A reaction between acid and marble chips release carbon dioxide gas. In chemistry terms,
`2HCl + CaCO_3 -> CaCl_2 + CO_2 + H_2O`
The reaction progress is observed by the amount of CO2 gas released. A change in concentration of acid will mean more or less particles and higher or lower reaction rate. In other words, an increase in acid concentration will increase the rate of generation of CO2.
The effect of change in concentration can be measured by determining the rate at which the CO2 is released or the amount of CO2 released in a certain amount of time. The amount of CO2 generated can be measured by using an upturned cylinder full of water or gas syringe method or simply by placing the setup on an electronic scale. This way, we can measure the quantity of CO2 released and relate it to the change in concentration of acid.