Chemical Reactions and Conservation of Mass

Chemical reactions

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 + OxygenMagnesium 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.

Conservation of Mass

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.

Diagram representing atoms as balls to show how the equation of one methane molecule + two oxygen molecules produces two water molecules + one carbon dioxide molecule.

This equation can also be shown as the word equation:

Methane + OxygenCarbon dioxide + Water

  • Methane and oxygen are on the left – the reactants
  • Carbon dioxide and water are on the right – the products

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.

  • 1 carbon atom 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 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.

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