Measurement of Rate of Reaction between an acid with Marble Chips

How to measure rate of reaction between an acid with marble chips?

First, we need to figure out the observable changes in quantity of reactant/product.

The observation mostly given by my class students are as follows :

1. marble chips dissapears.
2. gas bubbles produced.

Which quantity they would like to measure? Ha ha.. many students were stucked and were unable to given any response. Some might gave nonsence. The hand glove was purposely fixed on the mouth of a beaker filled with acid and marble chips. Soon, they realised it wasn't an accurate way of measurement the quantity of gas.   Slowly the students were guided to use the water displacement method to measure the changes in volume of gas released per unit time.