminate any hypothesis that mentions an outcome nobody measured or a factor that was kept the same. What remains should name exactly the independent and dependent variables.
Question 23 of 36
Fill inTemperature and Dissolving Tablets
Lena tested how water temperature affects the time it takes an antacid tablet to dissolve. For every trial she used tablets of the same brand and size and 150 mL of water in identical glasses. She did not stir the water. She ran three trials at each temperature and timed how long each tablet took to dissolve completely.
Water temperature (°C)
Trial 1 (s)
Trial 2 (s)
Trial 3 (s)
10
61
59
60
20
44
42
43
30
32
34
51
40
24
26
25
What is the mean (average) dissolving time of all three trials at 30 °C?
seconds
Type your answer in the box. Fractions like 3/4 are fine.
Hint
Add the three 30 °C times, then divide by the number of trials.
Step-by-step solution
Use the 30 °C row of the table: "| 30 | 32 | 34 | 51 |".
Step by step:
Add the three trials: 32 + 34 + 51 = 117 seconds.
Divide by the number of trials (3):
\text{mean} = \frac{117}{3} = 39\text{ s}
The mean of all three trials is 39 seconds.
Notice that the third trial (51 s) is much larger than the other two. Without it, the mean would be \dfrac{32 + 34}{2} = 33 s. That one unusual trial raises the average by 6 seconds, which is why scientists look closely at outliers.
Check:39 \times 3 = 117 ✓
Tip: Mean = sum ÷ count. Always check how many values you added so you divide by the right number.
Question 24 of 36
Temperature and Dissolving Tablets
Lena tested how water temperature affects the time it takes an antacid tablet to dissolve. For every trial she used tablets of the same brand and size and 150 mL of water in identical glasses. She did not stir the water. She ran three trials at each temperature and timed how long each tablet took to dissolve completely.
Water temperature (°C)
Trial 1 (s)
Trial 2 (s)
Trial 3 (s)
10
61
59
60
20
44
42
43
30
32
34
51
40
24
26
25
Which statement best explains why Lena should repeat the third trial at 30 °C?
Hint
Compare the 51-second result with the other two 30 °C trials and with the pattern across temperatures.
Step-by-step solution
Look at the table row "| 30 | 32 | 34 | 51 |". Trials 1 and 2 are close together (32 s and 34 s), but trial 3 is 51 s, about 17–19 seconds longer.
Step by step:
Compare within the row. At every other temperature, the three trials differ by only about 2 seconds (61/59/60, 44/42/43, 24/26/25). A 17-second gap is unusual.
Compare with the trend. As temperature rises, dissolving time falls: about 60 s → 43 s → ? → 25 s. A time of 51 s at 30 °C would be slower than at 20 °C, which breaks the pattern. The other two trials (about 33 s) fit it well.
Conclusion: trial 3 is probably an error (for example, a timing mistake or water that had cooled). Repeating it checks whether it was a mistake.
Choice A is wrong because a hotter temperature is not the issue; the 40 °C trials were fine. Choice C reverses the logic. Choice D is false: Lena did three trials.
The correct answer is B.
Tip: An outlier is a value far from the others under the same conditions. Do not just delete it; repeat the trial or look for a cause.
Question 25 of 36
Fill inGlow Sticks and Temperature
A glow stick gives off light because of a chemical reaction that starts when the stick is bent. Omar wants to find out how temperature affects how brightly glow sticks glow.
He places identical, unbent glow sticks in water baths at 5 °C, 20 °C, 35 °C and 50 °C for 5 minutes. He then bends each stick and, in a dark room, measures its brightness with a light meter 2 minutes later. Brightness is measured in lux. He tests three glow sticks at each temperature.
Water temperature (°C)
Stick 1 (lux)
Stick 2 (lux)
Stick 3 (lux)
5
12
14
13
20
31
29
33
35
52
55
49
50
74
70
72
What is the mean brightness of the three glow sticks tested at 35 °C?
lux
Type your answer in the box. Fractions like 3/4 are fine.
Hint
Add the three values in the 35 °C row, then divide by the number of sticks.
