Why do i need to control the volume variable in my science investigation? See details.?

In my science investigation, I was to investigate how the concentration of hydrochloric acid affects the rate of reaction with calcium carbonate. Why do I need to control the amount of volume of the solution? Help asap, explain in detail why? Thankyou.|||because the amount of volume could change how the chemicals react. each test needs to have the same volume so that the volume does not affect the results and your only testing the variable you want to test.