ACT Science practice test

Fine dust sometimes rises above an airless body’s surface near sunrise. Two planetary physicists proposed explanations.

Dr. Chen—Electrostatic Model

Ultraviolet (UV) light removes electrons from surface grains. Similarly charged grains repel one another, and small grains lift. Temperature is not the direct cause, so increasing UV intensity at constant temperature should increase lofting. With UV blocked, warming alone should not loft many grains.

Dr. Silva—Gas-Flow Model

Small amounts of gas trapped in pores expand as the surface warms, pushing grains upward. UV has no direct role if temperature remains fixed. Increasing surface temperature should increase lofting even in darkness.

Researchers placed equal masses of dust on identical porous rocks in a vacuum chamber. In Experiment 1, surface temperature remained −20°C while UV intensity was set at 0%, 50%, or 100%. In Experiment 2, UV was blocked while surface temperature was set at −20°C, 0°C, or 20°C. Grain size, exposure time, rock porosity, and chamber pressure were held constant. Each condition was repeated 4 times. Tables 1 and 2 give the mean number of grains lofted per minute.

Researchers studied whether biochar, a carbon-rich material made by heating plant matter with little oxygen, can remove nitrate from water.

In Study 1, each beaker received 500 mL of water containing 40 mg/L nitrate. The researchers added 0 g, 5 g, 10 g, or 15 g of biochar to separate beakers. The mixtures were stirred for 24 hours at 20°C and then filtered. Four beakers were tested at each biochar mass. Table 1 shows the mean nitrate concentrations remaining.

In Study 2, every beaker received 10 g of biochar, but contact time varied. Remaining nitrate was measured after 1, 4, 12, and 24 hours. The starting water and stirring speed matched those used in Study 1. Table 2 shows the mean nitrate concentrations remaining.

Researcher A proposed that nitrate attaches to binding sites on the biochar surface. Additional biochar supplies more binding sites but encounters progressively less nitrate remaining in the fixed water volume, so each additional gram of biochar should remove less nitrate than the preceding gram. Researcher B proposed that microorganisms carried on the biochar consume nitrate. According to this explanation, longer contact gives the microorganisms more time to act. Both researchers expected that adding more biochar would generally leave less nitrate in the water.

The studies did not distinguish conclusively between these mechanisms. The biochar was not sterilized, and neither the number of binding sites nor the abundance of living microorganisms was measured. As a comparison, the researchers repeated the 24-hour, 10 g treatment with washed gravel instead of biochar (Table 3). They also found that pH changed by less than 0.2 unit in every beaker.

Scientists tested how moths corrected their headings when a white lamp appeared. In Study 1, the lamp was 60° to the right of the moths’ starting direction; in Study 2, it was 60° to the left. Separate groups experienced no lamp or a lamp at 6, 12, or 24 lux. After 4 seconds, the researchers calculated the magnitude of each group’s mean turn toward the lamp. Positive values therefore indicate a correction toward the light in either study. Each group contained 40 moths. The chamber, lamp color, release point, and airflow were unchanged. The no-lamp measurements reflect ordinary turning, so treatment effects can be found by subtracting each study’s own no-lamp value. Each moth was tested only once.

Researchers observed that dry field soil sometimes warmed during the 3 hr after a brief rainfall, even though the rainwater was cooler than the upper soil. They placed probes at depths of 5 cm, 15 cm, and 30 cm. Before watering, all plots had the same temperature at each depth. They then added equal amounts of 18°C water at noon to 3 types of plots: sunlit soil containing its usual microorganisms, sunlit soil sterilized to reduce microbial activity, and unsterilized soil shaded since sunrise. Figure 1 shows each mean temperature change 3 hr after watering. Positive values indicate warming.

Three researchers offered explanations.

Researcher 1 proposed the retained-energy model. Before watering, evaporation from dry soil removed little energy. Watering created a wet surface, but during the next hours the darkened surface absorbed more incoming solar energy instead of reflecting it. The retained energy warmed the upper soil first and then moved downward. This model predicts stronger warming in sunlit than shaded plots and does not require microorganisms.

Researcher 2 proposed the conductivity model. Sunlight stored energy near the dry surface before watering. Water filled air spaces and increased the soil’s thermal conductivity, allowing that stored energy to move downward more quickly. This model also predicts stronger warming in sunlit plots, but it predicts that downward heat transfer delays maximum warming, so the warming peak should occur progressively later at greater depths. Sterilization should have little effect.

Researcher 3 proposed the microbial model. Dormant microorganisms became active after water was added, and their metabolism released heat. Because this activity increased only gradually, the model allows a delay before maximum warming as metabolic heat production rises. This model predicts more warming in unsterilized than sterilized soil under otherwise identical conditions. It can account for warming in shaded plots, although it predicts less warming there if cooler shade slows metabolism.

