State the law of conservation of mass. Give one example to illustrate this law.

The Law of conservation of mass given by Lavoiser states that "Mass can neither be created nor be destroyed in a chemical reaction". So, in a chemical reaction, the total mass of reactants must be equal to the total mass of products.

Example: When calcium carbonate is heated, a chemical reaction takes place to form calcium oxide and carbon dioxide.

$CaCO_3 \xrightarrow {heat} CaO + CO_2$

If 100 gms of calcium carbonate is decomposed completely, then 56 gms of calcium oxide and 44 gms of carbon dioxide are formed.

Therefore, the total mass of products = 56 gms (CaO) + 44gms (CO₂) = 100 gms.

LHS = RHS

As the total mass of products is equal to the total mass of reactant so, the law of conservation of mass is satisfied.