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ACT Science study materials
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ACT Science practice questions

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Question 1Science content (domains not enumerated in source)

Which statement best describes the role of a control group in an experiment?

  1. It increases the total number of trials so results are more precise.
  2. It eliminates the need to identify variables in the experiment.
  3. It provides a baseline for comparison without the treatment being tested. Correct answer.
  4. It is the group that receives the highest dose of the independent variable.

Explanation

A control group is treated the same as the experimental group except it does not receive the treatment, so any difference in outcome can be attributed to the treatment. It establishes a baseline rather than boosting sample size or removing variables.

Question 2Science content (domains not enumerated in source)

A car travels 150 meters in 25 seconds at a steady speed. What is its speed?

  1. 0.17 m/s
  2. 3750 m/s
  3. 6 m/s Correct answer.
  4. 25 m/s

Explanation

Speed equals distance divided by time, so 150 m ÷ 25 s = 6 m/s. The other values result from dividing or multiplying the wrong quantities.

Question 3Science content (domains not enumerated in source)

In a food web, which organisms convert solar energy into chemical energy stored in organic molecules?

  1. Apex predators
  2. Primary consumers
  3. Producers Correct answer.
  4. Decomposers

Explanation

Producers such as plants and algae carry out photosynthesis, capturing light energy and storing it in sugars. Consumers and decomposers obtain energy by eating or breaking down organic matter rather than capturing light directly.

Question 4Science content (domains not enumerated in source)

Which subatomic particle determines the identity of an element?

  1. Photon
  2. Electron
  3. Proton Correct answer.
  4. Neutron

Explanation

The number of protons in the nucleus is the atomic number, which uniquely identifies an element. Neutrons vary among isotopes, and electrons can change during ion formation.

Question 5Science content (domains not enumerated in source)

A student measures the mass of a sample three times and obtains 5.1 g, 5.0 g, and 5.2 g, but the accepted value is 4.0 g. Which statement best describes the results?

  1. They are accurate but not precise.
  2. They are neither precise nor accurate.
  3. They are precise but not accurate. Correct answer.
  4. They are both accurate and precise.

Explanation

Precision refers to how closely repeated measurements agree with each other, and the three values cluster tightly. Accuracy refers to closeness to the true value, and all three are far from 4.0 g.

Question 6Science content (domains not enumerated in source)

Which of the following is the best example of a hypothesis stated in testable form?

  1. Plants are interesting organisms.
  2. Plants should always be watered daily.
  3. If a plant receives more light, then it will grow taller. Correct answer.
  4. Some plants are greener than others.

Explanation

A testable hypothesis predicts a relationship between variables in an if-then form that can be supported or refuted by data. The other statements are opinions or observations without a testable prediction.

Question 7Science content (domains not enumerated in source)

Which process in the water cycle releases water vapor into the atmosphere from plants?

  1. Transpiration Correct answer.
  2. Condensation
  3. Runoff
  4. Precipitation

Explanation

Transpiration is the loss of water vapor from plant leaves through stomata, adding moisture to the atmosphere. Condensation forms clouds, precipitation returns water to the surface, and runoff moves water across land.

Question 8Science content (domains not enumerated in source)

A rock sample contains 25% of its original radioactive parent isotope. If the half-life is 10 million years, how old is the sample?

  1. 30 million years
  2. 10 million years
  3. 5 million years
  4. 20 million years Correct answer.

Explanation

Each half-life halves the remaining parent isotope. Going from 100% to 50% takes one half-life and from 50% to 25% takes a second, so 2 × 10 million = 20 million years.

Question 9Science content (domains not enumerated in source)

A ball rolls down a straight ramp, and its speed increases each second. What does this indicate about the ball's motion?

  1. It is accelerating. Correct answer.
  2. It is at rest.
  3. It is moving in a circle.
  4. It has a constant velocity.

Explanation

Acceleration is any change in velocity, including an increase in speed along a straight line. A constant velocity would mean the speed does not change from second to second.

Question 10Science content (domains not enumerated in source)

Two clear liquids are mixed in a beaker, and the mixture becomes warm while a solid forms. Which conclusion is best supported?

