Physical and chemical changes

When materials change, some changes can be undone and some can’t. Melting, freezing and dissolving are physical changes: no new substance is made, and they are reversible. Burning, rusting and cooking are chemical changes: new substances are made, and they are usually irreversible.

Key facts

  • Physical changes (reversible): melting, freezing, evaporating, condensing, dissolving and mixing. The substance stays the same, just in a different form.
  • Chemical changes (usually irreversible): burning, rusting, cooking, milk going sour, and vinegar fizzing with bicarbonate of soda. New substances are made.
  • Signs of a chemical change: bubbles of gas, a colour change, a temperature change, light or a flame, a new solid forming, or a new smell.
  • Burning needs fuel, oxygen and heat: the fire triangle. Take one away and the fire goes out.
  • Rusting: iron + oxygen + water → rust (hydrated iron oxide). Iron needs both air and water to rust.
  • Mass is conserved in every change. If a gas escapes (or joins from the air), the mass measured in an open container changes.

Worked examples

Example 1

Physical or chemical change? Melting chocolate, frying an egg, dissolving salt, burning wood.

  1. Melted chocolate sets again when it cools: no new substance. Physical.
  2. A fried egg can’t be made runny again: new substances form. Chemical.
  3. Salt can be got back by evaporating the water. Physical.
  4. Burning wood makes ash, smoke and gases, and gives out heat and light. Chemical.

Answer: melting chocolate: physical; frying an egg: chemical; dissolving salt: physical; burning wood: chemical

Example 2

Three iron nails are left for a week: A in tap water open to the air, B in boiled water with a layer of oil on top, C in dry air with a drying agent. Which nails rust?

  1. A has water and air: it rusts.
  2. B has water, but boiling removed the air and the oil stops more getting in: no rust.
  3. C has air, but the drying agent removes the water: no rust.

Answer: only A: rusting needs both water and oxygen (air)

Example 3

A beaker of vinegar and bicarbonate of soda has a mass of 152.0 g before they react and 149.8 g after the fizzing stops. Why is it lighter, and what mass of gas escaped?

  1. The fizzing is bubbles of carbon dioxide gas, a new substance.
  2. The beaker is open, so the gas escapes into the air and its mass is no longer weighed.
  3. The mass of gas is the mass lost. 152.0 − 149.8 = 2.2 g

Answer: carbon dioxide gas escaped into the air; 2.2 g

Example 4

Why does putting a fire blanket over a pan of burning oil put out the fire?

  1. A fire needs all three: fuel, oxygen and heat.
  2. The blanket stops air reaching the oil, so there is no oxygen.
  3. With one side gone, the fire goes out. (Never throw water on burning oil.)

Answer: it cuts off the oxygen, which is one side of the fire triangle