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Question 1 of 20 · Earth and its system components
Fill inLayers of the Atmosphere
Earth's atmosphere is a layer of gases, about 78% nitrogen and 21% oxygen, with small amounts of argon, carbon dioxide and water vapor. Scientists divide it into layers by how temperature changes with altitude (height above sea level).
In the troposphere, the lowest layer, air is heated mainly from below by the warm ground, so temperature drops as you go up. Nearly all water vapor and clouds, and so nearly all weather, are found here. In the stratosphere, the ozone layer absorbs ultraviolet (UV) light from the Sun. This warms the air, so temperature rises with altitude. In the mesosphere, temperature falls again; most meteors burn up here. In the thermosphere, the very thin air absorbs high-energy radiation from the Sun, and temperature climbs sharply.
The graph shows average temperatures. At sea level the average is 15 °C. At the top of the troposphere, about 11 km up, it is about −56.5 °C.
Use the passage and graph. On average, by how many degrees Celsius per kilometer does the temperature drop as you go up through the troposphere?
°C per km
Type your answer in the box. Fractions like 3/4 are fine.
Hint
Find the total temperature change across the troposphere and divide by its thickness in kilometers.
Step-by-step solution
Where the answer comes from: the passage says "At sea level the average is 15 °C. At the top of the troposphere, about 11 km up, it is about −56.5 °C."
Rate: \frac{71.5\ ^\circ\text{C}}{11 \text{ km}} = 6.5\ ^\circ\text{C per km}
The temperature drops by about 6.5 °C per kilometer.
Check:15 - 6.5 \times 11 = 15 - 71.5 = -56.5 °C ✓
Tip: Subtracting a negative number is adding. Going from 15 to −56.5 is a 71.5-degree drop, not a 41.5-degree drop.
Question 2 of 20 · Structure of the cosmos
Select allThe Dwarf Planets
A dwarf planet orbits the Sun and has enough mass for its own gravity to pull it into a round shape, but unlike a planet, it has not cleared other objects out of its orbital path. Five dwarf planets are officially recognized. Distances are given in astronomical units (AU); 1 AU is Earth's average distance from the Sun, about 150 million km. Light from the Sun takes about 8.3 minutes to travel 1 AU.
Dwarf planet
Average distance from Sun (AU)
Orbital period (Earth years)
Diameter (km)
Ceres
2.8
4.6
940
Pluto
39.5
248
2,377
Haumea
43.1
284
about 1,600*
Makemake
45.8
306
1,430
Eris
67.7
559
2,326
*Haumea is shaped like a stretched egg; the value is an average.
Which dwarf planets take more than 300 Earth years to orbit the Sun once? Select all that apply.
Select all that apply.
Hint
Use the orbital period column and compare each value with 300.
Step-by-step solution
Where the answer comes from: the table's orbital period column: Ceres 4.6, Pluto 248, Haumea 284, Makemake 306, Eris 559 Earth years.
Step by step:
Compare each period with 300 years.
Ceres (4.6), Pluto (248) and Haumea (284) are all less than 300.
Makemake (306) and Eris (559) are more than 300.
The answers are Makemake and Eris.
Check: These are also the two farthest dwarf planets (45.8 AU and 67.7 AU), which fits the pattern that farther objects take longer to orbit ✓.
Tip: In select-all items with a cutoff, check every row. Values close to the cutoff (284 and 306 here) are where mistakes happen.
Question 3 of 20 · Structure of the cosmos
Star Colors and Star Fates
A star's color depends on its surface temperature. The hottest stars glow blue; cooler stars look yellow, orange or red. Astronomers sort stars into classes. Temperatures are in kelvins (K).
Class
Surface temperature (K)
Color
Example star
O
above 30,000
Blue
(rare)
B
10,000–30,000
Blue-white
Rigel
A
7,500–10,000
White
Sirius
F
6,000–7,500
Yellow-white
Procyon
G
5,200–6,000
Yellow
The Sun
K
3,700–5,200
Orange
Arcturus
M
2,400–3,700
Red
Proxima Centauri
Every star begins in a nebula, a cloud of gas and dust, which collapses into a protostar. For stars in the long, stable main sequence stage, more massive stars are hotter, bluer and brighter, and they use up their fuel much faster. So massive blue stars live only millions of years, while small red stars can shine for many billions.
