1Atoms are rearranged
In a chemical reaction, bonds break and atoms join up in new ways. Count them: the same atoms are there before and after.
chemical reaction
Lesson 5.4 notes
Reactions and conservation of mass
In a reaction, atoms are rearranged, never made or destroyed. So the total mass stays the same.
2Word equations
Reactants (what you start with) go on the left of the arrow. Products (the new substances) go on the right.
reactantproduct
3Mass is conserved
In a sealed container nothing gets in or out, so the total mass stays the same.
conservation of mass
4When a gas escapes
In an open flask, the gas made in the reaction escapes, so the reading goes down. The atoms still exist, in the air.
gas
5When a gas joins in
Burning magnesium gets heavier: oxygen atoms from the air join on to make magnesium oxide.
magnesium oxide
6Changes of state
Melting, boiling and dissolving don't change the particles, so the mass stays the same too.
change of state
7Signs of a reaction
Fizzing, a colour change, a temperature change or a new solid are signs that a new substance has formed.
new substance
Explore: Reaction player
Choose a reaction, play it, and count the atoms before and after.
Common mistake: Burning doesn't destroy atoms
When wood or a candle burns, it seems to disappear. Its atoms join with oxygen and float away as carbon dioxide and water vapour.
Remember
- Reactants → products. Atoms are rearranged, never made or destroyed.
- Mass is conserved in reactions and in changes of state.
- If the mass seems to change, a gas has escaped or joined in.
Going further at GCSE
GCSE: Conservation of mass → GCSE: Mass changes involving gases →