Below are answer explanations to the full-length Science test of the previously released ACT from the current 2015-2016 “Preparing for the ACT Test” (form 1572CPRE) free study guide available here for free PDF download.

The ACT Science test explained below begins on page 40 of the guide. Please note that the 2015-2016 ACT practice test is the same one used in the 2016-2017 and 2017-2018 “Preparing for the ACT Test” guides. Other answer explanations in this series of articles:

When you’re finished reviewing this official practice ACT test, start practicing with our own 10 full-length practice ACT tests—absolutely free during the pandemic.

## Answer Explanations to the 2015-2018 ACT Practice Science Test

### Passage I

Question 1, “In which of Studies 1 and 2…” The answer is “Study 2; 5% SY medium.”

This question tests your ability to interpret data.

1. This question asks which study (Study 1 or Study 2) had fruit flies that lived for more than 75 days, and what diet these fruit flies ate.
2. Figure 1 shows the data from Study 1 and Figure 2 shows the data from Study 2. The number of days that the fruit flies lived is on the x-axis, and the percentage of fruit flies that were alive is plotted on the y-axis.
3. Looking at Figure 1, we can see that the “percent alive” reached zero for each medium after 55 days. This means that none of the fruit flies in Study 1 lived for more than 75 days.
4. Looking at Figure 2, we see that at the 75 day mark, there is only one data point that isn’t at zero. There is an open circle at ~5%, and the open circle corresponds to 5% SY medium in the Key for Figure 2.
5. Thus, we know that the fruit flies lived for more than 75 days in Study 2, but only in the 5% SY medium. The correct answer is “Study 2; 5% SY medium.”

Question 2, “During Studies 1 and 2, why did the size…” The answer is “The birthrate was 0, because the initial population contained only virgin females.”

This question tests your knowledge of scientific investigation.

1. The passage states that 200 virgin female Strain N fruit flies were added to the test tubes in Study 1 and Study 2. This means that the initial population only contained females and did not contain males.
2. Therefore, we can rule out the answer choices “the birthrate was 0, because the initial population contained only males” and “the death rate was 0, because the initial population contained only males.”
3. If the death rate was 0, then the population of the flies would have stayed the same, because none of them would have died. Therefore, we can rule out the answer choice “the death rate was 0, because the initial population contained only virgin females.”
4. If the birth rate was 0, then no new flies were being born. This means that as the fruit flies died, there were no newborn flies to counterbalance their population, so their population decreased instead of increasing.
5. Therefore, the correct answer is “the birthrate was 0, because the initial population contained only virgin females.”

Question 3, “Study 1 differed from Study 2 in which of the following ways?” The answer is “The SY medium tested in Study 1 contained a higher percent of sugar than did the SY medium tested in Study 2.”

This question tests your knowledge of scientific interpretation.

1. The passage states that in both Study 1 and Study 2, 200 virgin female Strain N fruit flies were added to each test tube.
2. This means that the answer choices “Female fruit flies were tested in Study 1, whereas male fruit flies were tested in Study 2” and “Male fruit flies were tested in Study 1, whereas female fruit flies were tested in Study 2” are incorrect.
3. The other answer choices describe the percent of sugar in the SY mediums that were tested in Studies 1 and 2.
4. Study 1 tested 15% SY medium (a diet of 15% sugar and 15% killed yeast), while Study 2 tested 5% SY medium (a diet of 5% sugar and 5% killed yeast). This means that Study 1 contained a greater percentage of sugar than did Study 2.
5. Therefore, the correct answer is “The SY medium tested in Study 1 contained a higher percent of sugar than did the SY medium tested in Study 2.”

Question 4, “Suppose that an additional trial in Study 3…” The answer is “between 55.6 days and 58.6 days.”

This question tests your ability to evaluate models and infer results.

1. The question asks what the average life span of Strain X fruit flies would have been if a 12% SY medium trial had been performed.
2. Looking at Table 1, the Strain X that lived in 10% SY medium had an average life span of 58.6 days. The Strain X that lived in 15% SY medium had an average life span of 55.6 days.
3. An additional trial of Strain X in 12% medium would most likely have an average life span that is between the 10% life span and the 15% life span. Therefore, the correct answer is “between 55.6 days and 58.5 days.”

