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Measurements on the 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.

It helps to recall the mean, median, mode, and range from ACT Math, since they can reappear in ACT Science questions on occasion. The median is the middle number when the data is ordered numerically from least to greatest. Get a sense of where the “middle” is. The range is the greatest value less the smallest value, or the span of the data. Which set has the widest range? Which has the smallest? The general distribution means how far apart each set of data is from each other. Let’s look an at example measurements question!


Passage I

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 our two columns of measurement are pH and Activity. We 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 we only need to look at those. Our new Activity is 1.05, which is more than .9, our largest value for activity for Protein 1.

We 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 we need the activity to increase by half as much as it increases when the pH decreases by 1 unit. We need the pH to change by half a pH unit, from 3 to 2.5. The answer is (C).


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