How should the initial volume of water be kept in an evaporation experiment?

Correct answer: The same

In an evaporation experiment, the initial amount of water is a variable that can affect the result. A larger volume generally provides more water to evaporate and may take a different amount of time than a smaller volume. If the starting volume changes from trial to trial, a difference in evaporation could be caused by volume rather than by the factor being tested, such as temperature. Therefore the initial volume should be kept the same in every trial, making option C correct.

The volume should also be measured accurately and recorded, while other relevant conditions, such as the container and observation time, are controlled as far as possible. Using different or doubled amounts would make the comparison unfair. Leaving the volume unrecorded prevents the researcher from checking whether the trials were comparable. Keeping it constant does not mean the volume is unimportant; it means its influence is controlled so the effect of the selected test factor can be judged fairly.