Question 5, “The researchers had predicted that decreasing…” The answer is “Yes; for each SY medium tested, the average life span of Strain X fruit flies was longer than the average life span of Strain N fruit flies.”

This question tests your ability to evaluate models and infer results.

1. The passage states that Strain X flies could not detect many odors. From Table 1, we can see that the average life span of Strain X fruit flies was greater than that of the Strain N fruit flies, regardless of the percentages of sugar or killed yeast in the SY medium.
2. Thus, the results of Study 3 indicate that having a decreased ability to smell odors resulted in an increased life span. These results are consistent with the researchers’ prediction.
3. The correct answer is “Yes; for each SY medium tested, the average life span of Strain X fruit flies was longer than the average life span of Strain N fruit flies.”

Question 6, “Suppose the researchers wanted to determine…” The answer is “Repeat Study 1 except with Strain X fruit flies.”

This question tests your knowledge of scientific investigation.

1. If the researches want to determine whether a defect in the ability to detect odors would affect the life span of fruit flies, they should test fruit flies with a defect in the ability to detect odors.
2. Therefore, researchers should test Strain X fruit flies, which lack the protein required to detect a wide range of odors, because these flies have a defect in their ability to detect odors.
3. If researchers want to determine the life span of fruit flies that are fed 15% SY medium, they should repeat Study 1 instead of Study 2.
4. This is because Study 1 tested the 15% SY medium, while Study 2 tested 5% SY medium.
5. Therefore, the correct answer is “Repeat Study 1 except with Strain X fruit flies.”

Question 7, “The results for which 2 tubes should be compared…” The answer is “Tube 1 and Tube 4.”

This question tests your knowledge of scientific investigation.

1. To determine how a reduced calorie diet affects life span of fruit flies, the two test tubes that are compared must have different caloric amounts.
2. This means that the two test tubes cannot contain the same % SY medium. Tubes 1 and 2 both contain 15% SY medium, while Tubes 5 and 6 both contain 5% SY medium.
3. Therefore, answer choices “Tube 1 and Tube 2” and “Tube 5 and Tube 6” can be ruled out.
4. If this comparison is being done for fruit flies in the absence of live yeast and additional odors from live yeast, then the test tubes cannot contain live yeast or additional odors from live yeast.
5. Tubes 2, 3, 5, and 6 all contain either live yeast or additional odors from live yeast. Answer choice “Tube 2 and Tube 5” can be ruled out.
6. Tubes 1 and 4 are the only two test tubes without additional substances that contain different SY mediums. Therefore, they are the two test tubes that should be compared.
7. The correct answer is “Tube 1 and Tube 4.”

### Passage II

Question 8, “Which hypothesis, if any, asserts that monarch butterflies store lipids…” The answer is “Hypothesis 1.”

This question tests your ability to interpret information.

1. Hypothesis 1 states that the butterflies first store lipids at the beginning of migration, and then again at the end of migration/before the beginning of the overwintering period.
2. Hypothesis 2 states that the butterflies store lipids at the beginning of migration, but there is no need for them to store lipids for the overwintering period.
3. Hypothesis 3 states that the butterflies do not store lipids at the beginning of migration or during the overwintering period, but they do store them during migration.
4. Hypotheses 2 and 3 both state that the butterflies only store lipids during one period.
5. Meanwhile, Hypothesis 1 states that the butterflies store lipids during 2 different periods. Therefore, the correct answer is “Hypothesis 1.”

Question 9, “Which hypothesis, if any, asserts that monarch butterflies require energy…” The answer is “None of the hypotheses.”

This question tests your ability to interpret information.

1. Hypothesis 1 claims that the butterflies require energy from stored lipids for migration and for the overwintering period.
2. Hypothesis 2 claims that the butterflies require energy from stored lipids for migration only.
3. Hypothesis 3 claims that the butterflies require energy from stored lipids during the overwintering period only.
4. None of these hypotheses claim that the butterflies do not require energy from stored lipids during migration or during the overwintering period.
5. Therefore, the correct answer is “none of the hypotheses.”

Question 10, “Based on Hypothesis 3, which of the following figures best depicts…” The answer is “J.

This question tests your ability to interpret data.