Step-by-step solution
The mean (average) is the sum of the values divided by how many values there are. Use only the 35 °C row: 52, 55 and 49 lux.
Step by step:
Add the three readings: 52 + 55 + 49 = 156.
Divide by the number of readings: \dfrac{156}{3} = 52.
The correct answer is 52 lux.
Check: The mean should fall between the smallest (49) and largest (55) values, and 52 does. Common slips: dividing by 4 (the number of temperatures) or adding a value from the 20 °C or 50 °C row.
Tip: Repeated trials are averaged to reduce the effect of random variation. Always average within one condition only, and divide by the number of trials in that condition.
Question 26 of 36
Glow Sticks and Temperature
A glow stick gives off light because of a chemical reaction that starts when the stick is bent. Omar wants to find out how temperature affects how brightly glow sticks glow.
He places identical, unbent glow sticks in water baths at 5 °C, 20 °C, 35 °C and 50 °C for 5 minutes. He then bends each stick and, in a dark room, measures its brightness with a light meter 2 minutes later. Brightness is measured in lux. He tests three glow sticks at each temperature.
Water temperature (°C)
Stick 1 (lux)
Stick 2 (lux)
Stick 3 (lux)
5
12
14
13
20
31
29
33
35
52
55
49
50
74
70
72
Omar's teacher suggests a change that would make his results more reliable. Which change is it?
Hint
Reliability improves when results are based on more data collected the same way.
Step-by-step solution
Results are more reliable when they are based on a larger number of trials, because random differences between individual glow sticks tend to cancel out in the average.
Step by step:
Omar tested only three sticks at each temperature. One unusual stick could noticeably change the mean.
Testing more identical sticks (for example, ten) at each temperature, with everything else the same, gives a more trustworthy average.
The new data would still answer the same question because nothing else in the design changes.
The correct answer is D.
Check: A adds brand as a second variable that differs with temperature. B reduces the amount of data, which makes results less reliable. C changes the units but not the quality of the data; converting lux to another unit does not reduce random variation.
Tip: On the GED, 'more reliable' almost always means more trials or a larger sample, done under the same conditions.
Question 27 of 36
Modeling Meteorite Craters
Craters on the Moon form when rocks from space, called meteorites, crash into its surface. To model this, Ana fills a tray with flour 8 cm deep and sprinkles a thin layer of cocoa powder on top so the craters are easy to see.
Ana wants to know whether the mass of a falling object affects the size of the crater it makes. She uses four balls that are all 2.5 cm across but are made of different materials, so they have different masses: 10 g (plastic), 20 g (glass), 64 g (steel) and 93 g (lead-filled). She drops each ball from a height of 1.0 m and measures the crater's diameter with a ruler. She smooths the flour and adds fresh cocoa powder after every drop, and she drops each ball three times.
Ball mass (g)
Average crater diameter (cm)
10
4.1
20
5.0
64
6.8
93
7.6
Ana chose four balls of equal size made of different materials. Which factor is she testing as the independent variable?
Hint
Reread the sentence that tells what Ana wants to know.
Step-by-step solution
The passage states Ana's question directly: she 'wants to know whether the mass of a falling object affects the size of the crater it makes.' The factor she changes on purpose is the mass of the ball.
Step by step:
What is different from one ball to the next? Mass: 10 g, 20 g, 64 g and 93 g.
What does Ana measure? Crater diameter, the dependent variable.
What stays the same? Ball size (2.5 cm across), drop height (1.0 m) and flour depth (8 cm), all controlled.
The correct answer is A, the mass of the ball.
Check: Ana deliberately chose balls of the same size so that only mass would differ. If the balls had different sizes too, she could not tell which factor caused bigger craters. The material differs only as a way to get different masses.
Tip: When a passage tells you what a scientist 'wants to know,' the independent variable is the cause named in that question, and the dependent variable is the effect.
Question 28 of 36
Drag and dropModeling Meteorite Craters
Craters on the Moon form when rocks from space, called meteorites, crash into its surface. To model this, Ana fills a tray with flour 8 cm deep and sprinkles a thin layer of cocoa powder on top so the craters are easy to see.
