Chemical Reactions and Conservation of Mass

Chemical reactions

Word equations are used to show us when a chemical reaction takes place.

For example, if we are told that magnesium reacts with oxygen to form magnesium oxide:

Magnesium + Oxygen ⮕ Magnesium oxide

Mg + O₂ ⮕ 2MgO

The molecules on the left-hand side of the arrow are called reactants and the molecules on the right-hand side are called products.

The molecules on the left-hand side of the arrow are called reactants and the molecules on the right-hand side are called products.

Conservation of Mass

There are chemical reactions taking place around us at all times, many of which result in chemical change.

As you can see below, methane molecules react with oxygen molecules (in the air) to form carbon dioxide and water.

This equation can also be shown as the word equation:

Methane + Oxygen → Carbon dioxide + Water

  • On the left are methane and oxygen – the reactants
  • On the right are carbon dioxide and water – the products

Although the reaction resulted in a chemical change, forming new compounds, there are the same number of each atom on both sides.

  • 1 carbon atoms on the left and 1 carbon atom on the right
  • 4 oxygen atoms on the left and 4 oxygen atoms on the right
  • 4 hydrogen atoms on the left and 4 hydrogen atoms on the right

This is the same in all chemical reactions. The number of atoms in the reactants is always the same as the number of atoms in the products, which means the total mass of the reactants is equal to the total mass of the product or products.

  • This means, if the combined mass of the reactants (methane and oxygen) in the equation above equals 100g, then the combined mass of the products (carbon dioxide and water) must also equal 100g.