Lesson 5.1 - Conservation Of Mass

Law of conservation of mass

There is a fundamental law in chemistry. It states that:

No atoms are lost or made in a chemical reaction, and therefore the mass of the products and reactants are equal.

Let's break that down. This means that:

• There are the same atoms on each side of the reaction

• The mass of the atoms on the left equals the mass of the atoms on the right

• The number of atoms never change - they are just transferred into a different compound

• Atoms cannot change elements

• Atoms cannot be created or destroyed

• The mass of the atoms cannot be changed

Balancing reactions

In order to 'comply' with the law of conservation of mass, we need to balance both sides of the reaction.

This means keeping the same number of each element on each side of the reaction.

Chemical reactions are balanced by inserting multipliers (the big numbers) before a compound.

Balanced reactions - examples

The following reactions are balanced:

C + O2 ---> CO2 2Mg + O2 ---> 2MgO

Note that only the 'big' numbers (before a compound) are used when balancing reactions. We cannot change the 'small' (subscript) numbers, as this would change the formula of the compound.