A star with more than about 8 times the Sun's mass ends differently from a Sun-like star. After the main sequence it swells into a red supergiant and fuses heavier and heavier elements, up to iron, in its core. Then the core collapses and the star explodes as a supernova, scattering these elements into space, where they can become part of new stars and planets. The core left behind becomes a neutron star, or, if the star began with more than roughly 20–25 times the Sun's mass, a black hole. A Sun-like star instead becomes a red giant and ends as a white dwarf.
According to the table, which of these stars has the hottest surface?
Hint
Find each star in the Example column and compare its temperature range.
Step-by-step solution
Where the answer comes from: the table rows "| B | 10,000–30,000 | Blue-white | Rigel |", "| G | 5,200–6,000 | Yellow | The Sun |", "| K | 3,700–5,200 | Orange | Arcturus |" and "| M | 2,400–3,700 | Red | Proxima Centauri |".
Step by step:
Find each star's class: Rigel B, the Sun G, Arcturus K, Proxima Centauri M.
Class B is 10,000–30,000 K, hotter than G (5,200–6,000 K), K (3,700–5,200 K) and M (2,400–3,700 K).
The passage also says "The hottest stars glow blue." Rigel is blue-white.
The hottest star is Rigel.
Check: Even the coolest B star (10,000 K) is hotter than the hottest G star (6,000 K) ✓.
Tip: Star colors run opposite to everyday hot-and-cold color labels: in stars, blue is hottest and red is coolest.
Question 4 of 20 · Life functions and energy intake
Hot spotAlgae and Carbon Dioxide
Photosynthesis needs light, water, and carbon dioxide (CO₂). When one of these is in short supply, it limits how fast photosynthesis can go, even if the others are plentiful. A factor that holds back the rate in this way is called a limiting factor. Temperature also matters, because the reactions of photosynthesis are controlled by enzymes, which work more slowly when they are cool.
A biology class put the same amount of green algae into 12 sealed flasks of water under the same bright light. They added different amounts of dissolved CO₂ to the flasks. Six flasks were kept at 15 °C and six at 25 °C. The class measured how much oxygen the algae released in one hour.
Click the point on the 15 °C line where the algae released the same amount of oxygen as the 25 °C algae released at a CO₂ level of 5 mg/L.
Click on the figure to mark your answer (or focus it and use the arrow keys).
Hint
First read the blue line at 5 mg/L. Then look along the red line for that same height.
Step-by-step solution
Evidence: the 25 °C line (solid blue) is at 12 mg per hour at 5 mg/L of CO₂. The 15 °C line (dashed red) reads 0, 6, 12, 14, 15, 15 at 0, 5, 10, 15, 20, and 25 mg/L.
Step by step:
Find the target value: at 5 mg/L, the 25 °C algae released 12 mg per hour.
Move along the red line to find 12 mg per hour. It happens at 10 mg/L.
Do not click the blue point at 5 mg/L; that is the reference value on the wrong line. Do not click the red point at 5 mg/L (6 mg per hour) or 15 mg/L (14 mg per hour).
The result makes sense: cooler algae needed twice as much CO₂ to reach the same rate, because enzymes work more slowly when cool.
The answer is the red square at 10 mg/L (12 mg per hour).
Tip: two-step graph questions ask you to read one value, then find it somewhere else. Write the first value down before you look for the second point.
Question 5 of 20 · Life functions and energy intake
Fill inAlgae and Carbon Dioxide
Photosynthesis needs light, water, and carbon dioxide (CO₂). When one of these is in short supply, it limits how fast photosynthesis can go, even if the others are plentiful. A factor that holds back the rate in this way is called a limiting factor. Temperature also matters, because the reactions of photosynthesis are controlled by enzymes, which work more slowly when they are cool.
A biology class put the same amount of green algae into 12 sealed flasks of water under the same bright light. They added different amounts of dissolved CO₂ to the flasks. Six flasks were kept at 15 °C and six at 25 °C. The class measured how much oxygen the algae released in one hour.
According to the graph, at a CO₂ level of 20 mg/L, how many more milligrams of oxygen per hour did the algae at 25 °C release than the algae at 15 °C?
mg per hour
Type your answer in the box. Fractions like 3/4 are fine.
Hint
Find 20 mg/L on the horizontal axis. Read the blue point and the red point straight above it, then subtract.