1. Hypothesis 3 states that the butterflies’ lipid mass continuously increases from the beginning of migration until the end of migration.
2. In the figure, B represents the beginning of migration and E represents the end of migration.
3. If the lipid mass continuously increases from B to E, this means that the slope of the line in the figure should always be positive.
4. Answer choice J is the only one where the figure contains a line that only increases.

Question 11, “Assume that changes in the body mass of a monarch butterfly…” The answer is “Hypotheses 1 and 2 only.”

This question tests your ability to interpret information.

1. The statement is “The percent of a monarch butterfly’s body mass that is made up of lipids is greater at the beginning of migration than at the end of migration.”
2. This statement would be supported by hypotheses that state that the butterflies’ lipid mass decreases during migration.
3. Hypothesis 1 states that the butterflies store lipids before migration, and then during migration, their lipid mass continuously decreases. This would indicate that the butterflies’ lipid body mass is greater at the beginning of migration than at the end of migration.
4. Therefore, Hypothesis 1 supports the statement.
5. Hypothesis 2 states that the butterflies store lipids before migration and that during migration, lipid mass continuously decreases. Again, this indicates that the butterflies’ lipid mass is greater at the beginning of migration than at the end of migration.
6. Therefore, Hypothesis 2 also supports the statement.
7. Hypothesis 3 states that lipids are stored during migration so that the butterflies’ lipid mass continuously increases from the beginning of migration until the end of migration.
8. This would indicate that the butterflies’ lipid body mass is greater at the end of migration than at the beginning of migration.
9. Therefore, Hypothesis 3 contradicts the statement. The correct answer is “Hypotheses 1 and 2 only.”

Question 12, “To store lipids, monarch butterflies convert sugar from nectar…” The answer is “Hypothesis 1.”

This question tests your ability to interpret information.

1. The question is asking which hypothesis claims that the butterflies store lipids for the overwintering period.
2. Hypothesis 1 states that the butterflies must store lipids again before beginning the overwintering period.
3. Meanwhile, Hypotheses 2 and 3 both state that the butterflies do not store lipids while at the overwintering sites.
4. Therefore, Hypothesis 1 would be most likely to claim that nectar must be present at the overwintering sites so that the butterflies can store lipids for the overwintering period.

Question 13, “Which of the following statements about lipids in monarch butterflies…” The answer is “The butterflies’ lipid masses change during migration.”

This question tests your ability to interpret information.

1. This question asks which statement is consistent with all 3 hypotheses.
2. The answer choice “The butterflies’ lipid masses do not change during the overwintering period” is incorrect. Hypothesis 1 states that the butterflies must store lipids before beginning the overwintering period, indicating that the butterflies use the lipids as energy during the overwintering period.
3. The answer choice “The butterflies use energy from stored lipids during the overwintering period” is incorrect because Hypothesis 2 states that “energy from stored lipids is not required during the overwintering period.” Therefore, this statement is not consistent with all 3 hypotheses.
4. The answer choice “The butterflies use energy from stored lipids for migration” is incorrect because Hypothesis 3 states that the butterflies require energy from stored lipids during the overwintering period but not for migration. Therefore, this statement is not consistent with all 3 hypotheses.
5. The correct answer choice is “The butterflies’ lipid masses change during migration.”
1. Hypothesis 1 states that lipid mass continuously decreases during migration. This is a change in lipid mass during migration.
2. Hypothesis 2 also states that lipid mass continuously decreases during migration, which is also a change in lipid mass during migration.
3. Hypothesis 2 states that lipid mass continuously increases during migration. This indicates that the lipid masses change during migration.
4. Therefore, this statement is consistent with all 3 hypotheses.

Question 14, “When the monarch butterflies use their stored lipids…” The answer is “ATP.”

This question tests your knowledge of fundamental science concepts.

1. The question asks which molecule is produced as a direct result of the breakdown of lipids. The question also states that the lipids are broken down into “energy-rich molecules that can be readily used by cells.”
2. Therefore, we must figure out which answer choice is a molecule that is energy-rich and can be used by cells.
3. The correct answer is “ATP,” which is a molecule that carries energy and is found in the cells of all living things.
4. The answer choices “starch,” “DNA,” and “amino acids” are incorrect because these molecules are not direct sources of energy for cells.
1. Starch can be broken down to produce ATP, which then serves as the true energy source for cells.
2. DNA is not an inherent source of energy. Rather, it is the source of genetic material for cells.
3. Amino acids can be broken down to produce energy, but cells cannot use amino acids as energy-rich molecules themselves.

