If you burn 10 kilograms of wood in a fire (combustion) what is the weight of the products after the fire has finished burning the wood?

The answer is: the weight of products is is equal the weight of the wood plus the weight of oxygen that was used to burn that wood, so weigh of the product is greater than 10 kilograms.

Conservation of mass (mass is never lost or gained in chemical reactions), during chemical reaction no particles are created or destroyed, the atoms are rearranged from the reactants to the products.

In this example wood (mostly carbon) and oxygen are reactants and carbon dioxide (mostly) is product of reaction.