Step-by-step solution
Evidence: at 20 mg/L of CO₂, the 25 °C line (solid blue) is at 26 mg per hour and the 15 °C line (dashed red) is at 15 mg per hour.
Step by step:
Locate 20 on the x-axis.
Read up to the blue circle: 26 mg per hour.
Read up to the red square: 15 mg per hour.
Subtract: 26 - 15 = 11\text{ mg per hour}
Be careful not to read the 15 °C value at 15 mg/L (14) by mistake; the 15 on the axis and the 15 °C label are different things.
The answer is 11 mg per hour.
Check:15 + 11 = 26 ✓
Tip: for "how many more" questions on a two-line graph, read both values at the same x-value, then subtract the smaller from the larger.
Question 6 of 20 · Life functions and energy intake
Select allIndicator Tubes
Bromothymol blue (BTB) is a dye that shows how much CO₂ is dissolved in water. It is blue when there is little CO₂. As more CO₂ dissolves, the water becomes more acidic, and the dye turns green and then yellow. When CO₂ is removed from the water, the color shifts back toward blue.
A student filled six tubes with the same green BTB solution, which means a medium amount of CO₂. She added pond organisms, sealed the tubes, and placed three tubes in bright light and three in a dark cabinet at the same temperature. After 4 hours, she recorded the color of each tube.
Tube
Contents
Condition
Color after 4 hours
1
water snail
light
yellow
2
pond plant
light
blue
3
water snail and pond plant
light
green
4
water snail
dark
yellow
5
pond plant
dark
yellow
6
nothing added
light
green
In which tubes did living things add more CO₂ to the water than they removed? Select all that apply.
Select all that apply.
Hint
Yellow means CO₂ went up. Check each tube's final color, and remember what each organism can do in the dark.
Step-by-step solution
Evidence: the passage says the dye turns "green and then yellow" as more CO₂ dissolves and shifts "back toward blue" when CO₂ is removed. Final colors: Tube 1 yellow, Tube 2 blue, Tube 3 green, Tube 4 yellow, Tube 5 yellow, Tube 6 green (no living things).
Step by step:
Tube 1 (snail, light) — yes. Yellow: the snail's respiration added CO₂, and nothing removed it.
Tube 2 (plant, light) — no. Blue: photosynthesis removed more CO₂ than respiration added.
Tube 3 (snail and plant, light) — no. Green: CO₂ added and removed were about equal.
Tube 4 (snail, dark) — yes. Yellow: the snail respires in the dark too.
Tube 5 (plant, dark) — yes. Yellow: with no light, the plant cannot photosynthesize, but it still respires, so it adds CO₂.
The correct choices are A, D, and E.
Tip: plants respire all the time but photosynthesize only in light. In the dark, a plant behaves like an animal in terms of CO₂.
Question 7 of 20 · Life functions and energy intake
Indicator Tubes
Bromothymol blue (BTB) is a dye that shows how much CO₂ is dissolved in water. It is blue when there is little CO₂. As more CO₂ dissolves, the water becomes more acidic, and the dye turns green and then yellow. When CO₂ is removed from the water, the color shifts back toward blue.
A student filled six tubes with the same green BTB solution, which means a medium amount of CO₂. She added pond organisms, sealed the tubes, and placed three tubes in bright light and three in a dark cabinet at the same temperature. After 4 hours, she recorded the color of each tube.
Tube
Contents
Condition
Color after 4 hours
1
water snail
light
yellow
2
pond plant
light
blue
3
water snail and pond plant
light
green
4
water snail
dark
yellow
5
pond plant
dark
yellow
6
nothing added
light
green
Tube 1, which held only a water snail, turned yellow. Which gas did the snail release that caused this change?
Hint
Yellow means more of one gas dissolved in the water. Which gas does an animal give off when it carries out cellular respiration?
Step-by-step solution
Evidence: the passage says BTB is blue with little CO₂, and "As more CO₂ dissolves, the water becomes more acidic, and the dye turns green and then yellow." Tube 1 (snail, light) went from green to yellow.
Step by step:
Yellow means the CO₂ in the water increased.
A snail is an animal. Animals carry out cellular respiration, which uses oxygen and glucose and releases carbon dioxide and water.
Snails do not photosynthesize, so nothing in Tube 1 removed the CO₂, even in the light.
Eliminate A (oxygen is taken in during respiration, not released), C (nitrogen is not part of respiration), and D (glucose is a sugar the snail uses up, not a gas it releases).