### Passage III

Question 15, “According to Figure 2, the solar radiation intensity 8,000 years ago…” The answer is “500 watts/m².”

This question tests your ability to evaluate models.

1. Figure 2 plots the solar radiation intensity and the methane concentration in the atmosphere over the last 10,000 years. The solar radiation intensity is indicated by the dotted line.
2. The x-axis of Figure 2 is given in units of “thousands of years ago.” Therefore, data for 8,000 years ago can be found at the eighth tick (between 10 and 5) on the x-axis.
1. Note that the x-axis goes from right to left; the origin of the axis is located in the bottom right corner and marks the “present.”
3. Looking at the eighth tick, we can see that the solar radiation intensity line is at ~500 watts/m². Therefore, the correct answer is “500 watts/m².”

Question 16, “According to Figure 2, if the trend in the $CH_{4}$ concentration had continued…” The answer is “less than 550 ppb.”

This question tests your ability to evaluate models and infer results.

1. The question asks what the  $CH_{4}$ concentration at present would be if the $CH_{4}$ concentration trendline had followed the trend in the solar radiation intensity.
2. Looking at Figure 2, we can see that the solar radiation intensity (the dotted line) continued to decrease from 10,000 years ago to now.
3. However, the $CH_{4}$ concentration did not follow this trend. Instead of continuously decreasing over the past 10,000 years, the $CH_{4}$ concentration decreased for 5,000 years and then continuously increased for the next 5,000 years.
4. If the $CH_{4}$ concentration followed the “continuously decreasing” trend of the solar radiation intensity, then the $CH_{4}$ concentration in the present would be less than the $CH_{4}$ concentration 5,000 years ago.
5. Therefore, we can infer that the $CH_{4}$ concentration in the present would be less than 550 ppb if the $CH_{4}$ concentration trend had followed the solar radiation intensity trend.
6. The correct answer is “less than 550 ppb.” The other answer choices are incorrect because they include values that are higher than the $CH_{4}$ concentration 5,000 years ago.

Question 17, “Suppose that whenever the $CH_{4}$ concentration increases…” The answer is “B.

This question tests your ability to evaluate models and infer results.

1. The question states that whenever $CH_{4}$ concentration increases, the average global temperature also increases, and whenever $CH_{4}$ concentration decreases, the average global temperature also decreases.
2. This indicates that the average global temperature follows the same trend as the trend in $CH_{4}$ concentration.
3. Therefore, a graph of average global temperature over the past 11,000 years would look similar to the graph of $CH_{4}$ concentration over the past 11,000 years.
4. Looking at Figure 2, we can see that $CH_{4}$ concentration decreased for the first half of that 11,000 year period, and then increased for the next 5,000 years.
5. Answer choice B is correct because it is the only answer choice that contains a graph where the average global temperature decreases for the first half and increases for the second half of the 11,000 year period.

Question 18, “Based on Figure 1, the average solar radiation intensity over the past 250,000 years…” The answer is “480 watts/m².”

This question tests your ability to interpret a graph.

1. To find the average solar radiation intensity over the past 250,000 years from looking at Figure 1, we must find the midpoint between the maxima (the crests) and the minima (the troughs) of the solar radiation trendline, which is indicated by the dotted line.
2. The midpoint would roughly represent the “average” solar radiation intensity because the midpoint value would average the highest values and the lowest values of solar radiation intensity.
3. The answer choice “400 watts/m²” can be ruled out because this value does not even appear on the graph. The y-axis only ranges from 440 watt/m² to 540 watts/m².
4. The answer choice “440 watts/m²” can be ruled out because this value corresponds to many of the minima on the solar radiation line.
5. The answer choice “520 watts/m²” is incorrect because this value corresponds to many of the maxima on the solar radiation line.
6. The correct answer is “480 watts/m²” because this value occurs at the approximate midpoint between the minima and the maxima of the solar radiation trendline.

Question 19, “One solar radiation cycle is the time between a maximum in the solar radiation intensity and the next maximum…” The answer is “between 15,000 years and 35,000 years.”

This question tests your ability to interpret a graph.