Ana wants to know whether the mass of a falling object affects the size of the crater it makes. She uses four balls that are all 2.5 cm across but are made of different materials, so they have different masses: 10 g (plastic), 20 g (glass), 64 g (steel) and 93 g (lead-filled). She drops each ball from a height of 1.0 m and measures the crater's diameter with a ruler. She smooths the flour and adds fresh cocoa powder after every drop, and she drops each ball three times.
Ball mass (g)
Average crater diameter (cm)
10
4.1
20
5.0
64
6.8
93
7.6
Ana kept some factors fixed, changed one and measured one. Place each factor from her crater model in the correct column.
Independent variable
Dependent variable
Controlled variables
Tap a tile, then tap a box to place it.
Hint
One factor was changed on purpose, one was measured and the rest were kept the same.
Step-by-step solution
Sort each factor by asking how Ana treated it.
Step by step:
Changed on purpose → independent: ball mass (10, 20, 64, 93 g).
Measured → dependent: crater diameter, measured with a ruler.
Kept the same → controlled: drop height (1.0 m), ball diameter (2.5 cm across) and flour depth (8 cm).
The correct sorting is ball mass → independent; crater diameter → dependent; drop height, ball diameter and flour depth → controlled.
Check: Ball diameter is the trickiest tile. It might seem to vary because the balls are made of different materials, but the passage says they 'are all 2.5 cm across.' Ana kept size constant so that mass is the only difference between balls.
Tip: Watch for factors that sound like they should vary but are described as equal. The passage, not your expectation, decides which box a factor goes in.
Question 29 of 36
Modeling Meteorite Craters
Craters on the Moon form when rocks from space, called meteorites, crash into its surface. To model this, Ana fills a tray with flour 8 cm deep and sprinkles a thin layer of cocoa powder on top so the craters are easy to see.
Ana wants to know whether the mass of a falling object affects the size of the crater it makes. She uses four balls that are all 2.5 cm across but are made of different materials, so they have different masses: 10 g (plastic), 20 g (glass), 64 g (steel) and 93 g (lead-filled). She drops each ball from a height of 1.0 m and measures the crater's diameter with a ruler. She smooths the flour and adds fresh cocoa powder after every drop, and she drops each ball three times.
Ball mass (g)
Average crater diameter (cm)
10
4.1
20
5.0
64
6.8
93
7.6
Ana now wants to test whether the height from which a ball is dropped affects crater diameter. Which plan is the best design?
Hint
Only one factor should change. Which plan changes the height and keeps the ball and the flour the same?
Step-by-step solution
To test drop height, height must be the only factor that changes. Everything else, including the ball, should stay the same, and each height should be repeated.
Step by step:
Choose one ball so mass and size are controlled (for example, the 64 g steel ball).
Drop it from several heights, such as 0.5 m, 1.0 m, 1.5 m and 2.0 m.
Smooth the flour after every drop, do three drops at each height and measure each crater's diameter.
The correct answer is A.
Check: B changes both the ball and the height, so a bigger crater could come from either. C never changes the height, so it cannot test height. D changes the surface along with the height (flour for some heights, sand for others), adding a second variable.
Tip: For 'best design' questions, check three things: only one variable changes, it is the one in the question, and there are repeated trials.
Question 30 of 36
Caffeine and Daphnia Heart Rate
Daphnia are tiny freshwater animals with see-through bodies, so their beating hearts can be watched under a microscope. Daphnia cannot control their own body temperature; their bodies are always the same temperature as the water around them, and their heart rate rises as their body temperature rises.
A student group wanted to know whether caffeine changes the heart rate of Daphnia. On Monday morning, in a cool lab (18 °C), they placed 5 Daphnia one at a time in plain pond water at room temperature. For each one, they counted heartbeats for 10 seconds and multiplied by 6 to get beats per minute. On Wednesday afternoon, with the same lab heated to 25 °C, they placed 5 different Daphnia, one at a time, in room-temperature pond water containing caffeine and counted the same way.
