Question 5
What has to be done in activity 17.1 to be sure the result is caused by the surface area?
The right answer is the one marked above.
Both beakers must hold equal volumes of HCl acid, and the CaCO3 masses must be equal too. Then the only thing that differs is whether the CaCO3 is in lumps or as powder.
If the temperature or the amount of acid also differed, we could not say what caused the difference in time. Only one thing may be varied at a time.