1. This question asks how much time elapses between two maxima on the solar radiation intensity trendline.
2. Looking at Figure 1, we can see that the first maximum of solar radiation intensity occurred 240,000 years ago and the next maximum after that occurred a little less than 220,000 years ago.
3. This indicates that the time interval between two maxima of solar radiation intensity is about 20,000 years.
4. The correct answer is “between 15,000 years and 35,000 years” because it includes the 20,000 year time interval.
5. The other answer choices are incorrect because their time ranges are too small or too large.

Question 20, “Which of the following statements best describes the primary effect of…” The answer is “$CH_{4}$ absorbs heat that comes up from Earth’s surface, warming Earth’s climate.”

This question tests your knowledge of fundamental scientific principles.

1. The passage states that greenhouse gases such as methane ($CH_{4}$) warm Earth’s climate.
2. We can rule out answer choices “$CH_{4}$ gives off visible light to space, cooling Earth’s climate” and “$CH_{4}$ absorbs heat as it enters Earth’s atmosphere from space, cooling Earth’s climate” because these both incorrectly state that methane cools Earth’s climate.
3. Greenhouse gases are gases that absorb radiation, contributing to a greenhouse effect in Earth’s atmosphere.
4. We know that greenhouse gases do not give off UV radiation, therefore the answer choice “$CH_{4}$ gives off ultraviolet radiation to space, warming Earth’s climate” is incorrect.
5. The correct answer is “$CH_{4}$ absorbs heat that comes up from Earth’s surface, warming Earth’s climate” because $CH_{4}$ is a greenhouse gas that absorbs heat and radiation, which contributes to the warming greenhouse effect in Earth’s atmosphere.

### Passage IV

Question 21, “If a block was pulled toward the east, the frictional force exerted on the block…” The answer is “west.”

This question tests your knowledge of fundamental scientific principles.

1. Friction works against the pulling force.
2. Therefore, if the block is being pulled across a flat surface toward the east, then there is also a frictional force being exerted on the box in the opposite direction.
3. The opposite direction of east is the “west,” which is the correct answer.

Question 22, “Based on Figure 2, what is the order of the 3 blocks, from the block that required the shortest time…” The answer is “2.00 kg block, 2.50 kg block, 3.00 kg block.”

This question tests your ability to evaluate models and infer results.

1. Figure 2 plots the speed of the blocks versus the time as the blocks are pulled across a surface.
2. Looking at the 15 m/sec mark on the y-axis, we can see that the 2.00 kg line reached 15 m/sec in about 1.5 seconds.
3. The 2.50 kg reached 15 m/sec in about 2 seconds, and the 3.00 kg block reached 15 m/sec in about 3 seconds.
4. Therefore, the 3 blocks listed in order of shortest time to reach 15 m/sec to longest time to reach 15 m/sec are the “2.00 kg block, 2.50 kg block, and the 3.00 kg block.”

Question 23, “Based on Figure 2, what was the approximate value of the acceleration…” The answer is “5 m/sec².”

This question tests your knowledge of fundamental scientific principles.

1. This question asks for the acceleration of the 3.00 kg block.
2. Acceleration is equal to change in velocity divided by change in time.
3. We can find the acceleration of the 3.00 kg block by taking its total change in velocity and dividing that by its total change in time.
4. The 3.00 kg block’s velocity increased from 0 m/sec to 15 m/sec over a time interval of 3 seconds.
5. The block’s acceleration = 15 m/sec divided by 3 seconds, which gives us 5 m/sec/sec.
6. This can also be written as 5 m/sec², which is the correct answer.

Question 24, “Based on Figure 1, the results of Experiment 1 are best modeled by which…” The answer is “Pulling force (N) = 5.0 x block mass (kg).”

This question tests your ability to evaluate models and infer results.

1. Figure 1 plots the pulling force in newtons versus the block mass in kilograms.
2. Because Figure 1 and Experiment 1 do not include time as a variable, we can rule out answer choices “block speed (m/sec) = 0.2 x time (sec)” and “block speed (m/sec) = 5.0 x time (sec).”
3. The other two answer choices state that Pulling force in N equals ___ times the block mass in kg.
4. Looking at Figure 1, we can see that when the block mass was 1 kg, the pulling force was 5 N. When the block mass was 2 kg, the pulling force was 10 N.
5. The pulling force is equal to the block mass multiplied by 5.
6. Therefore, the correct answer is “Pulling force (N) = 5.0 x block mass (kg).”

