ACT Science Practice Questions: 15 Questions With Explanations

Passage 1: Dissolved Oxygen in Lake Water

A limnologist measured the dissolved oxygen (DO) concentration in a lake at various depths during summer and winter. DO is the amount of oxygen available in water for aquatic organisms. Higher DO levels generally support more diverse aquatic life. The results are shown in the table below.

Question 1

According to the table, at a depth of 10 m, the dissolved oxygen concentration in summer was approximately:

A) 4.3 mg/L    B) 6.1 mg/L    C) 10.8 mg/L    D) 11.2 mg/L

Answer: B) 6.1 mg/L

Find the row for 10 m depth, then read across to the Summer DO column: 6.1 mg/L. Answer C (10.8) is the winter value at the same depth, the most common trap on Data Representation questions.

Strategy: Always check that you are reading the correct column. On the ACT, wrong answers often come from reading the right row but the wrong column.

Question 2

Based on the data, as depth increases during summer, dissolved oxygen concentration:

A) increases only    B) decreases only    C) increases then decreases    D) remains constant

Answer: B) decreases only

Follow the Summer DO column from top to bottom: 8.2, 7.8, 6.1, 4.3, 2.1, 1.4. Each value is lower than the one above it, so the trend is a consistent decrease.

Strategy: For trend questions, scan the entire column, not just a few values. If there is even one exception, "only" answers are wrong.

Question 3

At which depth is the difference between summer and winter DO concentrations the greatest?

A) 0 m    B) 10 m    C) 20 m    D) 25 m

Answer: D) 25 m

Calculate the difference at each depth: 0 m: 11.5 − 8.2 = 3.3. 10 m: 10.8 − 6.1 = 4.7. 20 m: 10.1 − 2.1 = 8.0. 25 m: 9.8 − 1.4 = 8.4. The greatest difference is at 25 m.

Strategy: When a question asks about differences, you must calculate, not estimate visually. The answer is not always where one value is highest or lowest.

Question 4

Based on the data, if the DO concentration were measured at 12 m depth during winter, the value would most likely be closest to:

A) 5.0 mg/L    B) 7.5 mg/L    C) 10.6 mg/L    D) 11.0 mg/L

Answer: C) 10.6 mg/L

This is an interpolation question. 12 m falls between 10 m (winter DO = 10.8) and 15 m (winter DO = 10.5). Since 12 m is closer to 10 m, the value should be slightly below 10.8 but above 10.5. The best answer is 10.6. Answer A is a summer value trap; answer D is too high.

Strategy: For interpolation questions, bracket the unknown between the two nearest known values, then estimate proportionally.

Question 5

A fish species requires a minimum DO concentration of 5.0 mg/L to survive. Based on the data, during summer this species could survive at depths down to approximately:

A) 5 m    B) 10 m    C) 13 m    D) 20 m

Answer: C) 13 m

Look at the summer column. At 10 m, DO = 6.1 (above 5.0). At 15 m, DO = 4.3 (below 5.0). So the 5.0 threshold is crossed somewhere between 10 m and 15 m. Since the DO drops from 6.1 to 4.3 over that 5 m range, and 5.0 is closer to 4.3 than to 6.1, the crossover is closer to 15 m, roughly around 13 m.

Strategy: This combines interpolation with application. First find where the threshold falls in the data, then estimate the depth. Questions that apply data to a real-world scenario are common on the harder end of Data Representation.

Passage 2: Water Filtration Experiment

Students investigated how different filter materials affect the clarity of turbid (cloudy) water. They used turbidity measured in NTU (Nephelometric Turbidity Units), where higher values mean cloudier water.

Experiment 1
Students prepared 5 identical samples of turbid water (initial turbidity: 150 NTU). Each sample was passed once through a filter made of a single material. The turbidity of the filtered water was measured.

Experiment 2
Using the most effective filter material from Experiment 1, students tested whether passing water through the filter multiple times further reduced turbidity. They started with a fresh 150 NTU sample each time.

Question 6

In Experiment 1, the independent variable was:

A) the initial turbidity of the water    B) the filter material    C) the filtered turbidity    D) the volume of water

Answer: B) the filter material

The independent variable is what the experimenter deliberately changes between trials. In Experiment 1, the only thing that changed was the filter material. Initial turbidity (150 NTU) was held constant. Filtered turbidity (answer C) is the dependent variable, what was measured as a result.