  1. No change occurred because no gas was released.
  2. The liquids boiled because the beaker became warm.
  3. A chemical reaction occurred because heat and a new solid were produced. Correct answer.
  4. A physical change occurred because the liquids were both clear.

Explanation

The release of heat and the formation of a solid are evidence of a chemical change producing new substances. Physical mixing of unchanged liquids would not spontaneously generate heat and a new solid.

Question 11Science content (domains not enumerated in source)

A population of insects develops resistance to a pesticide over several generations. Which mechanism best explains this outcome?

  1. The pesticide causes mutations that make all insects resistant at once.
  2. Insects with naturally resistant traits survive and reproduce more successfully. Correct answer.
  3. The pesticide loses potency each time it is applied.
  4. Individual insects learn to tolerate the pesticide during their lifetime.

Explanation

Natural selection favors individuals that already carry traits allowing survival under pesticide exposure, so those individuals reproduce and pass the traits on. Resistance spreads through the population over generations, not within a single lifetime.

Question 12Science content (domains not enumerated in source)

A sealed syringe contains gas at constant temperature. If the volume is halved, what happens to the pressure of the gas?

  1. It stays the same.
  2. It is halved.
  3. It is doubled. Correct answer.
  4. It becomes zero.

Explanation

At constant temperature, pressure and volume are inversely related (Boyle's law). Halving the volume doubles the pressure because the same number of particles collide with the walls more frequently in a smaller space.

Question 13Science content (domains not enumerated in source)

A student graphs data and finds that as the independent variable increases, the dependent variable decreases at a steady rate. What type of relationship is shown?

  1. Inverse linear Correct answer.
  2. No relationship
  3. Direct linear
  4. Exponential growth

Explanation

A steady decrease of the dependent variable as the independent variable increases produces a straight line with a negative slope, which is an inverse linear relationship. A direct linear relationship would show a steady increase.

Question 14Science content (domains not enumerated in source)

Which structure in a eukaryotic cell is primarily responsible for generating ATP through aerobic respiration?

  1. Nucleus
  2. Ribosome
  3. Mitochondrion Correct answer.
  4. Golgi apparatus

Explanation

The mitochondrion houses the electron transport chain and ATP synthase that produce most of the cell's ATP during aerobic respiration. Ribosomes build proteins, the nucleus stores DNA, and the Golgi modifies and packages proteins.

Question 15Science content (domains not enumerated in source)

In an experiment, a plant is placed in darkness and its oxygen production is measured. Which explanation best accounts for a near-zero oxygen reading?

  1. Oxygen is converted directly into carbon dioxide by chlorophyll.
  2. The plant's stomata permanently close in darkness.
  3. The plant stops respiring entirely in the dark.
  4. Photosynthesis requires light to split water and release oxygen. Correct answer.

Explanation

The light-dependent reactions of photosynthesis use light energy to split water, releasing oxygen. Without light, this stage cannot proceed, so oxygen production falls near zero.

Question 16Science content (domains not enumerated in source)

A student wants to test whether fertilizer affects plant height. Which is the independent variable?

  1. The type of soil used
  2. The final height of each plant
  3. The amount of fertilizer applied Correct answer.
  4. The number of leaves on each plant

Explanation

The independent variable is the factor the experimenter deliberately changes, which here is the amount of fertilizer. Plant height is the dependent variable measured as the outcome.

Question 17Science content (domains not enumerated in source)

Two solutions are tested with pH paper. Solution X has a pH of 3 and solution Y has a pH of 6. Which statement is correct?

  1. Both solutions are basic.
  2. Solution Y is more acidic than solution X.
  3. Solution X has a lower hydrogen ion concentration than solution Y.
  4. Solution X is ten times more acidic in hydrogen ion concentration than solution Y. Correct answer.

Explanation

The pH scale is logarithmic, so each unit represents a tenfold change in hydrogen ion concentration. A pH of 3 is three units below neutral and one unit below pH 6, making it ten times more concentrated in hydrogen ions.

Question 18Science content (domains not enumerated in source)

A geologist observes that a layer of sedimentary rock contains fossils of fish, while the layer directly above contains fossils of land plants. What can be inferred?

  1. The rock layers were deposited upside down.
  2. Fish and land plants lived at exactly the same time.
  3. The environment changed from aquatic to terrestrial over time. Correct answer.
  4. The land plants are older than the fish.

