Physical & chemical change
1.7 · Chemical change
Physical & chemical change
By the end of this section:
- I can tell a physical change from a chemical change
- I can give examples of physical and chemical changes
- I can write a word equation for a reaction, showing reactants turning into products (extension: a balanced formula equation)
1 · Read
The notes, the key idea and the words to knowIn 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.
Everyday examples. Physical changes: melting ice, boiling water, dissolving sugar in water (you can get the sugar back by evaporating the water), snapping a stick. Chemical changes: burning (for example a firework), rusting an iron nail, cooking (frying an egg).
We write reactions as reactants → products and add state symbols to show each substance’s physical state:
- (s) = solid
- (l) = liquid
- (g) = gas
- (aq) = dissolved in water
A word equation names the reactants and products. Magnesium burning in oxygen:
magnesium + oxygen → magnesium oxide
The same reaction as a balanced formula equation, with state symbols: 2 Mg(s) + O2(g) → 2 MgO(s)
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.
- Count the atoms on each side. Every big number is 1 for now. Count each kind of atom in the reactants on the left and in the products on the right.
- Balance it with the big numbers. Make the two bars for each element equal. Change only the big numbers in front. The small numbers are locked.
- H₂ + O₂ → H₂O: Balanced: 2 H₂(g) + O₂(g) → 2 H₂O(g)
- Mg + O₂ → MgO: Balanced: 2 Mg(s) + O₂(g) → 2 MgO(s)
- Na + Cl₂ → NaCl: Balanced: 2 Na + Cl₂ → 2 NaCl
- Li + H₂O → LiOH + H₂: Balanced: 2 Li + 2 H₂O → 2 LiOH + H₂
- Al + S → Al₂S₃: Balanced: 2 Al + 3 S → Al₂S₃
- Cs + N₂ → Cs₃N: Balanced: 6 Cs + N₂ → 2 Cs₃N
- NH₃ + O₂ → N₂ + H₂O: Balanced: 4 NH₃ + 3 O₂ → 2 N₂ + 6 H₂O
- H₃PO₄ + KOH → K₃PO₄ + H₂O: Balanced: H₃PO₄ + 3 KOH → K₃PO₄ + 3 H₂O
- Al(OH)₃ + HBr → AlBr₃ + H₂O: Balanced: Al(OH)₃ + 3 HBr → AlBr₃ + 3 H₂O
- H₃PO₄ + Mg(OH)₂ → Mg₃(PO₄)₂ + H₂O: Balanced: 2 H₃PO₄ + 3 Mg(OH)₂ → Mg₃(PO₄)₂ + 6 H₂O
- CH₄ + O₂ → CO₂ + H₂O: Balanced: CH₄ + 2 O₂ → CO₂ + 2 H₂O
- C₂H₆ + O₂ → CO₂ + H₂O: Balanced: 2 C₂H₆ + 7 O₂ → 4 CO₂ + 6 H₂O
- C₃H₈ + O₂ → CO₂ + H₂O: Balanced: C₃H₈ + 5 O₂ → 3 CO₂ + 4 H₂O
- Same atoms before and after. Same atoms before and after, so the mass is the same.
Why mass is conserved — and why we balanceTap to open
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).
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.
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Not thisMass is lost when a gas is given off.
ThisNo — the gas still has mass; it just leaves an open container. In a sealed container the mass stays the same.
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Not thisYou balance an equation by changing the little subscripts.
ThisNever — that changes the substance (H2O vs H2O2). Only change the coefficients in front.
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Not thisDissolving sugar is a chemical change.
ThisIt is physical — the sugar is still sugar and can be recovered by evaporating the water. No new substance forms.
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Not thisA colour change always means a chemical reaction.
ThisIt is a strong clue, but check for a new substance. Always weigh the evidence (gas, precipitate, temperature change) together.
- physical change
- particles rearrange but stay the same substance; usually reversible (e.g. melting)
- chemical change
- particles rearrange into one or more new substances; usually hard to reverse
- reactant / product
- the substances that react / the new substances formed
- state symbol
- (s), (l), (g) or (aq), showing a substance’s physical state in an equation
- law of conservation of mass
- total mass of products = total mass of reactants (Lavoisier; explained by Dalton)
- coefficient
- the number in front of a formula, adjusted to balance an equation
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- Booklet pp. 29–34
- Knowledge Book pp. 27–30
- In your class workbook: Chapter 1.5 Chemical change 1: Recognizing & describing reactions (pp. 40–44) · Revise this section
- In your class workbook: Chapter 1.6 Chemical change 2: Conservation of mass (pp. 45–48) · Revise this section
- In your class workbook: Chapter 1.7 Chemical change 3: Reaction cycles (pp. 49–52) · Revise this section
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