Chemical Reactions & Equations (Years 7–11) — Science

Strike a match, bake a cake, or watch iron rust — in each case a chemical reaction is turning one set of substances into completely new ones. The starting materials, the reactants, become the finished materials, the products. Yet nothing is ever truly gained or lost.

Why this matters: chemical reactions build every material around you and power your own body. Once you grasp that atoms only rearrange — never vanish — you can predict masses, balance equations and recognise a reaction when you see one.

Same atoms, rearranged — so total mass stays the same

Reactants and products

Reactants sit before the arrow; the product comes after.

A reaction is written with an arrow, and the arrow points from what you start with to what you end up with:

Reactants — the starting substances, written before the arrow.
Products — the new substances made, written after the arrow.

For example, in magnesium + oxygen → magnesium oxide, the reactants are magnesium and oxygen, and the single product is magnesium oxide.

Conservation of mass

Here is the golden rule. In a reaction the atoms are only rearranged — like the same set of Lego bricks taken apart and rebuilt into a new model. No bricks are added, none thrown away. So the total mass of the products always equals the total mass of the reactants.

Conservation of mass: total mass of products = total mass of reactants. Nothing is created or destroyed.

The scale balances: reactant mass in = product mass out.

Signs a reaction has happened

Four tell-tale signs: a colour change, gas bubbles, a temperature change or a precipitate.

How do you know a genuine chemical change has occurred, rather than something just melting or dissolving? Look for these tell-tale signs:

Sign	Example
Colour change	Iron turning orange as it rusts
Bubbles of gas	Fizzing when acid meets a metal
Temperature change	Getting hot or cold to the touch
A precipitate forms	A solid appearing in a clear liquid

Worked reasoning. 24 g of magnesium reacts with 16 g of oxygen — what mass of magnesium oxide forms? Because mass is conserved, the products must weigh exactly what the reactants weighed. Add the reactant masses: 24 + 16 = 40 g. You never need to know the chemistry in detail — the conservation rule alone gives the answer.

Common mix-up: thinking mass "disappears" when a gas is given off — for instance a burning candle seeming to lose mass. It hasn't: the gases (carbon dioxide and water vapour) simply float away. Weigh them too and the total is unchanged. Mass is always conserved.

Memory hook: "Reactants Ready, Products Produced" — reactants before the arrow, products after. And remember the Lego rule: same atoms, rearranged, so same total mass.