Explanation

In undisturbed sedimentary sequences, lower layers are older than upper layers. Fish in the lower layer and land plants above suggest a shift from a water environment to a land environment over time.

Question 19Science content (domains not enumerated in source)

Which statement correctly compares mass and weight?

  1. Mass is the amount of matter, while weight is the gravitational force on that matter. Correct answer.
  2. Mass changes with location, while weight stays constant.
  3. Weight is measured in kilograms, while mass is measured in newtons.
  4. Mass and weight are identical quantities.

Explanation

Mass measures the quantity of matter and is independent of location, while weight is the force of gravity acting on that mass and depends on the local gravitational field. They are related but distinct quantities.

Question 20Science content (domains not enumerated in source)

In a predator-prey graph, the prey population rises sharply, followed a short time later by a rise in the predator population. What best explains the delay?

  1. Predators require time to reproduce in response to increased food. Correct answer.
  2. Prey reproduce only when predators are absent.
  3. The delay is caused by measurement error in the graph.
  4. Predator populations never respond to prey numbers.

Explanation

Predators respond to more abundant prey by surviving and reproducing better, but reproduction takes time, so the predator peak lags behind the prey peak. This lag is a standard feature of predator-prey cycles.

Question 21Science content (domains not enumerated in source)

A sample of an unknown liquid boils at 78 °C and freezes at -114 °C. A student claims it is pure water. Which observation refutes the claim?

  1. The liquid was heated in a glass beaker.
  2. The liquid is clear and colorless.
  3. The liquid boils at a temperature different from water's boiling point. Correct answer.
  4. The liquid's mass was measured with a balance.

Explanation

Pure water at standard pressure boils at 100 °C and freezes at 0 °C. A boiling point of 78 °C and freezing point of -114 °C differ from these values, indicating the liquid is not pure water.

Question 22Science content (domains not enumerated in source)

A circuit contains a 12-volt battery and a 4-ohm resistor. What current flows through the circuit?

  1. 3 A Correct answer.
  2. 48 A
  3. 16 A
  4. 0.33 A

Explanation

Ohm's law states that current equals voltage divided by resistance, so 12 V ÷ 4 Ω = 3 A. Multiplying or inverting the quantities gives the other values.

Question 23Science content (domains not enumerated in source)

A researcher finds that a drug reduces symptoms in 70% of patients who receive it and in 65% of patients who receive a placebo. Which conclusion is most reasonable?

  1. The placebo caused the symptoms to worsen.
  2. The drug is highly effective because most patients improved.
  3. The drug's effect is largely explained by the placebo response. Correct answer.
  4. The drug is completely ineffective in all patients.

Explanation

Because the placebo group improved at nearly the same rate as the drug group, most of the improvement is attributable to the placebo response rather than the drug. The small difference does not support a strong drug effect.

Question 24Science content (domains not enumerated in source)

Enzymes catalyze reactions in cells. Which statement best explains how an enzyme increases reaction rate?

  1. It changes the equilibrium constant of the reaction.
  2. It raises the activation energy required for the reaction.
  3. It lowers the activation energy by binding substrates in a favorable orientation. Correct answer.
  4. It is consumed in the reaction and must be replaced each time.

Explanation

Enzymes provide an alternative reaction pathway with a lower activation energy and bring substrates together in the proper orientation. They are not consumed and do not alter the overall equilibrium position.

Question 25Science content (domains not enumerated in source)

A population of bacteria is exposed to an antibiotic. Most cells die, but a few survive and multiply. After several generations, the population is largely resistant. Which statement best describes the change in allele frequencies?

  1. Stabilizing selection kept allele frequencies unchanged.
  2. Disruptive selection produced two equally common phenotypes.
  3. Genetic drift eliminated all resistance alleles.
  4. Directional selection increased the frequency of resistance alleles. Correct answer.

Explanation

Directional selection favors one extreme phenotype, here the resistant cells, shifting allele frequencies toward resistance over generations. Stabilizing selection would favor intermediate phenotypes and disrupt drift would not consistently increase resistance.

Question 26Science content (domains not enumerated in source)

A student measures the mass of a metal block as 54.0 g and its volume as 20.0 cm³. What is the density of the metal?

