Physical and Chemical Change, Revisited
Matter undergoes two kinds of change. A physical change forms no new substance — freezing, evaporation and sublimation are all physical changes; when liquid water freezes to ice, it’s still the same substance in a different state. A chemical change forms a genuinely new substance, by breaking molecules apart and rearranging them into different ones, and is not easily reversed — burning wood, rusting iron, and fermenting meat are all chemical changes.
Experiment: Iron and Sulphur — Mixture vs. Compound
Requirements: iron filings, powdered sulphur, a bar magnet, a crucible and lid, Bunsen burner.
Procedure: A mixture of iron filings and powdered sulphur is placed in a crucible, and a magnet is waved over it, with the result noted. The same mixture is then heated strongly in the crucible, watching for any colour change, and afterwards the magnet is waved over the resulting solid.
Observation: before heating, the magnet attracts the iron filings out of the mixture easily. As heating proceeds, the yellow sulphur and silvery iron disappear and a black solid forms; afterwards, the magnet no longer attracts anything from this solid.
Conclusion: simply mixing iron and sulphur is a physical change — the iron can still be separated out with a magnet. But heating them together produces a new substance, iron(II) sulphide, which is not magnetic — so this is a chemical change.
Chemical Equations
A chemical equation represents a chemical reaction using symbols and formulae. The starting substances (reactants) are written on the left, the substances formed (products) on the right, joined by an arrow meaning “produces” or “yields”. A plus sign between reactants means “reacts with”; between products it just means “and”. The physical state of each substance is shown in brackets as a subscript: solid (s), liquid (l), gas (g), or aqueous — dissolved in water — (aq).
Balancing an Equation
A balanced equation has equal numbers of each type of atom on both sides, in line with the Law of Conservation of Mass: matter can be neither created nor destroyed in a chemical reaction, so the total mass of reactants must equal the total mass of products.
- Write the reactants and products as a word equation, e.g. Iron + Sulphur → Iron sulphide.
- Write the correct formula for each substance: Fe + S → FeS.
- Add the physical states: Fe(s) + S(s) → FeS(s).
- Balance by adding whole-number coefficients in front of formulae as needed — never by changing a formula itself.
Example: magnesium burning in oxygen. Written as Mg(s) + O₂(g) → MgO(s), the left side totals 24g + 32g = 56g, but the right side is only 40g — unbalanced. Adding coefficients gives 2Mg(s) + O₂(g) → 2MgO(s), which balances both the atom count and the mass (2×24 + 32 = 2×40 = 80g).
A reversible reaction — one that can run in both directions — is shown with a double arrow (⇌) instead of a single one, e.g. NH₃(g) + HCl(g) ⇌ NH₄Cl(s).
Common Types of Chemical Reaction
- Decomposition: a larger substance breaks down into smaller ones. When caused by heat, it’s called thermal decomposition, e.g. CaCO₃(s) → CaO(s) + CO₂(g).
- Combustion: a substance burns in oxygen, e.g. cooking gas (butane): 2C₄H₁₀(g) + 13O₂(g) → 8CO₂(g) + 10H₂O(g).
- Direct combination: two or more simple substances combine to form a larger one, e.g. Fe(s) + S(s) → FeS(s).
- Neutralisation: an acid reacts with a base to form only a salt and water, e.g. NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l).
- Precipitation: two soluble substances react to form an insoluble solid, e.g. AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq).