The answer is B. Carbon dioxide.
Tip: link the indicator color to a gas first (yellow = more CO₂), then ask which process in that tube adds that gas.
Question 8 of 20 · Organization of life: cells, tissues, organisms
Four Mystery Samples
A lab technician examined cells from four unlabeled samples with a powerful microscope. She knows that one sample is a bacterium, one is from an animal, one is from the leaf of a green plant, and one is from a mushroom (a fungus). Fungus cells have a nucleus and a cell wall but no chloroplasts. She recorded which structures she saw and the typical width of the cells.
Structure
Sample 1
Sample 2
Sample 3
Sample 4
Cell membrane
Yes
Yes
Yes
Yes
Ribosomes
Yes
Yes
Yes
Yes
Nucleus
No
Yes
Yes
Yes
Mitochondria
No
Yes
Yes
Yes
Cell wall
Yes
Yes
Yes
No
Chloroplasts
No
No
Yes
No
Typical width (micrometers)
2
8
40
20
Which sample is most likely the bacterium, and what evidence in the table supports this?
Hint
Bacteria are prokaryotes. Which structure do prokaryotic cells lack that all the other cells have?
Step-by-step solution
Where the answer comes from: in the table, Sample 1 is the only sample with "No" in the Nucleus row (and also "No" for mitochondria). It is also the smallest, about 2 micrometers wide.
Step by step:
Bacteria are prokaryotes: their DNA floats in the cytoplasm instead of sitting inside a nucleus, and they have no membrane-covered organelles such as mitochondria.
Scan the Nucleus row: Samples 2, 3 and 4 have one; Sample 1 does not.
Samples 2, 3 and 4 are therefore eukaryotes (fungus, plant and animal).
Answer: Sample 1, because it has no nucleus.
Check: bacteria are usually only a few micrometers long, and Sample 1 is the smallest cell (2 micrometers), which fits.
Tip: good evidence must belong to only one choice. A cell wall or a missing chloroplast is shared by several samples, so it cannot single one out.
Question 9 of 20 · Heredity and genetics
Select allABO Blood Types
Human ABO blood type is controlled by one gene with three alleles: I^A, I^B and i. Every person has two of these alleles, one from each parent.
Alleles I^A and I^B are codominant: when a person has both, both are fully expressed, and the person has type AB blood. Both I^A and I^B are dominant over i.
Blood type
Possible genotypes
A
I^A I^A or I^A i
B
I^B I^B or I^B i
AB
I^A I^B
O
i\,i
A mother has type AB blood and a father has type O blood. Select all the blood types their children could have.
Select all that apply.
Hint
Write each parent's genotype from the table. Each child gets one allele from the mother and one from the father.
Step-by-step solution
Where the answer comes from: the table shows type AB has only one genotype, I^A I^B, and type O has only one genotype, i\,i.
Step by step:
The mother can pass on I^A or I^B. The father can pass on only i.
Possible children: I^A i → type A, and I^B i → type B.
Type AB would need both I^A and I^B, but the father has neither. Type O would need i from both parents, but the mother has no i.
Answer: type A and type B.
Check: none of the children would have the same blood type as either parent, a surprising but correct result that blood-type tests confirm.
Tip: list every allele each parent can pass on, then combine them one from each side. Do not assume children can match a parent.
Question 10 of 20 · Energy: conservation, transformation, flow
Drag and dropWhere Millbrook Gets Its Electricity
The town of Millbrook (a fictional town) published how much electricity it used last year from each energy source. Electrical energy is measured here in gigawatt-hours (GWh).
Energy source
Electricity (GWh)
Natural gas
420
Coal
180
Nuclear (uranium)
150
Wind
120
Hydroelectric (river dam)
70
Solar
60
Total
1,000
Renewable energy sources are replaced naturally in a human lifetime, such as sunlight, wind and flowing water. Nonrenewable sources, such as coal, natural gas, oil and uranium, exist in limited amounts and take millions of years to form or cannot be replaced at all. The town council wants at least 40% of its electricity to come from renewable sources within ten years.
Use the definitions in the passage. Drag each energy source into the correct category.
Renewable
Nonrenewable
Tap a tile, then tap a box to place it.
Hint
Ask: will nature replace this source within a human lifetime?
