ACT science practice test 21

Directions: Each passage is followed by several questions. After reading a passage, choose the best answer to each question and fill in the corresponding oval on your answer document. You may refer to the passages as often as necessary.

You are NOT permitted to use a calculator on this test.


Methane (CH4) is an important energy source and a powerful greenhouse gas. CH4 levels in the atmosphere are increasing, largely as a result of increasing livestock populations and energy emissions. Two scientists debate possible consequences of rising levels of atmospheric methane.


Scientist 1

Increasing CH4 levels are a serious concern because, in the atmosphere, CH4 can be converted into formaldehyde (H2CO). H2CO is a dangerous chemical, banned in some countries and used as an embalming fluid in others.

When ozone (O3) is struck by solar radiation (light) in the presence of water, hydroxyl radicals (·OH) are created (Reaction 1):

light + O3 + H2O → 2·OH + O2

When OH comes into contact with CH4, another radical, ·CH3, is formed (Reaction 2):

OH + CH4 → ·CH3 + H2O

In the presence of oxygen (O2) and nitric oxide (NO), the highly reactive ·CH3 is converted into H2CO (Reaction 3):

·CH3 + NO + 2O2 → H2CO + NO2 + HO2

The product HO2 is unstable and reacts with NO, yielding more ·OH (Reaction 4):

HO2 + NO → NO2 + ·OH

Together, Reactions 2?4 are called a chain reaction because the OH formed in Reaction 4 can react with another CH4 molecule in Reaction 2:

·OH + CH4 → ·CH3 + H2O

·CH3 + NO + 2O2 → H2CO + NO2 + HO2

HO2 + NO → NO2 + ·OH

As a result, one ·OH can convert a great deal of CH4. At current CH4 levels, this chain reaction is the primary fate of atmospheric ·OH, making the formation of H2CO an urgent concern.

Scientist 2

H2CO is a dangerous chemical, but atmospheric formaldehyde levels will not increase dramatically due to methane emissions. Carbon monoxide (CO) generation may be the greater concern. Hydroxyl radicals can break down methane, leading to the formation of H2CO and nitric oxide, as in Reactions 1?4; in the presence of light, however, H2CO quickly decomposes to CO and hydrogen, H2 (Reaction 5):

H2CO → H2 + CO

Furthermore, the OH generated by Reactions 1 and 4 will react rapidly with any H2CO in the atmosphere to produce CO and water: (Reaction 6)

H2CO + 2·OH → CO + 2H2O

In addition to reducing the amount of H2CO by breaking down the H2CO molecule, this reaction removes OH from the atmosphere, inhibiting the chain reaction of Reactions 2?4.

1. Which of the following substances do the two scientists agree must be present in order for ·CH3 to be generated by atmospheric methane?

A. H3O+
B. NO2
C. HNO3
D. O3

2. Which of the following graphs reflects Scientist 1's hypothesis of how levels of H2CO in the atmosphere will change as more CH4 is released into the atmosphere?

F.
G.
H.
J.

3. A student suggested that the molecular mass of either product in Reaction 5 would be greater than the molecular mass of the reactant in Reaction 5. Is he correct?

A. No; H2CO is composed not of molecules, but of atoms.
B. Yes; the mass of a molecule of H2CO is greater than the mass of either reactant.
C. No; the mass of a molecule of H2CO is greater than the mass of either product.
D. Yes; the mass of a molecule of CO is greater than the mass of a molecule of H2.

4. In certain parts of the atmosphere, the amount of O3 is decreasing. As O3 levels decrease, which of the following would Scientist 1 most strongly agree with regarding the levels of ·CH3 and H2CO in the atmosphere?

F. The amount of ·CH3 would increase and the amount of H2CO would decrease.
G. The amount of ·CH3 would decrease and the level of H2CO would remain constant.
H. The amounts of ·CH3 and H2CO would both decrease.
J. The amounts of ·CH3 and H2CO would both increase.

5. Of the following statements, with which would Scientist 2 most strongly disagree?

A. O3 is involved in the generation of H2CO in the atmosphere.
B. ·OH is contributing to the formation of carbon monoxide in the atmosphere.
C. Solar radiation contributes to the break down of CH4.
D. As CH4 emissions increase, levels of H2CO will rise dramatically.

6. After examining Scientist 1's hypothesis, Scientist 2 claimed that Reaction 3 would lead to increased levels of carbon monoxide. By which of the following explanations would Scientist 2 most likely support this argument?

F. Reaction 3 reduces the amount of NO present, inhibiting Reaction 4.
G. Reaction 3 produces H2CO, which can react in Reaction 5 and Reaction 6.
H. Reaction 3 produces HO2, which can react with H2CO to produce CO.
J. Reaction 3 reduces the amount of O2 present, making it more difficult for CO to form.

7. Further investigation has shown that Reaction 6 occurs on a large scale. Which of the following statements explains how the new evidence most weakens the argument of Scientist 1?

A. The OH produced in Reaction 4 reacts with CH4.
B. The OH produced in Reaction 4 reacts with H2CO.
C. The H2O produced in Reaction 6 reacts with light and O3.
D. The OH produced in Reaction 6 reacts with H2CO.

A Carnot heat engine is an engine which runs by compressing and expanding a gas and transferring heat.

Figures 1 and 2 show the changes in pressure, P, and volume, V, that occur as two Carnot heat engines, A and B,?run. For every gas, PV = ΩT, where Ω is a constant and T represents the time.

The cycle begins as the gas is at its highest temperature and pressure. First, the gas expands, so volume increases while pressure decreases. As the gas expands, it can do work, such as pushing a piston. After the gas has run out of thermal energy and can no longer do work it is at its lowest temperature and pressure and the gas begins to be compressed, for instance a piston falling back down on the gas. As the gas is compressed, pressure increases while volume decreases and temperature begins to rise. In every Carnot heat engine, the gas ends at the same pressure, temperature, and volume as it began, thus completing a cycle.

Figure 1

Figure 2

8. According to Figure 2, for Carnot heat engine B, when V was decreasing from its largest value and had a value of 1.5 mL, P had a value closest to:

F. 10 Pa.
G. 30 Pa.
H. 50 Pa.
J. 70 Pa.

9. For a new Carnot heat engine, F, a partial graph of V versus P is obtained.

If Carnot heat engine F behaves like Carnot heat engines A and B, the remainder of the graph of V versus P for Carnot heat engine F will look most like which of the following?

A.
B.
C.
D.

10. For Carnot heat engine A, the minimum value of P was obtained at a V closest to:

F. 0.5 mL.
G. 2.0 mL.
H. 3.5 mL.
J. 5.0 mL.

11. Consider the largest value of V and the smallest value of V on the graph in Figure 2. How are these values related?

A. The smallest value of V is -1 times the largest value of V.
B. The smallest value of V is 1/3 times the largest value of V.
C. The smallest value of V is 1 times the largest value of V.
D. The smallest value of V is 2 times the largest value of V.

12. The reversible isothermal expansion step of a Carnot heat engine cycle takes place when P is decreased from its highest value and V is increased from its lowest value. According to Figure 1, the reversible isothermal expansion step for Carnot heat engine A begins when V is closest to:

F. 1.0 mL.
G. 2.25 mL.
H. 3.0 mL.
J. 3.5 mL.