GED Science Practice Test 3

The passage explains: "Sea otters eat large numbers of sea urchins. Where otters are missing, sea urchin populations can grow quickly and graze the kelp down to bare rock."

Step by step:

1. Fewer sea otters means fewer predators eating sea urchins.
2. With less predation, the sea urchin population increases.
3. Sea urchins eat kelp (arrow from Kelp to Sea urchin), so more urchins means more kelp is eaten. The kelp decreases, and urchin barrens can form.
4. This chain of effects running down the food web is called a trophic cascade. It is why the passage calls sea otters a keystone species.

Correct answer: Sea urchins would increase, and kelp would decrease.

Tip: To predict the effect of removing a predator, move one step down the food web at a time: prey goes up, and whatever the prey eats goes down.

The passage says: "Once a week, the class added a different amount of garden fertilizer to each tank." That is the only factor the class deliberately set to a different value in each tank.

Step by step:

1. Independent variable (changed on purpose): amount of fertilizer added per week, 0, 5, 10 or 20 g.
2. Dependent variables (measured to see the effect): the amount of algae (chlorophyll) and dissolved oxygen at dawn.
3. Controlled variables (kept the same): 100 liters of the same pond water, the same amount of sunlight, the same 4-week time.

Correct answer: the amount of fertilizer added to each tank each week.

Tip: Look in the table's first data column or the graph's horizontal axis. The values the experimenter chose (0, 5, 10, 20) are the independent variable; values that came out of measurement (8.1, 6.4 …) are dependent variables.

The passage gives the key facts: "During the day, algae release oxygen into the water through photosynthesis. At night, photosynthesis stops, but algae and other organisms keep using oxygen for cellular respiration. When algae die, bacteria decompose them, and these bacteria also use up dissolved oxygen."

Step by step:

1. Afternoon (11.0 mg/L): sunlight drives photosynthesis by the huge number of algae, so they add a lot of oxygen. The pond's oxygen can rise very high.
2. Dawn (2.5 mg/L): all night there was no photosynthesis, but respiration by algae and by bacteria decomposing dead algae kept using oxygen. By dawn the oxygen is at its lowest.
3. Tanks: the class measured at dawn. More fertilizer → more algae → more oxygen used overnight → lower dawn oxygen (8.1 → 6.4 → 4.2 → 1.9 mg/L).
4. So the findings do not conflict: a pond with many algae can swing from very high oxygen in the afternoon to dangerously low oxygen at dawn. 2.5 mg/L is well below the 5 mg/L level at which many fish become stressed.

Correct answer: Algae add oxygen by photosynthesis in daylight, but at night respiration by algae and bacteria uses it up.

Tip: When two findings seem to disagree, check whether they were measured under different conditions (time of day, place, method). The difference in conditions often explains the difference in results.

Combine the data with the passage.

Step by step:

1. Data: Tank 4 received the most fertilizer (20 g per week) and had by far the most algae, 62 µg/L of chlorophyll. Its dawn oxygen was the lowest, 1.9 mg/L.
2. Passage: at night "algae and other organisms keep using oxygen for cellular respiration," and "When algae die, bacteria decompose them, and these bacteria also use up dissolved oxygen."
3. The tank with the most algae has the most respiration overnight and the most dead algae for bacteria to decompose, so it loses the most oxygen.

Correct answer: Tank 4 had the most algae, so nighttime respiration and decomposition used up the most oxygen.

Note that the passage never says fertilizer itself reacts with oxygen; the effect works through the living things in the water.

Tip: A "best supported" explanation must be backed by both the data and the background text. Reject choices that add a new mechanism the passage never mentions or that contradict the table.

An electric current flowing through a wire produces a magnetic field that circles around the wire. A compass needle is a small magnet, so it lines up with that field. That is why the needle swung away from north when the current started.

Step by step:

1. The direction of the magnetic field around a wire depends on the direction of the current.
2. Reversing the battery connections reverses the current.
3. So the magnetic field under the wire reverses too, and the needle swings the opposite way: toward the west instead of the east.

Correct answer: It will swing to point toward the west, the opposite direction.

This link between electricity and magnetism is the same idea behind electromagnets and electric motors: change the current's direction and you change the magnetic field's direction.

Tip: In "what happens if you reverse…" questions, ask whether the effect depends on direction. Magnetic fields from currents do, so reversing the cause reverses the effect.

To compare foods fairly, divide each food's Calories by its serving size (the "Serving size (g)" column).

