Measurements on ACT Science Test

Measurements questions on the ACT Science Test involve the numbers and corresponding labels that appear in the graphs and tables of the Science passages. Questions that involve measurements sometimes allow you to approximate if the answer choices are far apart. Measurements questions are almost like Math questions with a Science theme.

Concepts like mean, median, mode, and range can appear in ACT Science questions on occasion. The median is the middle number when the data is ordered numerically from least to greatest. The range is the greatest value less the smallest value, or the span of the data.

Example Passage

Pepsin is an enzyme in the human stomach whose function is to break down proteins in food into smaller pieces. Table 1 shows how the activity of pepsin (the rate at which it breaks down another compound referred to as the substrate) is affected by pH. (In each case, 100 units of pepsin were added to 1 ml of solution containing 1 mg/ml of one of two proteins and adjusted to a particular pH. After 1 minute, the mg of protein that had been broken down after 1 minute was measured.) It can be seen that this enzyme functions best in acidic (low pH) environments, and this fact may explain why the stomach also secretes acid.

Question #1

When 100 units of pepsin are added to a solution containing 1 mg/ml of Protein 1, the enzyme’s activity is measured at 1.05. Based on the results in Table 1, the pH of the solution is likely to be closest to:

  1. 1.5
  2. 2.0
  3. 2.5
  4. 3.0

Here, two columns of measurement are pH and Activity. You can see that the protein and the pepsin concentration described in the question are the same as in solutions 1 through 4 in Table 1, so you only need to look at those. New Activity is 1.05, which is more than .9, the largest value for activity for Protein 1.

You can see that for Protein 1’s distribution, the activity increases by 0.3 with each decrease of 1 pH unit. An activity of 1.05 is 0.15 higher than the activity of 0.9 at pH 2, so you need the activity to increase by half as much as it increases when the pH decreases by 1 unit. You need the pH to change by half a pH unit, from 3 to 2.5. The answer is (C).