Free ACT Experimental Design and Methods practice questionsEnhanced ACT, 2026 format
What changed between experiments, which variable was held constant, and why a particular step in the procedure was taken.
Answer each question, then read why the right answer is right and why the tempting one is wrong.
Question 1
Science
Transpiration is the loss of water vapor from a plant, mostly through pores (stomata) in its leaves. Students measured the transpiration rate of cut stems of the same species using a potometer, a device that records how much water the stem draws up over time. Each stem had 6 leaves of similar size. Unless noted, conditions were: 22°C, 50% relative humidity, still air, and moderate light. Rate is given in milliliters of water per hour (mL/h).
Experiment 1
Stems were placed in front of a fan at different speeds.
Table 1
Fan setting | Air speed (m/s) | Transpiration rate (mL/h)
Off | 0 | 0.42
Low | 1.0 | 0.71
Medium | 2.5 | 0.98
High | 4.0 | 1.05
Experiment 2
Stems were placed in sealed chambers at different relative humidities, in still air.
Table 2
Relative humidity (%) | Transpiration rate (mL/h)
20 | 0.88
40 | 0.55
60 | 0.31
80 | 0.12
Experiment 3
Half of a stem's leaves were removed, or the lower surfaces of all leaves were coated with petroleum jelly (which blocks stomata), in still air at 50% humidity.
Table 3
Treatment | Transpiration rate (mL/h)
No treatment (6 leaves) | 0.42
3 leaves removed | 0.22
Lower leaf surfaces coated | 0.06
Upper leaf surfaces coated | 0.39
A student claims that the fan in Experiment 1 increased transpiration mainly by warming the leaves. Which of the following additions to Experiment 1 would best test this claim?
Question 2
Science
Students investigated how the pressure of a fixed amount of air in a sealed metal cylinder changes with temperature and with volume. A piston allowed the volume to be set, and a pressure sensor and thermometer recorded conditions. Pressure is given in kilopascals (kPa) and temperature in kelvins (K). (0°C = 273 K.)
Experiment 1
The volume was held at 1.00 L while the cylinder was warmed in a water bath.
Table 1
Temperature (K) | Pressure (kPa)
273 | 101
293 | 108
313 | 116
333 | 123
353 | 131
Experiment 2
The temperature was held at 293 K while the piston changed the volume.
Table 2
Volume (L) | Pressure (kPa)
0.50 | 216
0.75 | 144
1.00 | 108
1.50 | 72
2.00 | 54
Experiment 3
Experiment 1 was repeated with the volume held at 0.50 L.
Table 3
Temperature (K) | Pressure (kPa)
273 | 202
293 | 216
313 | 232
333 | 246
353 | 262
Experiments 1 and 3 were identical except for one factor. Which factor was different, and what was the effect of the difference?
Question 3
Science
Amylase breaks starch into shorter sugars. Its activity can be followed with iodine solution, which turns blue-black in the presence of starch and stays amber when the starch has been broken down. The time taken for a sample to stop turning blue-black is the clearing time; a shorter clearing time means faster enzyme activity.
Students used amylase purified from a soil bacterium. Unless noted, every trial used 2.0 mL of enzyme solution, 5.0 mL of 1% starch solution, a buffer at pH 7.0, and a water bath at 35 °C. Each value below is the mean of 5 trials.
Study 1
The pH of the buffer was varied. All other conditions were held at the values above.
Table 1
pH | Clearing time (s)
4.0 | no clearing after 600
5.0 | 310
6.0 | 122
7.0 | 88
8.0 | 145
9.0 | no clearing after 600
Study 2
The water-bath temperature was varied. The buffer was held at pH 7.0.
Study 3
Enzyme solution volume was varied from 0.0 mL to 4.0 mL and made up to a constant total of 4.0 mL with buffer, so that total volume was the same in every trial. Temperature was 35 °C and pH was 7.0.
Table 3
Enzyme volume (mL) | Clearing time (s)
0.0 | no clearing after 600
0.5 | 352
1.0 | 176
2.0 | 88
4.0 | 44
Study 4
Samples of enzyme solution were held at 80 °C for 10 minutes, cooled back to 35 °C, and then used in a standard trial at pH 7.0.
Table 4
Treatment | Clearing time (s)
Untreated | 88
Held at 80 °C, then cooled | no clearing after 600
| Temperature (°C) | Clearing time (s) |
|---|---|
| 10 | 405 |
| 20 | 210 |
| 35 | 88 |
| 50 | 96 |
| 65 | no clearing after 600 |
| 80 | no clearing after 600 |
A student claims that Study 2 shows enzyme activity is fastest at exactly 35 °C. Which change to Study 2 would best test that claim?
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