  1. 74.0 g/cm³
  2. 0.37 g/cm³
  3. 1080 g/cm³
  4. 2.70 g/cm³ Correct answer.

Explanation

Density equals mass divided by volume, so 54.0 g ÷ 20.0 cm³ = 2.70 g/cm³. The units of grams per cubic centimeter follow directly from dividing a mass by a volume.

Question 27Science content (domains not enumerated in source)

Which of the following best describes a controlled experiment?

  1. Only one variable is deliberately changed while other conditions are held constant Correct answer.
  2. The results are predicted before the data are collected
  3. No measurements are recorded during the procedure
  4. Several variables are changed at once to save time

Explanation

A controlled experiment isolates the effect of the independent variable by keeping all other conditions the same. This allows any change in the dependent variable to be attributed to the variable being tested.

Question 28Science content (domains not enumerated in source)

A student observes that a sample of matter has a fixed volume but takes the shape of its container. In which state of matter is the sample most likely?

  1. Solid
  2. Gas
  3. Plasma
  4. Liquid Correct answer.

Explanation

A liquid has a definite volume but no fixed shape because its particles can flow past one another while remaining close together. Solids keep both a fixed shape and a fixed volume.

Question 29Science content (domains not enumerated in source)

Which of the following is a chemical change rather than a physical change?

  1. Sugar dissolving in water
  2. Iron forming rust when exposed to moist air Correct answer.
  3. Glass shattering into pieces
  4. Ice melting into liquid water

Explanation

Rusting produces a new substance with different properties, which is the hallmark of a chemical change. The other examples change only the form or arrangement of the same substance.

Question 30Science content (domains not enumerated in source)

Which organ system is primarily responsible for transporting oxygen and nutrients throughout the human body?

  1. Circulatory system Correct answer.
  2. Digestive system
  3. Respiratory system
  4. Nervous system

Explanation

The circulatory system uses blood and blood vessels to carry oxygen and nutrients to cells and remove wastes. The respiratory system exchanges gases, but transport through the body is the job of the circulatory system.

Question 31Science content (domains not enumerated in source)

A food chain in a grassland includes grass, grasshoppers, frogs, and snakes. Which organism in this chain is the primary producer?

  1. Grasshopper
  2. Frog
  3. Snake
  4. Grass Correct answer.

Explanation

A primary producer makes its own food through photosynthesis and forms the base of the food chain. Grass captures sunlight directly, while the animals obtain energy by eating other organisms.

Question 32Science content (domains not enumerated in source)

Which of the following best describes the role of the Sun in Earth's climate system?

  1. It cools the atmosphere by absorbing heat from the surface
  2. It is the primary source of energy that drives weather and climate patterns Correct answer.
  3. It affects only ocean currents and not the atmosphere
  4. It has no measurable effect on atmospheric temperature

Explanation

The Sun supplies nearly all of the energy that heats Earth's surface and atmosphere, driving winds, evaporation, and ocean currents. Uneven heating across the globe creates the temperature differences that produce weather.

Question 33Science content (domains not enumerated in source)

A car travels 150 meters in 10 seconds at a constant speed. What is its speed?

  1. 15 m/s Correct answer.
  2. 160 m/s
  3. 1500 m/s
  4. 0.067 m/s

Explanation

Speed equals distance divided by time, so 150 m ÷ 10 s = 15 m/s. Constant speed means the value does not change during the interval.

Question 34Science content (domains not enumerated in source)

A student places a plant in a sealed clear container with a carbon dioxide sensor and exposes it to bright light. Over several hours, the carbon dioxide level inside the container falls. Which process best explains this decrease?

  1. The container is leaking carbon dioxide through the glass
  2. The plant is absorbing carbon dioxide through its roots only
  3. Respiration is releasing carbon dioxide faster than it is used
  4. Photosynthesis is removing carbon dioxide from the air inside the container Correct answer.

Explanation

In bright light, photosynthesis outpaces cellular respiration, so the plant takes in more carbon dioxide than it releases. The net removal of carbon dioxide lowers the measured concentration inside the sealed container.

