The law of conservation of mass states that in a closed system, matter cannot be created or destroyed during a chemical reaction. This principle means that the total mass of the reactants—that is, the starting materials—will always equal the total mass of the products produced by the reaction.
For example, when sodium reacts with chlorine gas to form sodium chloride (table salt), the mass of the sodium and chlorine combined before the reaction will precisely match the mass of the sodium chloride after the reaction. If you start with 23 grams of sodium and 35.5 grams of chlorine, the resulting sodium chloride will weigh 58.5 grams.
This concept helps chemists predict the amounts of substances involved in reactions and is foundational for balancing chemical equations. It highlights the importance of systematic observation and measurement in chemistry, reinforcing that every atom in reactants must be accounted for among the products. Understanding this law is crucial for anyone studying chemistry or involved in chemical processes.