This is an example of a chemical change because a new substance, carbon dioxide, is being formed. Marble is calcium carbonate, with the formula CaCO3. The equation for the chemical reaction is:
`CaCO_3 + 2 HCl -> CaCl_2 + H_2O + CO_2`
The three products are different substances from the two reactants because they have different chemical formulas. The atoms that make up the molecules have been rearranged into new molecules.
In general, signs that a chemical reaction has taken place include:
1. Production of gas, as you would see here - bubbles are formed by HCl on marble.
2. A color change
3. A solid precipitate forming (clear solution becomes cloudy)
4. Heat and light produced, as in combustion
Sometimes bubbles are produced when a physical reaction takes place, as when water boils, but the evolution of gas without the addition of heat usually indicates a new substance.