Key Concepts: Conservation of Mass

The concept of conservation of mass originates from the principle that matter can neither be created nor destroyed. It can, however, be transformed from one form into another.
In a closed system, irrespective of the physical changes or chemical reactions that may happen, the total mass of the reactants (substances before reaction) always equals the total mass of the products (substances after reaction).
This concept can be used to explain why, in a chemical equation, the number of atoms for each element is identical on both the reactant and product side. This balancing act is critical for accurately depicting chemical reactions.
Even though a substance’s appearance, properties, or state may transform, the underlying atoms making up the substance do not alter in terms of number and type.
Energy, however, is not subject to this same rule. Energy can travel into or out of a system, and hence a system’s overall energy might not remain constant.
An exception to the principle of conservation of mass occurs in nuclear reactions where small amounts of mass can be converted into energy, in accordance with Einstein’s E=mc^2 equation.
During a reaction in a non-closed system, it might seem like the mass is not conserved. This is because substances might have entered or left the system, hence the system’s mass appears to change. However, if these inputs and outputs are taken into account, the law of conservation of mass still applies.

Remember to apply this principle when predicting the amount of product or reactant in any given chemical reaction.