Step-by-step solution
Evidence: The passage defines renewable sources as those "replaced naturally in a human lifetime, such as sunlight, wind and flowing water," and names "coal, natural gas, oil and uranium" as nonrenewable.
Step by step:
Wind — wind keeps blowing; it is replaced naturally → renewable.
Solar — sunlight arrives every day → renewable.
Hydroelectric — rivers are refilled by rain and snow (flowing water) → renewable.
Natural gas and coal — fossil fuels that took millions of years to form → nonrenewable.
Nuclear (uranium) — uranium is a mined element in limited supply → nonrenewable, even though nuclear plants release no carbon dioxide while running.
Tip: "Renewable" is about whether the supply is replaced, not about pollution. Do not move nuclear into the renewable box just because it is low-carbon.
Question 11 of 20 · Work, motion and forces
Testing a Delivery Drone
A delivery company tested a drone on a straight route between its warehouse and a customer's house. The table shows the three legs of the test trip.
Leg
What the drone did
Distance (m)
Time (s)
1
Flew from the warehouse to the house
1,800
120
2
Hovered while lowering the package
0
60
3
Flew back to the warehouse against the wind
1,800
180
Speed is distance divided by time: v = \dfrac{d}{t}. The average speed for a whole trip is the total distance traveled divided by the total time, including any time spent not moving.
What was the drone's speed during Leg 1 of the test trip?
Hint
Use only the Leg 1 row of the table. Speed = distance ÷ time.
Step-by-step solution
Evidence: The table shows Leg 1: "Flew from the warehouse to the house," 1,800 m in 120 s. The passage gives v = \dfrac{d}{t}.
Step by step:
Use only Leg 1's numbers: d = 1{,}800 m, t = 120 s.
Divide distance by time:
v = \frac{1{,}800\text{ m}}{120\text{ s}} = 15\text{ m/s}
The drone flew at 15 m/s during Leg 1.
Check:15\text{ m/s} \times 120\text{ s} = 1{,}800 m, matching the table. Flipping the fraction (120 ÷ 1,800) gives about 0.067, which would be seconds per meter, not meters per second.
Tip: The units tell you the order: m/s means meters on top, seconds on the bottom.
Question 12 of 20 · Work, motion and forces
Pulling a Sled
A parent pulls a child on a sled across level snow, holding the rope parallel to the ground. Together, the sled and child have a mass of 25 kg. The force diagram shows all the forces acting on the sled.
The net force is the combined effect of all the forces. Forces in opposite directions are subtracted. Newton's first law says an object keeps moving at a constant velocity unless a net force acts on it. Newton's second law says F_{\text{net}} = ma. Newton's third law says that when one object pushes or pulls on a second object, the second object pushes or pulls back on the first with a force of equal size in the opposite direction.
The parent is pulling the sled forward across the snow. When all four forces in the diagram are combined, what is the net force acting on the sled?
Hint
The up and down forces cancel. Combine the two horizontal forces, which point in opposite directions.
Step-by-step solution
Evidence: The diagram shows a rope pull of 150 N to the right and friction of 40 N to the left. Gravity (245 N down) and the normal force (245 N up) are equal and opposite. The passage says "Forces in opposite directions are subtracted."
Step by step:
Vertical:245\text{ N up} - 245\text{ N down} = 0. The sled does not move up or down.
Horizontal: the forces point in opposite directions, so subtract:
F_{\text{net}} = 150\text{ N} - 40\text{ N} = 110\text{ N}
The larger force points right, so the net force points right, the direction of the pull.
The net force is 110 N to the right.
Check: If the forces were added (190 N), friction would be helping the pull, but friction always opposes the sliding motion.
Tip: Find the net force one direction at a time. Same direction: add. Opposite directions: subtract, and the net force points the way of the larger force.
Question 13 of 20 · Work, motion and forces
Pulling a Sled
A parent pulls a child on a sled across level snow, holding the rope parallel to the ground. Together, the sled and child have a mass of 25 kg. The force diagram shows all the forces acting on the sled.
The net force is the combined effect of all the forces. Forces in opposite directions are subtracted. Newton's first law says an object keeps moving at a constant velocity unless a net force acts on it. Newton's second law says F_{\text{net}} = ma. Newton's third law says that when one object pushes or pulls on a second object, the second object pushes or pulls back on the first with a force of equal size in the opposite direction.
Use the force diagram and Newton's second law, F_{\text{net}} = ma.