Group
Average heart rate (beats per minute)
Plain pond water (Monday, 18 °C room)
210
Pond water with caffeine (Wednesday, 25 °C room)
290
The group concluded that caffeine raises the heart rate of Daphnia.
Besides caffeine, which factor could best explain why the Daphnia in the caffeine group had higher heart rates?
Hint
Compare the conditions on Monday and Wednesday. Then reread what the first paragraph says about Daphnia body temperature.
Step-by-step solution
A confounding variable is a factor other than the one being tested that differed between groups and could cause the results. The plain-water group was tested in an 18 °C room, but the caffeine group was tested in a 25 °C room, using room-temperature water.
Step by step:
List what differed between groups: caffeine, day, time of day and room (and water) temperature.
The passage says Daphnia 'are always the same temperature as the water around them, and their heart rate rises as their body temperature rises.'
So the warmer water on Wednesday could raise heart rate even with no caffeine. The 80-beat difference cannot be credited to caffeine alone.
The correct answer is B.
Check: A is not a difference: both groups were counted the same way. C is a weakness, but a small sample alone does not explain why one group was consistently higher. D is the same for both groups.
Tip: When a passage gives you a background fact (such as the link between Daphnia body temperature and heart rate), it is usually the key to spotting the confound.
Question 31 of 36
Caffeine and Daphnia Heart Rate
Daphnia are tiny freshwater animals with see-through bodies, so their beating hearts can be watched under a microscope. Daphnia cannot control their own body temperature; their bodies are always the same temperature as the water around them, and their heart rate rises as their body temperature rises.
A student group wanted to know whether caffeine changes the heart rate of Daphnia. On Monday morning, in a cool lab (18 °C), they placed 5 Daphnia one at a time in plain pond water at room temperature. For each one, they counted heartbeats for 10 seconds and multiplied by 6 to get beats per minute. On Wednesday afternoon, with the same lab heated to 25 °C, they placed 5 different Daphnia, one at a time, in room-temperature pond water containing caffeine and counted the same way.
Group
Average heart rate (beats per minute)
Plain pond water (Monday, 18 °C room)
210
Pond water with caffeine (Wednesday, 25 °C room)
290
The group concluded that caffeine raises the heart rate of Daphnia.
Which redesign would best allow the students to test whether caffeine alone affects Daphnia heart rate?
Hint
The fix should make caffeine the only difference between the groups.
Step-by-step solution
To make a fair test, caffeine must be the only difference between the groups. That means testing both groups under the same conditions, ideally at the same time.
Step by step:
Keep the water temperature the same for both groups, for example 20 °C, measured with a thermometer.
Test plain-water and caffeine Daphnia on the same day, alternating between them so time of day is also the same.
Use more Daphnia per group (for example 15) so the averages are more reliable.
The correct answer is D.
Check: A removes the comparison group, so there is nothing to compare with. B keeps the original confound (different temperatures). C changes the caffeine amount but keeps the temperature difference, so the confound remains.
Tip: A good redesign removes the confound and keeps a control group. Check that every answer you choose still compares a treatment with a non-treatment under identical conditions.
Question 32 of 36
Caffeine and Daphnia Heart Rate
Daphnia are tiny freshwater animals with see-through bodies, so their beating hearts can be watched under a microscope. Daphnia cannot control their own body temperature; their bodies are always the same temperature as the water around them, and their heart rate rises as their body temperature rises.
A student group wanted to know whether caffeine changes the heart rate of Daphnia. On Monday morning, in a cool lab (18 °C), they placed 5 Daphnia one at a time in plain pond water at room temperature. For each one, they counted heartbeats for 10 seconds and multiplied by 6 to get beats per minute. On Wednesday afternoon, with the same lab heated to 25 °C, they placed 5 different Daphnia, one at a time, in room-temperature pond water containing caffeine and counted the same way.
Group
Average heart rate (beats per minute)
Plain pond water (Monday, 18 °C room)
210
Pond water with caffeine (Wednesday, 25 °C room)
290
The group concluded that caffeine raises the heart rate of Daphnia.
The students counted heartbeats for only 10 seconds and multiplied by 6. Why is this a possible source of measurement error?
