1.5 Chemical change 1: Recognizing & describing reactions

Two kinds of change

In a physical change, the arrangement and movement of the particles change, but the particles themselves stay the same. Changes of state — melting, freezing, boiling — are the classic examples: no new substance is made and the change is usually reversible.

In a chemical change, the particles rearrange and one or more new substances form. It is usually hard to reverse. Tell-tale signs: a colour change, a gas given off, a temperature change, or a solid (precipitate) forming.

The law of conservation of mass: the total mass of the products always equals the total mass of the reactants. Around 1789 the French chemist Antoine Lavoisier established it by careful weighing; decades later John Dalton explained why — atoms are only rearranged, never created or destroyed.

Why mass is conserved — and why we balance

A chemical reaction does not make or destroy atoms. It takes the atoms in the reactants, separates them, and recombines them into the products. So the number and type of each atom is exactly the same before and after — and since each atom keeps its mass, the total mass cannot change.

That is the whole reason we balance equations. A balanced equation is simply a statement that the atoms tally on both sides. We make them tally by changing coefficients (the big numbers in front) — never subscripts, because changing a subscript would change the substance itself (1.2).

Balance H₂ + O₂ → H₂O.

2 H₂(g) + O₂(g) → 2 H₂O(g)

Tally: 4 H and 2 O on each side. Same atoms before and after — mass is conserved.

Why does a flask of fizzing acid seem to lose mass — but a sealed one does not? When hydrochloric acid reacts with sodium hydrogen carbonate, carbon dioxide gas escapes from an open flask, so the balance reads lighter. Seal the flask (e.g. with a balloon) and the gas is trapped: the mass stays the same, exactly as Lavoisier's law predicts. The mass was never lost — it just floated away.

A rusting iron nail gets heavier, not lighter — because iron atoms combine with oxygen from the air, and that oxygen's mass is now part of the rust. It looks like mass appears from nowhere until you remember the air. This is the law of conservation of mass in everyday life: mass is never lost or gained, only rearranged. The same accounting — atoms in = atoms out — scales all the way up to industrial chemistry and even (with energy added in) to the reactions inside stars.