What is the acceleration of the sled?
Hint
Use the net force, not just the rope's pull. Use the mass in kilograms, not the weight in newtons.
Step-by-step solution
Evidence: The diagram shows a 150 N pull to the right and 40 N of friction to the left; the vertical forces (245 N each) cancel. The passage gives the mass as "25 kg" and Newton's second law as F_{\text{net}} = ma.
Step by step:
Net force: 150 - 40 = 110 N to the right.
Solve for acceleration: a = \dfrac{F_{\text{net}}}{m}.
Substitute:
a = \frac{110\text{ N}}{25\text{ kg}} = 4.4\ \text{m/s}^2
The sled accelerates at 4.4 m/s² to the right.
Check:25 \times 4.4 = 110 N, which equals the net force. ✓
Tip: Newton's second law uses the net force. Find it first, then divide by the mass.
Question 14 of 20 · Chemical properties and reactions
Where Did the Mass Go?
A student mixed vinegar and baking soda, two substances that react to produce carbon dioxide gas, water and a dissolved salt. She ran the reaction twice, measuring the total mass of everything on a balance before and after.
Trial 1 (open flask): She placed a flask of vinegar and a small cup of baking soda on the balance: 250.0 g in total. She poured the baking soda into the flask. The mixture fizzed and bubbled for a minute. When the bubbling stopped, the balance read 247.8 g.
Trial 2 (sealed flask): She repeated the experiment with the same amounts, but this time she stretched a balloon over the mouth of the flask before tipping the baking soda in from inside the balloon. The total mass, including the balloon, was 252.0 g before. The mixture fizzed and the balloon inflated. When the bubbling stopped, the balance still read 252.0 g.
The law of conservation of mass states that in a chemical reaction, matter is neither created nor destroyed: the total mass of the products equals the total mass of the reactants.
Which statement best explains the different results of Trial 1 and Trial 2?
Hint
What could leave the open flask but not the sealed one?
Step-by-step solution
Evidence: In Trial 1 (open flask) the mass dropped from 250.0 g to 247.8 g. In Trial 2 (sealed with a balloon) "the balloon inflated" and the balance "still read 252.0 g." The passage states that "matter is neither created nor destroyed."
Step by step:
Both trials used the same reaction, which produces carbon dioxide gas.
In Trial 1 the gas escaped into the air, so the balance no longer measured it.
In Trial 2 the balloon trapped the gas (that is why it inflated), so everything stayed on the balance and the mass did not change.
Together, the trials show that mass is conserved; the open flask only seemed to lose mass.
The answer is The gas produced escaped from the open flask but was trapped by the balloon, so mass was conserved in both trials.
Tip: When two trials differ in only one condition (open vs. sealed), that condition explains the difference in results.
Question 15 of 20 · Chemical properties and reactions
Testing Garden Soil
Different plants grow best in soil with different pH values. A community garden posted this chart for its members.
Plant
Soil pH the plant prefers
Blueberries
4.5 to 5.5
Potatoes
5.0 to 6.0
Tomatoes
6.0 to 6.8
Asparagus
6.5 to 7.5
Lilacs
7.0 to 8.0
A pH of 7 is neutral; a pH below 7 is acidic and a pH above 7 is basic. The pH scale is logarithmic: each drop of one pH unit means the soil is 10 times more acidic. Garden lime (calcium carbonate) is a base, so mixing it into soil neutralizes some of the acid and raises the pH. Garden sulfur slowly forms an acid in moist soil and lowers the pH.
Ana tested the soil in her plot. Its pH was 4.8.
How many times more acidic is Ana's soil than soil with a pH of 6.8, the top of the tomato range?
Hint
Find the difference in pH units. Each unit is a factor of 10, and the factors multiply.
Step-by-step solution
Evidence: The passage says "The pH scale is logarithmic: each drop of one pH unit means the soil is 10 times more acidic." Ana's soil is pH 4.8.
Step by step:
Difference in pH: 6.8 - 4.8 = 2 units.
Each unit multiplies the acidity by 10, so two units multiply it by
10 \times 10 = 10^2 = 100
Ana's soil has the lower pH, so it is the more acidic one.
Ana's soil is 100 times more acidic.
Check: Step by step: pH 6.8 → 5.8 is 10 times more acidic; 5.8 → 4.8 is another 10 times; 10 \times 10 = 100. ✓
Tip: For pH comparisons, subtract the pH values and raise 10 to that power. A difference of 1 means 10 times, 2 means 100 times, 3 means 1,000 times.
