No atoms are created or destroyed in a chemical reaction. Instead, they just join together in a different way than they were before the reaction, and form products. This means that the total mass of the products in a chemical reaction will be the same as the total mass of the reactants.
The mass of one substance in a reaction can be calculated if the masses of the other substances are known. For example:
No substances can enter or leave a closed system. A simple closed system could be a sealed container such as a stoppered flask. Sometimes reactions that happen in open beakers are closed systems. These include:
In both examples, the reactants and products stay in the beaker. The total mass of the beaker and the substances it contains stays the same during the reaction.
Substances can enter or leave a non-enclosed system. These systems are often open flasks or crucibles that let gases enter or leave. For example:
If a gas escapes, the total mass will look as if it has decreased. If a gas is gained, the total mass will look as if it has increased. However, the total mass stays the same if the mass of the gas is included.