Equations are used to represent the chemical reactions that take place. For example, if we are told that magnesium reacts with oxygen to form magnesium oxide, we can write the word equation as:
Magnesium + Oxygen → Magnesium oxide
And the symbol equation as:
Mg + O2 → 2MgO
The molecules on the left-hand side of the arrow are the reactants and the molecules on the right-hand side are the products.
Chemical reactions are constantly taking place around us, leading to chemical changes.
For example, let’s take a look at the combustion of methane. Methane reacts with oxygen (in the air) to form carbon dioxide and water vapour.
This equation can also be shown as the word equation:
Methane + Oxygen → Carbon dioxide + Water
Although the reaction results in a chemical change and forms new compounds, the number of each type of atom remains the same on both sides.
This is true for all chemical reactions. The number of atoms in the reactants is always the same as the number of atoms in the products. This means that the total mass of the reactants will always be equal to the total mass of the products.
Therefore, if the combined mass of the reactants (methane and oxygen) in the equation above is 100 g, the combined mass of the products (carbon dioxide and water) will also be 100 g.