Question 16 of 20 · Reasoning from data
Heating a Pot of Water
A student heated 1 liter of water in a pot on a stove that stayed at the same setting the whole time. She measured the water's temperature with a thermometer every 2 minutes, starting when she turned on the burner. The experiment took place at sea level, where pure water boils at 100 °C. The water began to boil at the 10-minute reading.
The student did not take a reading at 5 minutes. Based on the graph, what was the water temperature most likely at 5 minutes?
Hint
5 minutes is halfway between two readings. What temperature is halfway between them?
Step-by-step solution
This is interpolation: estimating a value between two measured points. The graph shows 52 °C at 4 minutes and 68 °C at 6 minutes. Because the points lie on a straight line, the temperature at 5 minutes should be halfway between them.
Step by step:
Find the readings on each side of 5 minutes: 52 °C (4 min) and 68 °C (6 min).
Find the halfway value: \dfrac{52 + 68}{2} = 60 °C.
On the graph, the line crosses the 5-minute gridline exactly at the 60 °C gridline.
The correct answer is 60 °C.
Check: The water warms 8 °C per minute. From 20 °C, after 5 minutes: 20 + 5 \times 8 = 60 °C. Same answer.
Tip: For a value between two data points, use the points on both sides. Picking just the nearest point gives a value that is too high or too low.
Question 17 of 20 · Evaluating conclusions with evidence
Drop-downDo Heavier Objects Fall Faster?
A student claimed: "Heavier objects fall faster than lighter ones." His class tested the claim with three balls that were the same size and shape but had different masses. Each ball was dropped from a height of 2.0 meters, and its fall was timed with a video recording. The class ran 5 trials for each ball.
Ball
Mass (g)
Mean fall time (s)
Plastic
10
0.65
Rubber
45
0.64
Steel
110
0.64
Later, the class dropped a flat sheet of paper (5 g) from the same height. It took 1.9 seconds to land. They then crumpled the same sheet into a tight ball and dropped it again. This time it took 0.67 seconds.
Complete the statement.
The results for the three balls the student's claim, because .
Choose an answer from each menu.
Hint
Compare the mean fall times. Did the steel ball (11 times heavier than the plastic one) land much sooner?
Step-by-step solution
Use the table to test the claim. If heavier objects fell faster, the 110 g steel ball should take noticeably less time than the 10 g plastic ball.
Evidence: Mean fall times — plastic 0.65 s, rubber 0.64 s, steel 0.64 s.
Step by step:
The masses differ a lot: 10 g, 45 g, 110 g.
The fall times differ by only 0.01 s — a gap smaller than normal timing error.
So mass made almost no difference: the data do not support the claim.
The correct answers are do not support and the balls took nearly the same time to fall even though their masses were very different.
Check: The balls were the same size and shape, so air resistance affected them about equally. That makes mass the only real difference, and it barely changed the times.
Tip: A tiny difference in results (0.01 s here) is not strong evidence of an effect, especially when the variable being tested changed by a large amount.
Question 18 of 20 · Working with findings
Testing an Antibiotic Against Bacteria
A hospital lab tested how much of an antibiotic is needed to stop the growth of a common bacterium. Technicians added the same number of bacteria to six tubes of nutrient broth. Each tube received a different concentration of the antibiotic, measured in micrograms per milliliter (µg/mL). After 24 hours at body temperature (37 °C), they measured how cloudy each tube was. Cloudier broth means more bacteria grew. The lab counts a cloudiness reading below 0.10 as "no growth."
The readings were 0.90 at 0 µg/mL, 0.85 at 1 µg/mL, 0.60 at 2 µg/mL, 0.05 at 4 µg/mL, 0.04 at 8 µg/mL and 0.04 at 16 µg/mL.
Bacteria can change over time. Some bacteria carry a gene that lets them break down or pump out an antibiotic. Bacteria with such a gene are called resistant.
The lab later tested bacteria of the same species taken from a different patient. In the same test, these bacteria made the broth very cloudy (0.82) even at 16 µg/mL. Which explanation is best supported by the passage?
Hint
The first sample stopped growing at 4 µg/mL. What does the passage say can make some bacteria survive an antibiotic?
