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During a chemical reaction, matter is not created or destroyed. The basic understanding that all of the chemicals interacting in a chemical reaction are still present at the end is called the conservation of mass.
The principle of conservation of mass states that the total mass of the reactants at the beginning of a chemical interaction are equal to the total mass of the products.
It doesn't always seem like the matter in a chemical reaction is being conserved. When wood is burned in a fire, much of the matter in the fire is able to escape into the environment. If a dome was placed over a wood fire and it contained all of the gases and products of the fire, the mass of the wood before the fire and the products of the fire would have the same mass.