Question 35Science content (domains not enumerated in source)

A student measures the boiling point of an unknown clear liquid three times and obtains 98.1 °C, 98.2 °C, and 98.1 °C. The accepted value for this liquid is 100.0 °C. Which statement best describes the measurements?

  1. They are neither accurate nor precise
  2. They are accurate but not precise
  3. They are precise but not accurate Correct answer.
  4. They are both accurate and precise

Explanation

Precision refers to how closely repeated measurements agree with each other, while accuracy refers to closeness to the true value. The three readings cluster tightly together but are consistently about 2 °C below the accepted value.

Question 36Science content (domains not enumerated in source)

A 2 kg object is pushed with a net force of 6 N across a frictionless surface. What is the acceleration of the object?

  1. 3 m/s² Correct answer.
  2. 0.33 m/s²
  3. 12 m/s²
  4. 8 m/s²

Explanation

Newton's second law states that acceleration equals net force divided by mass, so 6 N ÷ 2 kg = 3 m/s². The result carries units of meters per second squared.

Question 37Science content (domains not enumerated in source)

A beaker of water is heated on a hot plate. After several minutes, the temperature stops rising at 100 °C even though heating continues. Which explanation best accounts for this observation?

  1. The added energy is being used to change liquid water into vapor at the boiling point Correct answer.
  2. The water has reached its freezing point
  3. The thermometer can no longer register temperatures above 100 °C
  4. The hot plate has stopped transferring energy to the water

Explanation

At the boiling point, added energy goes into breaking intermolecular attractions during vaporization rather than raising temperature. This is why the temperature stays constant during a phase change.

Question 38Science content (domains not enumerated in source)

A population of rabbits lives in an area where a new predator is introduced. Over many generations, the average running speed of the rabbits increases. Which explanation best fits the theory of natural selection?

  1. Individual rabbits train themselves to run faster and pass the improvement to offspring
  2. The predator causes mutations that make rabbits faster
  3. Rabbits choose to move faster to avoid being eaten
  4. Faster rabbits survive and reproduce more often, passing on traits that favor speed Correct answer.

Explanation

Natural selection acts on heritable variation: individuals with traits that improve survival leave more offspring, so those traits become more common. The change in the population results from differential reproduction, not from individual effort.

Question 39Science content (domains not enumerated in source)

A student tests how the temperature of a solvent affects how quickly sugar dissolves. She records the time for complete dissolving at four temperatures. Which variable is the dependent variable?

  1. The temperature of the solvent
  2. The type of container used
  3. The time required for the sugar to dissolve Correct answer.
  4. The amount of sugar used

Explanation

The dependent variable is the measured outcome that responds to the change being tested. Here the time to dissolve is measured as the temperature is varied.

Question 40Science content (domains not enumerated in source)

A block of wood floats in water, while a block of the same size made of iron sinks. Which property best explains the difference?

  1. The density of each material compared with the density of water Correct answer.
  2. The color of each material
  3. The temperature of each block
  4. The shape of each block

Explanation

An object floats if its density is less than the density of the fluid and sinks if its density is greater. Wood is less dense than water, while iron is more dense.

Question 41Science content (domains not enumerated in source)

A solution has a hydrogen ion concentration of 1 × 10⁻⁴ M. What is the pH of the solution?

  1. 4 Correct answer.
  2. −4
  3. 10
  4. 14

Explanation

pH is defined as the negative logarithm of the hydrogen ion concentration, so pH = −log(1 × 10⁻⁴) = 4. A pH of 4 indicates an acidic solution.

Question 42Science content (domains not enumerated in source)

A student builds a series circuit with two identical resistors and a battery. If one resistor is removed so the circuit is broken, what happens to the current?

  1. The current stays the same because the battery is unchanged
  2. The current doubles because fewer resistors remain
  3. The current increases slightly because resistance decreases
  4. The current stops because a series circuit has only one path Correct answer.

Explanation

In a series circuit, components form a single path for charge flow. Breaking that path at any point stops the current everywhere in the circuit.

Question 43Science content (domains not enumerated in source)

A scientist finds that a new fertilizer increases crop yield in a greenhouse study. Before recommending it to farmers, which additional step is most important?