Question 25, “At each of the times plotted in Figure 2 (except 0.00 sec)…” The answer is “decreased only.”

This question tests your ability to interpret data.

1. In Figure 2, we can see the different block speeds for different block masses.
2. The 2.00 kg block reached the highest speeds, while the 3.00 kg block reached the lowest speeds.
3. As the block mass increased, we can see that the block speed decreased.
4. The correct answer is “decreased only” because the block speed decreased for blocks with more mass.
5. The answer choice “increased only” is incorrect because the greatest block mass did not result in increased block speed.
6. The answer choice “varied, but with no general trend” is not correct because there is a trend in block speed and block mass.
7. The answer choice “remained the same” is incorrect because the block speeds were not the same between the three different block masses.

Question 26, “Based on Figure 1, an applied force of 30.00 N would…” The answer is “6.00 kg.”

This question tests your ability to evaluate models and interpret results.

1. From Question 24, we know that the “Pulling force (N) = 5.0 x block mass (kg).”
2. If the pulling force is 30 N, then we can find the block mass by dividing 30 N by 5.
3. The block mass = 30/5 = 6 kg.
4. The correct answer is “6.00 kg.”

### Passage V

Question 27, “One way Experiment 2 differed from Experiment 3 was that in Experiment 2…” The answer is “the solutions to which indicators were added were of known pH.”

This question tests your knowledge of scientific investigation.

1. Looking at Table 2 and Table 3, metanil yellow is present as an indicator for both. Therefore, both experiments 2 and 3 used metanil yellow.
2. Answer choices “metanil yellow was used” and “metanil yellow was not used” can be ruled out because if both experiments used metanil yellow, then they cannot differ in this way.
3. The passage states that in Experiment 3, students were given 4 solutions of unknown pH. In Experiment 2, the solutions had a pH of 8 or greater, and Table 2 shows that there was a solution of each pH value from 8-14.
4. This means that the solutions in Experiment 2 were of known pH, but the solutions in Experiment 3 were of unknown pH.
5. The answer choice “the solutions to which indicators were added were of unknown pH” is incorrect because Experiment 2 contained solutions of known pH, not unknown pH.
6. The correct answer is “the solutions to which indicators were added were of known pH.”

Question 28, “Based on the description of the well plate and how it was used, the empty…” The answer is “White.”

This question tests your knowledge of scientific investigation.

1. This question is asking what color the empty well plate is.
2. If the well plate is being used to hold colorless solutions and to see what color they turn into when the indicator is added, then the well plate cannot be any of the colors that the solution might turn into.
3. Therefore, the answer choices “blue” and “red” can be ruled out, because these colors would make it difficult to see when an indicator turns a solution blue or red.
4. Additionally, the answer choice “black” is incorrect because colors are very difficult to see against a black background.
5. The well plate needs to contrast against the color of the solution, so it is most likely that the well plate is white.
6. The correct answer is “white.”

Question 29, “Based on the results of Experiments 1 and 2, which of the following…” The answer is “pH = 7.4 to pH = 8.6.”

This question tests your ability to evaluate models and infer results.

1. The passage states that the transition range of an indicator is the pH range where “the indicator’s color will be an intermediate of its other 2 colors.”
2. Looking at Table 1, curcumin resulted in a yellow color for solutions with pHs from 0 to 7.
3. Looking at Table 2, curcumin resulted in an orange color for the solution with a pH of 8, and curcumin also resulted in a red color for solutions with pHs greater than 8.
4. Orange is the intermediate color between yellow and orange, so the pH at which curcumin turned orange should be included in curcumin’s transition range.
5. From this, we know that curcumin’s transition range must include a pH of 8.
6. Looking at the answer choices, the correct answer is “pH = 7.4 to pH = 8.6” because this is the only range of pHs that includes a value of 8.

Question 30, “A chemist has 2 solutions, one of pH = 1 and one of pH = 6….” The answer is “No; indigo carmine is blue at both pH = 1 and pH = 6.”

This question tests your ability to evaluate models and interpret results.