Step-by-step solution
Where the answer comes from: the first sample showed "0.04 at 16 µg/mL" (no growth). The passage also states: "Some bacteria carry a gene that lets them break down or pump out an antibiotic. Bacteria with such a gene are called resistant."
Step by step:
Same species, same test, very different result: 0.82 versus 0.04 at 16 µg/mL.
The passage gives a ready explanation: some bacteria carry a resistance gene. The second patient's bacteria most likely have it.
The idea that the antibiotic helps bacteria grow is contradicted by the first test, where 16 µg/mL stopped growth.
Nothing in the passage says the temperature changed; the test used 37 °C, and "the same test" was run.
Correct answer: B.
Check: a good explanation of an unexpected result fits both the old and new findings. Resistance explains why one sample died and the other did not.
Tip: When a result surprises you, do not throw out the earlier data. Look for a difference between the two samples that the passage gives you.
Question 19 of 20 · Scientific theories
Fill inPushing Carts: Newton's Second Law
Newton's second law of motion states that the acceleration of an object depends on the net force on it and on its mass:
F = ma
where F is the net force in newtons (N), m is the mass in kilograms (kg), and a is the acceleration in meters per second squared (m/s²).
Students pulled carts along a nearly frictionless track with a spring scale and measured the acceleration with a motion sensor.
Trial
Cart mass (kg)
Pulling force (N)
Acceleration (m/s²)
1
2.0
6.0
3.0
2
4.0
6.0
1.5
3
2.0
12.0
6.0
The students also studied work, the energy transferred when a force moves an object. Work is found with W = Fd, where W is work in joules (J), F is force in newtons (N), and d is the distance moved in meters (1 m = 100 cm).
A student pulls a cart with a constant force of 6.0 N for a distance of 150 cm along the track. How much work does she do on the cart?
J
Type your answer in the box. Fractions like 3/4 are fine.
Hint
The formula needs distance in meters. Convert first.
Step-by-step solution
Where the answer comes from: the passage gives W = Fd, "where … d is the distance moved in meters (1 m = 100 cm)."
Step by step:
Convert the distance: 150\text{ cm} \div 100 = 1.5 m.
Substitute: W = Fd = 6.0\text{ N} \times 1.5\text{ m}.
Multiply: 6.0 \times 1.5 = 9.0.
Units: newton × meter = joule (J).
W = 6.0\text{ N} \times 1.5\text{ m} = 9.0\text{ J}
Correct answer: 9 J.
Check: If you forgot to convert, you would get 6.0 \times 150 = 900 — a hundred times too large. That is the most common trap on this kind of question.
Tip: Before substituting, make every value match the units the formula expects (here, newtons and meters).
Question 20 of 20 · Probability and statistics in science
Hot spotAntacid Tablets and Water Temperature
A chemistry student timed how long one antacid tablet took to dissolve completely in 200 mL of water at four temperatures. She repeated each temperature six times.
On the graph, each bar shows the mean dissolving time. The error bar shows the range within which the true mean is likely to fall. A longer error bar means the six trials at that temperature were less consistent.
How to read error bars: if the error bars of two groups overlap, the difference between their means may be due to chance. If they do not overlap, the difference is more likely to be real.
Click the bar for the temperature whose mean dissolving time is not clearly different from the mean time at 25 °C.
Click on the figure to mark your answer (or focus it and use the arrow keys).
Hint
Write down the 25 °C error bar's range (41 to 49 s). Which other error bar overlaps it?
Step-by-step solution
Where the answer comes from: the graph shows 10 °C: 62 s (56 to 68 s); 25 °C: 45 s (41 to 49 s); 40 °C: 38 s (30 to 46 s); 55 °C: 24 s (22 to 26 s). The passage says that if error bars "overlap, the difference between their means may be due to chance."
Step by step:
The 25 °C error bar runs from 41 to 49 s.
10 °C: lowest value 56 s is above 49 s → no overlap → clearly different.
40 °C: 30 to 46 s. The top part (41 to 46 s) lies inside the 25 °C range → overlap → not clearly different.
55 °C: highest value 26 s is below 41 s → no overlap → clearly different.
Correct answer: the 40 °C bar.
Check: the 40 °C bar also has the longest error bar (16 s), showing its six trials were the least consistent, which is why its mean is uncertain.
Tip: Compare the ends of the error bars, not the tops of the bars. Two means can look different and still overlap once their uncertainty is included.