Strategy: Independent = what changes on purpose. Dependent = what is measured. Control = what stays the same. This distinction appears on nearly every Research Summary passage.

Question 7

Which filter material in Experiment 1 reduced turbidity the most?

A) Sand    B) Gravel    C) Activated charcoal    D) Cotton fabric

Answer: C) Activated charcoal

The starting turbidity was 150 NTU. Activated charcoal reduced it to 18 NTU (a reduction of 132 NTU), which is the largest reduction. Sand brought it to 42, cotton to 65, and gravel to 98. The lowest filtered turbidity means the most effective filter.

Strategy: "Reduced the most" means find the lowest result, not the highest. Read the question carefully to know whether you are looking for the maximum or minimum value.

Question 8

Why did the students include a "no filter" control in Experiment 1?

A) To test whether the turbidity meter was accurate
B) To show what the turbidity level would be without any filtration
C) To demonstrate that all filter materials are equally effective
D) To measure the volume of water lost during filtration

Answer: B)

A control group provides a baseline for comparison. Without the "no filter" condition, you would not be able to confirm that the turbidity reduction was caused by the filter materials and not by some other factor (like the water settling). The control confirms that the initial turbidity was 150 NTU and that filtration caused the observed changes.

Strategy: Questions about controls test whether you understand experimental design. The control always exists to provide a baseline or to isolate the effect of the independent variable.

Question 9

Based on Experiment 2, what is the relationship between the number of filter passes and turbidity reduction?

A) Each additional pass reduces turbidity by the same amount
B) Each additional pass reduces turbidity, but by a decreasing amount
C) Turbidity increases with more passes
D) There is no clear relationship

Answer: B)

Look at the reductions between passes: 1st pass: 150 to 18 (reduction of 132). 2nd pass: 18 to 7 (reduction of 11). 3rd pass: 7 to 3 (reduction of 4). 4th pass: 3 to 2 (reduction of 1). Each additional pass still reduces turbidity, but the amount of reduction gets smaller each time. This is a diminishing returns pattern.

Strategy: Do not just look at the final values. Calculate the change between consecutive data points to identify the pattern. "Decreasing rate of change" is a concept the ACT tests frequently.

Question 10

A student claims that passing water through an activated charcoal filter 5 times would reduce the turbidity to 0 NTU. Based on the data from Experiment 2, is this claim supported?

A) Yes, because each pass reduces turbidity
B) Yes, because the turbidity is already close to 0 after 4 passes
C) No, because the rate of reduction is decreasing with each pass, so reaching 0 is unlikely
D) No, because the experiment only tested up to 4 passes

Answer: C)

The data shows diminishing returns: reductions of 132, 11, 4, and 1. The turbidity approaches but does not reach zero, and each pass removes less than the one before. Extrapolating this pattern, a 5th pass would likely reduce turbidity by less than 1 NTU, bringing it to about 1 to 2 NTU, not 0. Answer D is tempting but weak: the data does allow reasonable extrapolation, and the better reason is the diminishing returns pattern itself.

Strategy: "Is this claim supported by the data?" questions require you to evaluate a conclusion. Look at the trend in the data, not just individual values. The ACT rewards students who can distinguish between what the data shows and what someone assumes.

Passage 3: The Extinction of Large Ice-Age Mammals

Around 12,000 years ago, many species of large mammals (megafauna), including woolly mammoths, saber-toothed cats, and giant ground sloths, went extinct across North America. Scientists continue to debate the primary cause.

Scientist 1 (Climate Change Hypothesis)

The extinctions were driven primarily by rapid climate change at the end of the last Ice Age. As glaciers retreated, temperatures rose sharply, and habitats transformed. Forests replaced open grasslands that megafauna depended on for food. These large animals could not adapt quickly enough to the changing vegetation and temperature. Fossil pollen records show a dramatic shift from grassland to forest species between 13,000 and 11,000 years ago, coinciding precisely with the extinction timeline. Smaller mammals survived because they could adapt to a wider range of habitats and had shorter generation times, allowing faster evolutionary response.

Scientist 2 (Human Overkill Hypothesis)

The extinctions were caused primarily by overhunting by humans who first arrived in North America approximately 13,000 years ago. The Clovis people were highly efficient hunters who used sophisticated stone-point weapons. Megafauna had no evolutionary experience with human predators and were therefore easy targets. In Australia, a similar wave of megafauna extinctions occurred shortly after human arrival around 45,000 years ago, during a period of stable climate. In Africa, where megafauna co-evolved with humans over millions of years, large mammals survived the same climate shift. The pattern of extinction following human arrival, regardless of climate conditions, points to hunting as the primary cause.