Hint
What happens to a small counting mistake when it is multiplied by 6?
Step-by-step solution
Daphnia hearts beat very fast (about 3–5 beats per second), so counting is difficult. The passage says the students 'counted heartbeats for 10 seconds and multiplied by 6.' Any counting mistake is multiplied, too.
Step by step:
Suppose a student miscounts by just 1 beat in 10 seconds.
Multiplying by 6 turns that 1-beat slip into an error of 1 \times 6 = 6 beats per minute.
A miscount of 3 beats would become an 18 bpm error. Counting for a longer time, or recording video and counting it slowly, would reduce this error.
The correct answer is C.
Check: A is wrong because the arithmetic is correct: 10 seconds × 6 = 60 seconds. B is not supported by anything in the passage. D confuses measurement error with a confounding variable; the counting method was the same for both groups.
Tip: Measuring for a short time and then scaling up magnifies small mistakes. Longer measurements or better tools (such as video) reduce this kind of error.
Question 33 of 36
Temperature and Dissolving Tablets
Lena tested how water temperature affects the time it takes an antacid tablet to dissolve. For every trial she used tablets of the same brand and size and 150 mL of water in identical glasses. She did not stir the water. She ran three trials at each temperature and timed how long each tablet took to dissolve completely.
Water temperature (°C)
Trial 1 (s)
Trial 2 (s)
Trial 3 (s)
10
61
59
60
20
44
42
43
30
32
34
51
40
24
26
25
Lena now wants to test whether stirring affects how fast a tablet dissolves. Which plan is the best design?
Hint
Only one variable should change. Which plan keeps temperature constant and compares stirred with unstirred tablets?
Step-by-step solution
In the new investigation the independent variable is stirring (stirred or not stirred), and the dependent variable is still dissolving time. Everything else must stay the same, including temperature, which Lena already knows affects dissolving time (her table shows times falling from about 60 s at 10 °C to about 25 s at 40 °C).
Step by step:
Choice D keeps temperature fixed at 20 °C, keeps tablet size and water volume the same, compares stirred with unstirred tablets, and uses several trials of each. Only stirring changes, and repeated trials make it reliable.
Choice A changes two variables at once: stirred tablets are in 40 °C water and unstirred tablets in 10 °C water. If the stirred tablets dissolve faster, there is no way to tell whether stirring or heat caused it.
Choice B stirs every tablet, so there is no unstirred group to compare with. It tests temperature again, not stirring.
Choice C changes stirring speed AND temperature, with only one trial each.
The correct answer is D.
Tip: For any "best design" question, check three things: one independent variable, a comparison (control) group, and repeated trials.
Question 34 of 36
Fertilizer and Algae in Pond Water
Algae are tiny plant-like organisms that grow in water. Fertilizer from lawns can wash into ponds after rain. An environmental science class wondered whether fertilizer could make algae grow faster.
The class collected water from one pond and poured 1 L into each of 12 identical clear jars. They added 0, 1, 2 or 4 mL of liquid lawn fertilizer to the jars, three jars for each amount. All jars sat side by side on the same sunny windowsill for 14 days.
Algae turn water green and cloudy, so each day the students measured the water's turbidity (cloudiness) with a turbidity meter. Higher turbidity readings mean cloudier water and more algae. The table shows the average reading on day 14.
Fertilizer added (mL)
Average turbidity on day 14 (NTU)
0
5
1
18
2
34
4
61
Which choice describes what the class actually measured as the dependent variable?
Hint
Find the measuring tool named in the passage. What did it record?
Step-by-step solution
The class's question is about algae growth, but they did not count algae directly. The passage says the students 'measured the water's turbidity (cloudiness) with a turbidity meter,' and explains that higher readings mean more algae. Turbidity is the dependent variable they recorded.
Step by step:
Find the measurement: turbidity readings from a turbidity meter, each day for 14 days.
Confirm in the table: the results column is 'Average turbidity on day 14 (NTU).'
Turbidity is used as an indirect measure of how much algae grew.
The correct answer is B, the turbidity of the water in each jar.
