Conservation of mass. Reactants and products.
When elements are joined to cause a chemical reaction, no atoms are made or lost during the process. But at the end of it they are joined together differently from the way they were at the beginning.
This means that the mass of the substances at the start is the same as the mass of the substances at the end.
The substances at the start of a reaction are called the reactants. The substances at the end are called the reaction products.
Getting a balanced equation
Let's use the reaction of copper with oxygen to make copper oxide as an example. The word equation for this reaction is:
copper + oxygen copper oxide
Here, copper and oxygen are the reactants, and copper oxide is the product.
When we directly replace the word equation shown above by the chemical symbol equation, we may get an unbalanced equation:
Cu + O2 CuO
Here, the number of oxygen (O) atoms on the left-hand side (two) is different to that on the right-hand side (one).
To make things equal, we need to adjust the number of units of some of the substances until we get equal numbers of each type of atom on both sides of the arrow.
Here is the balanced and correct equation:
2Cu + O2 2CuO
We now have two copper atoms and two oxygen atoms on each side. This matches what actually happens in the reaction.
We use balanced equations to show exactly what happens to the different atoms in chemical reactions.