In chemical reactions, substances change to make new materials, similar to how a relay race involves changes in runners.

Just as a baton is passed from one runner to another without adding or losing any batons, in chemistry, the total number of atoms stays the same even though they change.

This key rule is known as the conservation of mass.

It means that atoms, like the batons in a relay race, are not made or destroyed; they simply move from one substance to another, joining in new ways.

Consequently, the mass of the starting substances, called reactants, is the same as the mass of the new substances formed, known as products.

This important rule, mirroring the constant number of batons in a relay race, helps us understand how chemical reactions work and is a basic part of learning about chemical equations and stoichiometry.