The burning of wood is a chemical reaction between cellulose molecules in the wood and oxygen the products are carbon dioxide and water what would you have to do in order to prove that this reaction obeys the law of conservation of mass?

Put the wood and excess oxygen in a closed container that contains some device that can create a spark (to start the reaction).

Determine the mass of container + contents.

Start the reaction and, after everything has cooled down, again determine the mass of container + contents.

If the two masses are the same, you have demonstrated that the reaction obeys the Law of Conservation of Mass.

they are all solids.