Step by step:

1. Granola bar: 29 \times 4 + 3 \times 4 + 7 \times 9 = 116 + 12 + 63 = 191 Calories; 191 \div 40 \approx 4.8 Calories per gram.
2. Greek yogurt: 6 \times 4 + 17 \times 4 + 4 \times 9 = 24 + 68 + 36 = 128 Calories; 128 \div 170 \approx 0.75 Calories per gram.
3. Peanut butter: 7 \times 4 + 8 \times 4 + 16 \times 9 = 28 + 32 + 144 = 204 Calories; 204 \div 32 \approx 6.4 Calories per gram.

Peanut butter has about the same Calories per serving as the granola bar, but its serving weighs less (32 g vs. 40 g), and most of its energy comes from fat at 9 Calories per gram.

Correct answer: peanut butter, about 6.4 Calories per gram.

Tip: "Per gram," "per hour" and "per person" all mean divide. Comparing totals without dividing by the amount is a common trap when servings are different sizes.

Read the white bread line (red squares) on the graph. Its values are 90, 150, 135, 110, 95, 85 and 88 mg/dL at 0, 30, 60, 90, 120, 150 and 180 minutes.

Step by step:

1. Make sure you are on the right line: the legend shows white bread as red squares and lentils as blue circles.
2. The highest red point is 150 mg/dL.
3. Read straight down to the time axis: that point is at 30 minutes.

Correct answer: 30 minutes.

The lentil meal peaked later and lower (118 mg/dL at 60 minutes), which is why 60 minutes is a tempting wrong answer if you read the wrong line.

Tip: With two lines on one graph, check the legend first and keep your finger on the correct line. Then read the axis directly below or beside the point.

The passage explains: "When blood glucose rises, the pancreas releases the hormone insulin. Insulin signals body cells, especially muscle, fat and liver cells, to take in glucose from the blood."

Step by step:

1. After the white bread meal, glucose rose quickly to 150 mg/dL at 30 minutes.
2. The rise triggered the pancreas to release insulin.
3. Insulin caused body cells to take glucose out of the blood, so blood glucose fell to 110 mg/dL by 90 minutes.
4. This is homeostasis: the body's feedback system moving blood glucose back toward its normal range.

Correct answer: The pancreas released insulin, which signaled body cells to take in glucose from the blood.

Tip: Insulin lowers blood glucose (glucose goes into cells); glucagon raises it (the liver releases glucose). Remember: insulin lets glucose in.

In living things, structure is related to function: the shape of a body part helps it do its job. The job described here is moving digested nutrients "through these cells into the blood," which is called absorption.

Step by step:

1. Folding a surface into millions of finger-like villi, each covered with microvilli, greatly increases the surface area of the lining.
2. More surface area means more cells are in contact with digested food at the same time.
3. So nutrients can be absorbed into the blood much faster and more completely.

Correct answer: The folds greatly increase the surface area, so more nutrients can be absorbed into the blood.

The same idea, a large surface packed into a small space, appears in the air sacs of the lungs and the root hairs of plants.

Tip: When a GED question describes folds, branches or tiny projections, think surface area. Then link it to the job the passage names, here absorbing nutrients.

A research question names what is changed and what is measured. The passage says the class "tested 10, 20, 30, 40 and 50 turns of wire, first with one D cell (1.5 V) and then with two D cells," and "counted how many identical small steel paper clips the nail could lift."

Step by step:

1. Changed on purpose (independent variables): the number of turns of wire and the number of D cells.
2. Measured (dependent variable): the number of paper clips lifted, a measure of the electromagnet's strength.
3. Kept the same: the nail ("the same large iron nail"), the wire and the paper clips ("identical").

Correct answer: How do the number of turns and the number of D cells affect how many clips are lifted?

Tip: A good test of a research question: every variable it names should have been changed or measured in the experiment. Anything that was kept the same (the nail, the clips) cannot be what the investigation is testing.

Use the two-D-cell line (red squares): 6, 11, 16, 21 and 27 paper clips at 10, 20, 30, 40 and 50 turns.

Step by step:

1. Find the pattern: each extra 10 turns adds about 5 clips (11 - 6 = 5, 16 - 11 = 5, 21 - 16 = 5, 27 - 21 = 6).
2. The line is nearly straight, so continue the trend one more step: 27 + 5 = 32.
3. A value of about 32 paper clips fits the trend.

Correct answer: about 32.