  1. Test the fertilizer only on the same greenhouse plants
  2. Repeat the study in field conditions with proper controls Correct answer.
  3. Publish the greenhouse results immediately without further testing
  4. Increase the fertilizer dose until the plants show damage

Explanation

Greenhouse conditions differ from field conditions in soil, weather, and pests, so results must be validated in realistic settings with controls. Replication under field conditions strengthens confidence before broad recommendations.

Question 44Science content (domains not enumerated in source)

A rock layer contains fossils of marine shells, and the layer above it contains fossils of land plants. Which conclusion is best supported by this evidence?

  1. The rock layers formed at exactly the same time
  2. The shells were carried into the area by wind
  3. The land plants lived before the marine shells
  4. The area was once covered by water and later became dry land Correct answer.

Explanation

In undisturbed sedimentary sequences, lower layers are older than upper layers. Marine fossils below land-plant fossils suggest a change from a water environment to a terrestrial one over time.

Question 45Science content (domains not enumerated in source)

A sample of gas is held at constant pressure while its temperature is increased. According to Charles's law, what happens to its volume?

  1. The volume becomes zero at high temperature
  2. The volume increases in proportion to the absolute temperature Correct answer.
  3. The volume decreases as temperature rises
  4. The volume stays the same regardless of temperature

Explanation

Charles's law states that at constant pressure, the volume of a gas is directly proportional to its absolute temperature. Heating the gas makes the particles move faster and occupy more space.

Question 46Science content (domains not enumerated in source)

Two students measure the same table length and get 1.52 m and 1.53 m. A third student measures it as 1.85 m. Which statement best describes the third measurement?

  1. It proves the first two measurements are wrong
  2. It is an outlier and likely reflects a measurement error Correct answer.
  3. It is the most reliable value because it is the largest
  4. It shows the table length varies over time

Explanation

An outlier is a value far from the cluster of other measurements and often indicates a procedural or reading error. The two close values support each other, making the distant value suspect.

Question 47Science content (domains not enumerated in source)

A student measures the mass of a reactant and the mass of the products in a closed container before and after a chemical reaction. The total mass is the same before and after. Which principle does this observation support?

  1. Conservation of energy
  2. The ideal gas law
  3. Conservation of mass Correct answer.
  4. Newton's third law

Explanation

The law of conservation of mass states that matter is neither created nor destroyed in a chemical reaction. In a closed system, the total mass therefore remains constant.

Question 48Science content (domains not enumerated in source)

A student graphs the absorbance of a colored solution versus concentration and obtains a straight line through the origin. A sample of unknown concentration has twice the absorbance of a 0.10 M standard. What is the concentration of the sample?

  1. 0.20 M Correct answer.
  2. 2.0 M
  3. 0.05 M
  4. 0.10 M

Explanation

A straight line through the origin shows that absorbance is directly proportional to concentration. Doubling the absorbance therefore doubles the concentration from 0.10 M to 0.20 M.

Question 49Science content (domains not enumerated in source)

A population of mice lives on a dark lava field, and a mutation produces some mice with dark fur. Over many generations, the proportion of dark-furred mice increases. Which mechanism most directly accounts for this change?

  1. The dark fur mutation is caused by the need for camouflage
  2. Dark-furred mice choose to have more offspring
  3. Predators remove light-furred mice more often, so dark-furred mice survive and reproduce more Correct answer.
  4. The lava field chemically darkens the fur of all mice

Explanation

Differential survival and reproduction favor the trait that provides better camouflage on the dark background. The frequency of the advantageous allele rises because its bearers leave more offspring.

Question 50Science content (domains not enumerated in source)

A student calculates the density of a sample as 8.0 g/cm³ using a measured mass of 40.0 g and a measured volume of 5.0 cm³. If the volume measurement has a 10% uncertainty, which statement best describes the effect on the calculated density?

  1. The density will be exactly 8.0 g/cm³ regardless of measurement error
  2. The density uncertainty will be 1% because volume is divided
  3. The density will have no uncertainty because the mass is exact
  4. The density will also have roughly a 10% uncertainty because it is directly proportional to volume Correct answer.

Explanation

Density equals mass divided by volume, so a fractional uncertainty in volume propagates to the same fractional uncertainty in density when mass is treated as exact. A 10% volume uncertainty therefore yields roughly a 10% density uncertainty.