Conservation of Mass

Science: KS3, topic 2.3.7

The Conservation of Mass

In a chemical reaction or a change of state, the total mass does not change. This is always the case. We call it 'The Conservation of Mass'.

A change of state is when a substance is heated or cooled, which causes the particles to move faster or slower and take a different arrangement in space.
When a substance changes state the number of particles stays the same.
If you count the particles at the end and at the beginning, then the number is exactly the same.
This is because nothing has happened to them apart from moving in a different way.

If you have some blocks on a balance they will have a certain mass. If you re-stack them in a different way, then will the mass change?
No, the order doesn’t matter. The left-hand side is 5kg, and so is the right-hand side.
The same can be said for rearranging particles in a change of state.

Look at the diagram for ice melting, each circle represents a water particle.
The number of particles are the same at the start and at the end.
Therefore the mass will not change.
If each particle here has a mass of 1kg on a balance, then the ice would have a mass of 15kg and the liquid water would have a mass of 15kg. The mass is conserved.
This is true for all changes of state: the mass is conserved for them all as particles are not created or destroyed.

In a chemical reaction, the atoms rearrange and join together in different ways.
As the same number of each type of atom is there at the end as there was at the start, then the mass doesn’t change.
You can think of this as rearranging blocks on a scale again.
If we think about the total number of each atom, we can see it stays the same.
So the mass will also stay the same – it is conserved.
This is true for any chemical reaction because atoms cannot be created or destroyed.

What is the law of conservation of mass?
If the total mass of reactants in a chemical reaction is 20g, what can we say about the total mass of products?