Check: A is the independent variable. C sounds like a direct measure of algae, but no one counted cells. D was the same for all jars (same windowsill), so it is controlled.
Tip: The dependent variable is what was actually recorded, which is sometimes an indirect measure (cloudiness, balloon size, color) of what the scientists care about.
Question 35 of 36
Drag and dropFertilizer and Algae in Pond Water
Algae are tiny plant-like organisms that grow in water. Fertilizer from lawns can wash into ponds after rain. An environmental science class wondered whether fertilizer could make algae grow faster.
The class collected water from one pond and poured 1 L into each of 12 identical clear jars. They added 0, 1, 2 or 4 mL of liquid lawn fertilizer to the jars, three jars for each amount. All jars sat side by side on the same sunny windowsill for 14 days.
Algae turn water green and cloudy, so each day the students measured the water's turbidity (cloudiness) with a turbidity meter. Higher turbidity readings mean cloudier water and more algae. The table shows the average reading on day 14.
Fertilizer added (mL)
Average turbidity on day 14 (NTU)
0
5
1
18
2
34
4
61
The class next wants to test whether the amount of light affects algae growth when fertilizer is present. Drag the steps of the best plan into the correct order.
Step 1
Step 2
Step 3
Step 4
Tap a tile, then tap a box to place it.
Hint
Set up identical jars first, then change only the light, then measure, then compare.
Step-by-step solution
A good plan sets up identical conditions, changes only the factor being tested, measures the outcome the same way and then compares averages.
Step by step:
Set up: put 1 L of the same pond water and 2 mL of fertilizer into each of 9 identical jars, so water and fertilizer are controlled.
Change only the independent variable: place 3 jars in full sun, 3 in partial shade and 3 in darkness, all in the same room so the other conditions stay as alike as possible.
Measure the dependent variable: record each jar's turbidity every day for 14 days with the turbidity meter.
Analyze: compare the average day-14 turbidity of the three light groups.
The correct order is set up identical jars → vary the light → measure turbidity daily → compare averages.
Check: Measuring before the light is changed, or comparing before any data exist, would make the plan impossible to follow.
Tip: Most investigation plans follow the same order: control everything, change one thing, measure, then compare.
Question 36 of 36
Fertilizer and Algae in Pond Water
Algae are tiny plant-like organisms that grow in water. Fertilizer from lawns can wash into ponds after rain. An environmental science class wondered whether fertilizer could make algae grow faster.
The class collected water from one pond and poured 1 L into each of 12 identical clear jars. They added 0, 1, 2 or 4 mL of liquid lawn fertilizer to the jars, three jars for each amount. All jars sat side by side on the same sunny windowsill for 14 days.
Algae turn water green and cloudy, so each day the students measured the water's turbidity (cloudiness) with a turbidity meter. Higher turbidity readings mean cloudier water and more algae. The table shows the average reading on day 14.
Fertilizer added (mL)
Average turbidity on day 14 (NTU)
0
5
1
18
2
34
4
61
In a follow-up investigation, the students set up 12 new jars, each with 1 L of water from the same pond and 2 mL of fertilizer. They place 3 jars in each of four incubators set to 10 °C, 15 °C, 20 °C and 25 °C, all with identical lamps, and measure turbidity daily. What is the independent variable in this follow-up investigation?
Hint
In the follow-up, which factor is different from one group of jars to another?
Step-by-step solution
Each new investigation has its own variables. In the follow-up, the fertilizer amount is now the same (2 mL) in every jar, and the factor that differs between groups is the water temperature (10, 15, 20 and 25 °C).
Step by step:
List what differs between groups in the follow-up: only the incubator temperature.
List what is the same: pond water, volume (1 L), fertilizer (2 mL), lamps.
List what is measured: turbidity (the dependent variable, as before).
The correct answer is A, the water temperature.
Check: C is the trap: fertilizer amount was the independent variable in the original study, but in the follow-up it is held at 2 mL, so it is now controlled. B is the dependent variable. D is controlled because all incubators have identical lamps.
Tip: When the GED describes a follow-up study, identify the variables again from scratch. A factor that was changed before may now be held constant.