Predictions outside the tested range (extrapolation) are less certain than predictions between tested points, but "most likely … based on the trend" asks you to extend the pattern you see.

Tip: For a prediction question, (1) pick the correct line, (2) find the change per step, and (3) add one more step. Reject answers that ignore the trend or use the wrong series.

Compare the two sets of findings at the same conditions (50 turns):

Step by step:

1. The only planned difference between the setups is the material of the nail (the core).
2. With an iron core, the electromagnet lifted 13 and 27 clips; with aluminum, only 0 and 1. The iron core made the electromagnet far stronger.
3. This fits what is known about magnetic materials: iron is strongly magnetic, so it concentrates the coil's magnetic field. Aluminum is not.
4. Both classes also found that two D cells lifted at least as many clips as one, so neither result says the cells have no effect.

Correct answer: For a coil like this, an iron core makes a much stronger electromagnet than an aluminum core does.

Tip: To reconcile two sets of results, line them up under the same conditions and find what differs between the setups. That difference is the best explanation for the different results.

Read the tropical line (red squares): 28 °C at 0 m, 25 °C at 100 m, 15 °C at 200 m, 11 °C at 300 m, 9 °C at 400 m, then 8, 7, 6 and 5 °C deeper down.

Step by step:

1. 0 to 100 m: 28 - 25 = 3 °C drop.
2. 100 to 200 m: 25 - 15 = 10 °C drop, the largest.
3. 300 to 400 m: 11 - 9 = 2 °C drop.
4. 800 to 1,000 m: 6 - 5 = 1 °C drop over 200 m.

The steepest drop, from 100 to 200 m, marks the thermocline, which the passage defines as "a layer in which the temperature drops quickly as depth increases." Above it is the warm mixed layer; below it is cold deep water.

Correct answer: between 100 m and 200 m.

Tip: "Most quickly" means the largest change for the same step. Compare changes between equally spaced points, and watch for uneven spacing (here 600 → 800 → 1,000 m are 200 m apart).

The passage gives two key ideas: "Cold water is denser than warm water, so cold water tends to sink beneath warmer water," and "Near the poles, surface water becomes very cold, sinks, and then slowly spreads along the deep ocean toward the equator." It also says "Sunlight warms only the top part of the ocean."

Step by step:

1. Near the poles, surface water is chilled. Cold water is dense, so it sinks.
2. That cold water then spreads slowly through the deep ocean, all the way to the tropics.
3. Sunlight's warming reaches only the upper layer, so it cannot heat the water 1,000 m down.
4. Result: even under a 28 °C tropical surface, the deep water is cold, about 5 °C.

Correct answer: Cold, dense water that sank near the poles spreads through the deep ocean, and sunlight warms only the upper layer.

This sinking and spreading is part of the global ocean circulation that moves heat around the planet.

Tip: For "best explains" questions, choose the option that uses the science ideas given in the passage (here density and sinking polar water). Reject options that reverse a fact (warm water sinking) or invent a cause.

Use Q = m c \Delta T with the values from the question.

Step by step:

1. Mass of water: m = 100 g.
2. Specific heat of water: c = 4.18\ \text{J/(g °C)}.
3. Change in temperature: \Delta T = 32.0 - 20.0 = 12.0 °C.
4. Multiply: Q = 100 \times 4.18 \times 12.0 = 5{,}016 J.

Correct answer: 5,016 J (about 5.0 kJ).

This is an example of conservation of energy: chemical energy stored in the food was changed to thermal energy that heated the water. In a real can setup, some heat escapes to the air and the can, so the true energy in the food is somewhat higher than 5,016 J.

Tip: The Greek letter delta (\Delta) always means "change in." Subtract first (final − initial); using a single temperature instead of the change is the most common trap.

The scatter plot shows the 8 sites: 0 weeks → 5%, 2 → 12%, 3 → 20%, 4 → 30%, 6 → 48%, 8 → 65%, 9 → 72%, 12 → 90% bleached.

Step by step:

1. As the number of warm weeks increases, the percent bleached increases at every step. This is a positive correlation.
2. The conclusion that matches the data is: sites where the water stayed warm longer had a greater percentage of bleached colonies.
3. Check the other choices against the passage. "Bleached corals are still alive and can recover if the water cools soon," so bleaching does not mean all the corals died. And the survey measured bleaching after the heat wave; nothing suggests bleaching warms the water.

Correct answer: Sites where the water stayed warm for more weeks had a greater percentage of bleached coral colonies.