Question 11

Both Scientist 1 and Scientist 2 would agree that:

A) climate change was the primary cause of megafauna extinction
B) large mammals went extinct in North America approximately 12,000 years ago
C) smaller mammals were also significantly affected by the extinctions
D) human hunting had no effect on megafauna populations

Answer: B)

Both scientists accept the basic fact: large mammals went extinct in North America around 12,000 years ago. They disagree about the cause, not the timing. Answer A is only Scientist 1's view. Answer C is contradicted by Scientist 1, who notes smaller mammals survived. Answer D is only consistent with Scientist 1's framing.

Strategy: "Both agree" questions test whether you can separate shared facts from disputed causes. Look for statements that are presented as background information in both viewpoints, not as arguments.

Question 12

According to Scientist 2, the survival of large mammals in Africa supports the overkill hypothesis because:

A) Africa experienced more severe climate change than North America
B) African megafauna had evolved alongside human hunters and developed defenses
C) Humans never hunted large mammals in Africa
D) African megafauna were smaller than North American megafauna

Answer: B)

Scientist 2 specifically states that in Africa, megafauna "co-evolved with humans over millions of years," implying they developed behavioral and physical defenses against human hunters. In North America, megafauna "had no evolutionary experience with human predators." This contrast explains why African megafauna survived the same climate shift that North American megafauna did not.

Strategy: When a question asks "according to Scientist X," your answer must come directly from that scientist's stated argument. Do not introduce your own reasoning or knowledge.

Question 13

Which of the following evidence, if true, would most weaken Scientist 1's hypothesis?

A) Fossil evidence shows that some grassland areas persisted after the Ice Age
B) Temperature records show the climate shift occurred gradually over 5,000 years
C) Several megafauna species went extinct before any significant climate change occurred
D) Pollen records show that forest expansion began 15,000 years ago

Answer: C)

Scientist 1 argues that climate change caused habitat loss, which caused the extinctions. If some megafauna went extinct before climate change occurred, the timeline does not match, and climate change cannot be the primary cause. Answer A would only slightly weaken the argument (some grasslands survived, but perhaps not enough). Answer B actually helps Scientist 1 (gradual change still causes habitat shifts). Answer D would slightly complicate the timeline but not fundamentally break it.

Strategy: "Weaken" questions ask you to find evidence that breaks the cause-effect link in the argument. The strongest weakener shows that the proposed cause did not precede the effect.

Question 14

Scientist 2 uses the example of Australian megafauna extinction to argue that:

A) climate change affected Australia and North America equally
B) megafauna extinctions follow human arrival regardless of climate conditions
C) Australian megafauna were genetically similar to North American megafauna
D) the Clovis people migrated from Australia to North America

Answer: B)

Scientist 2 explicitly states that Australian megafauna went extinct "shortly after human arrival around 45,000 years ago, during a period of stable climate." The point is that the pattern (humans arrive, megafauna die) repeats across continents regardless of whether climate was changing. This strengthens the argument that humans, not climate, were the primary driver.

Strategy: When a scientist cites an example from another context, ask yourself: "What pattern is this example supposed to demonstrate?" The answer is always about the pattern, not the specific details of the example.

Question 15

Suppose researchers discover that a population of woolly mammoths survived on an isolated island until 4,000 years ago, long after the continental mammoths went extinct. This island had a stable climate but no human inhabitants until 4,000 years ago. This finding would most directly support:

A) Scientist 1, because the island's stable climate preserved the mammoths
B) Scientist 1, because the mammoths adapted to the island's habitat
C) Scientist 2, because the mammoths survived without human presence and died after humans arrived
D) Scientist 2, because the island had different vegetation than the mainland

Answer: C)

This evidence shows mammoths surviving for thousands of years after the Ice Age climate shift, as long as humans were absent, and then going extinct precisely when humans arrived. This directly supports Scientist 2's argument: the key factor is human presence, not climate. Answer A might seem plausible, but the question specifies the island had "stable climate," which means climate stability alone does not explain the timing of extinction. The extinction aligned with human arrival, not climate change.

Strategy: "New evidence" questions are the hardest on Conflicting Viewpoints passages. Ask: does this new information strengthen the cause-effect link for Scientist 1 or Scientist 2? The answer supports whichever scientist's causal mechanism is demonstrated by the evidence.