A student decided to investigate the effect of pH on the activity of the enzyme lipase , five different test tubes are set up ,each test tube contains a mixture of the enzyme lipase and its substrate ,a lipid which condition should not be kept the same in each of the five test tubes

the pH

Explanation:

The condition that should not be kept the same in each of the five test tubes is the pH.

Since the aim of the experiment is to investigate the effect of pH on the activity of the lipase enzyme, the pH is, therefore, the independent variable. Thus, it must be controlled or varied in all the test tubes in order to see its effect on the activity of the enzyme, which, in this case, would be the dependent variable.