A student performs an experiment that produces solid sodium chloride. He uses four different catalysts and lets the reaction run for two minutes, and then removes the sodium chloride product. Which is the best way for him to make a consistent measurement that would show the different amounts of sodium chloride produced? A. Measure the product separately on four different scales. B. Measure all of the product together and divide by four. C. Measure the four products separately, but using the same scale each time. D. Measure the amount of sodium chloride in one sample every 10 seconds.

Answer: option C. Measure the four products separately, but using the same scale each time.

Justification:

Review the choices given one by one.

A. Measure the product separately on four different scales.

He should not include other variables in the experiment. Using four different scales migh give different results due to differences on the scales.

B. Measure all of the product together and divide by four.

By doing this he will just obtain an average of the amount produced, but in this way he cannot determine the production individually (with each catalyst).

C. Measure the four products separately, but using the same scale each time.

Indeed doing this he will be able to compare and rank the efficiency of each catalyst.

D. Measure the amount of sodium chloride in one sample every 10 seconds.

He cannot do this without affecting at the same time the evolution of the reaction, and of course by doing it on one sample only he will not be able to compare.