1. When added to a solution with a pH of 1, indigo carmine turned the solution blue.
2. When added to a solution of pH 6, indigo carmine turned the solution blue.
3. If indigo carmine turned both the pH = 1 solution and the pH = 6 solution blue, then it can’t be used to distinguish the two solutions. Both are blue, so we can’t know which pH is which.
4. Therefore, the correct answer is “No; indigo carmine is blue at both pH = 1 and pH = 6.”

Question 31, “The indicator propyl red has a transition range of…” The answer is “Resorcin blue.”

This question tests your ability to evaluate models and infer results.

1. This question asks which indicator has a similar transition range to that of the indicator propyl red.
2. Propyl red has a transition of pH = 4.6 to pH = 6.8. We would expect to see an indicator with a similar transition range to turn into intermediate colors at pH = 5 and pH = 6.
3. Looking at Table 1, the only indicator that turned into its intermediate color at pH = 5 and pH = 6 is resorcin blue, which turns from red to purple at pH = 5 and from purple to blue after pH = 6.
4. Indigo carmine and curcumin can be ruled out because they are both the same color for the entire pH = 1 to pH = 6 range.
5. Metanil yellow can be ruled out because it turns into its intermediate color, orange, at pH = 2.
6. Therefore, the correct answer is “resorcin blue.”

Question 32, “A student claimed that Solution III has a pH of 7.3…” The answer is “No, because in Solution III resorcin blue was red.”

This question tests your ability to evaluate models and infer results.

1. If Solution III has a pH of 7.3, then metanil yellow will be yellow, resorcin blue will be blue, curcumin will be yellow, and indigo carmine will be blue when added to Solution III. We know this by looking at the column for “color in solution with a pH of 7” in Table 1.
2. However, looking at Table 3, we can see that resorcin blue was red when added to Solution III, not blue.
3. All of the other indicators turned into the correct color except resorcin blue. However, because resorcin blue did not turn blue in Solution III, we know that Solution III can’t have a pH of 7.3.
4. The answer is “No,” the results of Experiments 1-3 are not consistent with the claim in the question.
5. Therefore, the answer choices “Yes, because in Solution III metanil yellow was yellow” and “Yes, because in Solution III resorcin blue was red” can be ruled out.
6. Furthermore, the reason that Solution III doesn’t have a pH of 7.3 is because resorcin blue was red when added to Solution III. If Solution III truly had a pH of 7.3, resorcin blue would have turned blue.
7. Therefore, the correct answer is “No, because in Solution III resorcin blue was red.”

Question 33, “Based on the results of Experiments 1-3, which of Solutions I-IV…” The answer is “Solution IV.”

This question tests your ability to evaluate data and infer results.

1. Looking at Table 1, we can see the colors for each indicator at the lower pHs. From a pH range of 0 to 2, resorcin blue is red, curcumin is yellow, indigo carmine is blue, and metanil yellow is either red or orange.
2. Looking at Table 3, only Solutions III and IV resulted in red for resorcin blue, yellow for curcumin, and blue for indigo carmine.
3. Therefore, the answer choices “Solution I” and “Solution II” can be ruled out because they both resulted in blue for resorcin blue and red for curcumin which would indicate a pH range of 9-11.
4. Solution III resulted in yellow for metanil yellow while Solution IV resulted in orange for metanil yellow.
5. Looking at Table 1, we can see that metanil yellow turns orange at pH = 2 and yellow at pH = 3 or greater.
6. Therefore, Solution IV must have a lower pH than Solution III.
7. The correct answer is “Solution IV” has the lowest pH.

Question 34, “Albedo was measured at noon because that time of day is when…” The answer is “most intense.”

This question tests your knowledge of fundamental scientific principles.

1. Albedo is defined by the passage as the “proportion of the total incoming solar radiation that is reflected from a surface.”
2. Since albedo is a proportion, we know that less than 100% of solar radiation reaching the ground is reflected, and therefore less than 100% of solar radiation reaching the ground is absorbed.
3. We can rule out answer choices “100% reflected” and “100% absorbed.”
4. This leaves us with the answer choices of “least intense” and “most intense.”
5. UV rays are strongest in the middle of the day, from 10 am to 4 pm because this is when the sun is directly overhead. This means that the sun has a more direct angle, so its radiation is more intense.
6. Therefore, we know that albedo is measured at noon because solar radiation reaching the ground is more intense at noon.
7. The correct answer is “most intense.”

