Like limestone, marble is composed of calcium carbonate -- and just like limestone, it reacts with hydrochloric acid to yield carbon dioxide gas and calcium chloride in solution. Although the progress of the reaction is more difficult to track than some other simpler experiments, the reaction between hydrochloric acid and marble can make the basis for an interesting science project or experiment in a lab class.
The rate of a reaction is determined by the rate-limiting step -- in other words, by the slowest step of a reaction. Each reaction has its own rate law, an equation that gives the rate of that reaction as a function of the concentration of the reactants. To take an example, consider the following imaginary reaction:
A + B --> C
A possible rate law for this reaction might be Rate = k x [concentration of A] x [concentration of B squared]. The k in this equation is a constant specific to this reaction; it varies with temperature and other factors. Only reactants that take part in the rate-limiting step appear in the rate law, so a rate law won't necessarily include all the reactants from the chemical equation. If the concentration of a reactant doesn't appear in the rate law, chemists say the rate law is 0-order in that reactant. If it does appear and the exponent on it is one, chemists say the rate law is first-order in that reactant. If the concentration of that reactant is squared, it's called second-order and so on.
Depending on your objective, there are several kinds of experiments you could design involving these two reactants. You might try to determine whether the reaction is first- or second-order in hydrochloric acid. You could also try to determine how it varies with temperature. Either way, you'll want to be able to measure the rate of the reaction, either by stopping the reaction at a given point or by measuring the amount of one reactant used up at various points throughout the reaction.
Any time you're designing an experiment, you want (if possible) to vary only ONE experimental variable between different treatments. If you're trying to determine how the concentration of acid affects the reaction rate, for example, you'll want to run several trials, each with a different concentration of acid but with all other variables (total volume, temperature, mass of the marble, etc.) as close to unchanged as possible. On the other hand, if you're measuring how temperature affects the reaction rate, you'll want to run several trials with temperature as the only factor that changes between them.
An important variable to monitor is the surface area of the marble. Surface area of a solid can have a significant effect on the rate of a reaction, because it helps determine how rapidly collisions between the molecules of the two reactants can occur. If you're using marble chips, it's important to make sure the surface area is more or less the same between your different trials -- and that the mass of the marble chips remains unchanged as well.
One way to track the rate of the reaction would be to stop it at a given point in time by pouring the reactants through a crucible or sieve, then washing the marble chips -- this will bring the reaction to a halt. You can then measure the amount of marble consumed by weighing it once it dries (unless you weighed it wet originally), or you can determine how much of the acid is left by titrating the remaining hydrochloric acid with a base such as sodium hydroxide. Another way to track reaction progress is through the amount of carbon dioxide produced; you can trap the carbon dioxide with a balloon and measure its volume. This approach may be more difficult, however, unless you have a way to measure the volume of the balloon precisely.
When designing your experiment, it's important to remember that hydrochloric acid and sodium hydroxide are strong chemicals. If you spill them or splash them on your skin or eyes, they can cause serious burns or even blindness. Make sure you wear goggles and gloves when performing this kind of experiment, and preferably a lab coat or something to protect your clothes as well. Don't allow these chemicals to come in contact with metals or other materials that can corrode, because hydrochloric acid can eat away at metal surfaces.