Tip: A supported conclusion describes the pattern in the data without going further than the data. Watch for choices that use absolute words (all, every), reverse cause and effect, or treat one term (bleached) as another (dead).

The passage says the average surface pH "has dropped from about 8.2 to about 8.1," and that "7 is neutral, values below 7 are acidic, and values above 7 are basic. A lower pH means more hydrogen ions and a more acidic solution."

Step by step:

1. Both 8.2 and 8.1 are above 7, so ocean water is still basic, not acidic.
2. The pH decreased. A lower pH means more hydrogen ions, so the water has become more acidic (less basic). This shift is called ocean acidification, even though the water has not crossed 7.
3. The cause is carbon dioxide forming carbonic acid: \text{CO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{CO}_3.

Correct answer: The ocean is still basic but has become slightly more acidic, with more hydrogen ions.

Tip: On the pH scale, "more acidic" describes a direction of change (pH going down), while "acidic" describes a value below 7. Read carefully to see which one a choice is claiming.

The law of conservation of mass says that matter is not created or destroyed in a chemical reaction. Atoms are only rearranged, so the total mass of the reactants equals the total mass of the products.

Step by step:

1. Mass of reactants: 16\ \text{g} + 64\ \text{g} = 80\ \text{g}.
2. The products must also total 80 g: 44\ \text{g (CO}_2\text{)} + \text{water} = 80\ \text{g}.
3. Water: 80 - 44 = 36 g.

Correct answer: 36 grams.

The balanced equation shows the same idea at the level of atoms: there are 1 C, 4 H and 4 O on each side of the arrow.

Tip: In mass questions, add everything on the left side of the arrow, then subtract the products you already know. The coefficients (the 2s) count molecules; they are not masses, so do not multiply grams by them.

Read the power curve at the two wind speeds in the claim.

Step by step:

1. At 10 m/s, the turbine produces 1,600 kW.
2. At 20 m/s, it produces 2,000 kW.
3. Twice 1,600 kW would be 3,200 kW. The actual output is only \dfrac{2{,}000}{1{,}600} = 1.25 times as much, a 25% increase.
4. The reason: above 12 m/s the curve is flat at 2,000 kW, the turbine's maximum output. Extra wind speed does not raise the power.

Correct answer: No. Output rises only from 1,600 kW to 2,000 kW because it levels off above 12 m/s.

The claim also fails at low speeds: doubling from 4 to 8 m/s raises the power from 80 kW to 900 kW, more than ten times as much. Power output is not simply proportional to wind speed.

Tip: To test a "doubles" claim, read both values and divide. One example that does not fit is enough to show that a general claim is not supported.

Use the graph and the formula in the passage, E = P \times t.

Step by step:

1. Power at 10 m/s (from the graph): P = 1{,}600 kW.
2. Energy in 6 hours: E = 1{,}600\ \text{kW} \times 6\ \text{h} = 9{,}600 kWh.
3. Number of homes: \dfrac{9{,}600\ \text{kWh}}{30\ \text{kWh per home}} = 320 homes.

Correct answer: about 320 homes.

Common errors: leaving out the 6 hours gives 1{,}600 \div 30 \approx 53; using the turbine's maximum, 2,000 kW, instead of the value at 10 m/s gives 12{,}000 \div 30 = 400.

Tip: Power (kW) is a rate; energy (kWh) is an amount. To get energy, multiply power by time. Write the units at each step: \text{kW} \times \text{h} = \text{kWh}, and \text{kWh} \div \text{kWh per home} = \text{homes}.

Natural selection works through four ideas: (1) individuals in a population vary; (2) some variations are inherited; (3) individuals with helpful traits survive and reproduce more; (4) over generations those traits become more common.

Step by step:

1. Before spraying began, a few beetles already carried inherited traits (gene variants) that let them survive the insecticide.
2. Spraying killed most of the other beetles, so the resistant ones survived and had more offspring.
3. The offspring inherited resistance. Generation after generation, resistant beetles made up a larger share of the population.
4. The test showing the insecticide has not changed rules out "the spray got weaker."

Correct answer: Some beetles already had inherited resistance; they survived, reproduced and passed it on.

The same process explains antibiotic resistance in bacteria and herbicide resistance in weeds.

Tip: In natural selection, the environment does not create the helpful trait; it selects among traits that already exist. Reject answers in which individuals change because they "need" to, or pass on changes gained during their lifetime.