Researchers investigated how filter pore diameter affects the time required to filter water. They tested papers with pore diameters of 2, 5, 10, or 20 micrometers. Because of how the papers were manufactured, papers with smaller pores were also thicker.

In Experiment 1, a researcher poured 200 mL of water at 20°C into each filter and measured the time until all the water passed through. Experiment 2 repeated the procedure with water at 40°C. For each experiment, 5 new filters of each pore diameter were tested, and the mean time was calculated. The water source, filter area, funnel, poured volume, and pouring method remained constant. Water temperature was verified before each trial, and pore-diameter order was randomized. Table 1 and Figure 1 give the results.

Researchers investigated how flower characteristics affected visits by bees and hoverflies in a meadow. Each experimental array contained 12 artificial flowers, all receiving the same treatment. Flowers had identical shapes and nectar openings. A visit was counted when an insect landed and inserted its mouthparts into an opening. Repeated visits by the same insect were counted separately; therefore, visit rates did not measure numbers of individual insects.

Each study lasted 6 days. All treatments within a study were observed simultaneously for 1 h daily. Treatment locations were randomly reassigned among plots each day, and fresh flower inserts were used. Each flower initially contained 0.20 mL of nectar solution, which was restored to that volume every 5 min. Sugar concentration is expressed as grams of sugar per liter of solution. Figures report mean visits per hour to an entire array, separately for each pollinator type.

Study 1 Researchers varied petal color among blue, yellow, and white. All flowers were unscented and contained nectar at 200 g/L. An identical pad beneath each flower received a fixed volume of scent-free carrier liquid. The pads allowed subsequent scent treatments without changing flower structure. Figure 1 shows the results. For any treatment, the combined visit rate is the sum of its bee and hoverfly rates.

Study 2 Researchers tested scent presence separately on blue and yellow flowers. Scent-present pads received carrier liquid containing a floral scent; scent-absent pads received the same volume of carrier alone. Nectar remained at 200 g/L. Thus, each color had its own scent-absent comparison. Table 1 shows the results. A proportional scent effect can be calculated by dividing the rate with scent present by the rate with scent absent for the same color and pollinator.

Study 3 Researchers varied nectar sugar concentration in yellow, scent-present arrays, retaining the scent dose used in Study 2. Initial nectar volume and replenishment procedures remained unchanged, so increasing concentration also increased the initial sugar mass available in each flower. Table 2 shows the results. The studies measured visitation rather than sugar consumption: an insect could leave after removing only part of the available nectar.

Trees of one maple species appear to begin changing color earlier in some years than in others. Researchers investigated whether day length, cool nights, or late-summer drought explains this variation. They used genetically identical saplings and defined color onset as the first day that yellow pigment was visible in a fixed proportion of marked leaves. All saplings were green on Day 0. Table 1 summarizes environmental records for four chamber treatments. Color-onset results were withheld from three scientists, who proposed the explanations below. All other growing conditions were identical, and observations were made daily using the same procedure.

Scientist 1

Day length alone sets color onset. On the first day with at most 12 h of light, an irreversible internal process begins; visible color appears exactly 4 days later. Neither cool nights nor drought changes this delay or initiates the process independently. Thus, onset occurs on Day P + 4. Reports of differences between years reflect inconsistent sampling of canopy positions, rather than different environmental triggers. With marked leaves and standardized scoring, identical saplings exposed to identical day-length schedules should begin changing color together, even when their temperature and watering schedules differ.

Scientist 2

Cool nights initiate the process. Three consecutive nights at or below 8°C are required, and visible color appears exactly 4 days after the third qualifying night ends: Day C + 4. A warmer night before the third qualifying night resets the count. Once three qualifying nights have occurred consecutively, subsequent warming cannot cancel or delay color onset. Day length and soil moisture have no direct effect. Their seasonal association with cooling can make either appear responsible. Consequently, shortening days without the required cool-night sequence will leave these saplings green.

Scientist 3

Day length establishes the baseline onset at Day P + 4, but drought can advance it. After five consecutive days with soil water content at or below 12%, measured as water volume divided by soil volume, drought-induced color appears 2 days later. This advance requires the soil to remain at or below 12% throughout those additional days. Rewatering before visible color appears cancels the drought pathway, leaving the day-length pathway intact. Rewatering after color appears does not restore green coloration. Without rewatering, onset is the earlier of Day P + 4 and Day D + 2. Cool nights have no direct effect. For Table 1, day length continues decreasing, nights remain cool after C, and soil remains dry after D unless a question specifies a change.