Question 35, “Why was the study designed so that the 3 plots had the same type…” The answer is “amount of DM sprayed.”

This question tests your knowledge of scientific investigation.

1. This question basically asks how the 3 plots differed.
2. The passage states that 40 cubic meters per hectare of DM was sprayed on Plot 2, while 80 cubic meters per hectare of DM was sprayed on Plot 3, and no DM was sprayed on Plot 1.
3. From this, we know that the amount of DM that was sprayed on each plot varied.
4. The correct answer is “amount of DM sprayed.”
5. The answer choice “type of soil present” is incorrect because the passage states that each plot contained semiarid grassland area.
6. The answer choice “plot area” is incorrect because the passage states that each plot was 10 m by 40 m.
7. The answer choice “plot slope” is incorrect because the passage states that each plot was unsloped.

Question 36, “On one day of the study period, a measurable rainfall occurred…” The answer is “July 26.”

This question tests your ability to evaluate models and infer results.

1. This question states that the albedo calculated for the cloudless day just after the rainy day was lower than the albedo calculated for the cloudless day just before the rainy day.
2. The rainy day is the day before the day that albedo decreased.
3. Looking at Figure 1, there is a dip in each of the trendlines for all three plots on July 27th.
4. This means that July 27th was the cloudless day just after the rainy day.
5. Therefore, “July 26th” was the day on which measurable rainfall occurred.

Question 37, “For each plot, the number of temperature readings recorded…” The answer is “12.”

This question tests your ability to interpret information.

1. This question asks how many temperature readings were recorded every minute.
2. The passage states that for each plot, the sensor recorded soil temperature every 5 seconds.
3. To find the number of readings in a minute, we simply divide 60 seconds by 5 seconds. This gives us 12, which is the correct answer.

Question 38, “According to Figure 1 and the description of the study, was July 20…” The answer is “No, because albedo data were not collected on that day.”

This question tests your ability to evaluate models and interpret results.

1. Albedo data was collected on cloudless days. Albedo data collection is indicated in Figure 1 by the circle, square, and triangle data points for Plots 1, 2, and 3 respectively.
2. This means that albedo data was only collected on days where the data is indicated by these specific data points instead of just a dotted or regular line.
3. Looking at July 20, we don’t see any circle, square, or triangle data points.
4. This indicates that albedo data was not collected on July 20th, so July 20th cannot have been a cloudless day.
5. The correct answer is “No, because albedo data were not collected on that day.”

Question 39, “According to the results of the study, did the presence of a cover of DM increase or decrease…” The answer is “D. albedo: decrease; soil temperature: increase.”

This question tests your ability to evaluate models and infer results.

1. Looking at Figure 1, we can see that Plot 1 had higher albedo values than Plot 2, and Plot 2 also generally had higher albedo values than Plot 3.
2. Plot 1 had no DM sprayed, while Plot 3 had the most DM sprayed.
3. From this, we can infer that the presence of DM resulted in decreased albedo values.
4. We can rule out the two answer choices that claim that albedo increased as a result of the presence of DM.
5. Looking at Figure 2, we can see that Plot 3 (indicated by the triangle data points) had higher soil temperatures than Plot 2 (indicated by square data points). Plots 2 and 3 both had higher soil temperatures than Plot 1 (indicated by circle data points).
6. Therefore, it seems that having more DM sprayed resulted in higher soil temperatures than having no DM sprayed.
7. The presence of DM resulted in increased soil temperature.
8. The correct answer is “albedo: decrease and soil temperature: increase.”

Question 40, “Based on Figure 1, on August 3, what percent of incoming solar radiation was NOT reflected from Plot 2?” The answer is “80%.”

This question tests your ability to interpret data.

1. This question asks what percent of incoming solar radiation was NOT reflected from Plot 2 on August 3.
2. Looking at Figure 1, we can see that on August 3rd, the albedo for Plot 2 (indicated by the black square data points) was 0.20.
3. Albedo is a measure of the proportion of incoming solar radiation that IS reflected from a surface.
4. Therefore, the proportion of incoming solar radiation that was NOT reflected from Plot 2 on August 3rd is 0.80 (which is found by subtracting 0.20 from 1).
5. This can be converted into a percentage by multiplying by 100.
6. This gives us 80%, which is the correct answer.