For some ACT Science Test questions, you need to answer questions on how certain experiments are set up. These questions can appear in any of the three Science passages: Data Representation, Research Summaries, and Conflicting Viewpoints, but are much more common in Research Summaries. It is important to extract the set-up from any experiment mentioned before moving on to the questions.
Soap molecules often consist of long chains of hydrocarbons ending in a negatively charged ion. In water, the soap molecules form clusters called micelles. Soaps are able to remove dirt particles from surfaces by trapping them in the centers of the micelles.
A science fair student designs an experiment to measure the cleaning power of sodium oleate, a chemical that can be used as a soap. In the experiment, eight white T-shirts were stained with either dirt or ketchup. After the shirts were cleaned under varying conditions, the student rated how visible the stains were.
The student wishes to demonstrate that the cleaning action is due to the oleate, and not to the sodium. Accordingly, the student measures the cleaning power of potassium oleate, using 4 scoops of the soap to clean a T-shirt stained with ketchup and a T-shirt stained with soil, to show that the same results are obtained under these circumstances.
Notice how both experiments dirtied shirts, then cleaned them. However, the number of shirts stained was different in each, and Experiment 2’s shirts were only cleaned to the extent that they matched the level of dirtiness left on the shirts from Experiment 1.
Experiment 2 differed from Experiment 1 in that:
Another way to think of this question is: “what changed between the experiments?” Looking at the notes, you can see that the cleaning solution changed (sodium oleate to potassium oleate), the number of shirts changed (from 8 to 4), and what was specifically being measured changed (in Exp. 1 it was the visibility of the stains, in Exp. 2 it was how many scoops to achieve the results of Exp. 2).
You can quickly eliminate (B) and (D), since the stains were different (ketchup and dirt) within each experiment. Since Experiment 2 measured how much soap was needed to achieve the results of Experiment 1, then it is likely different amounts were needed depending on the severity of the stain. In Experiment 1, it is implied that all of the shirts were uniformly washed with the same amount of soap, but the conditions of the washing were modified. Therefore the answer is (A).