Physical and chemical changes

O Level / IGCSE Chemistry · Chemical reactions. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

In a physical change no new substance is made. The substance only changes its state, its shape or how it is mixed, as in melting, boiling, dissolving and crushing. The particles themselves stay the same, and the change is usually easy to reverse.

In a chemical change one or more new substances form, with properties different from the starting materials. The atoms are rearranged into new combinations. Signs to look for are a lasting colour change, a gas given off, a precipitate, or heat given out or taken in.

In both kinds of change the total mass of all the substances stays the same.

Rules to remember

Common mistake

Students think any change that can be reversed must be physical. Heating blue hydrated copper(II) sulfate can be reversed by adding water, but it is a chemical change because a new substance, white anhydrous copper(II) sulfate, forms.

Worked example

Iron filings and sulfur powder are stirred together (stage 1). The mixture is then heated; it glows red and a black solid forms that a magnet does not attract (stage 2). Which stage is a chemical change?

  1. Stage 1: a magnet can still pull the iron out, so iron and sulfur keep their own properties. No new substance: physical change.
  2. Stage 2: the black solid has new properties (not magnetic, not yellow), so a new substance has formed.
  3. The glow shows energy being given out, which is further evidence of a reaction.

Answer: Stage 2 is the chemical change (iron + sulfur → iron(II) sulfide). Stage 1 is only mixing, a physical change.

Practice questions

Try each one, then open the answer.

1. In a burning candle, solid wax first melts and then the molten wax burns. Which statement is correct?

  1. A
    melting is a chemical change and burning is a physical change
  2. B
    both are chemical changes
  3. C
    both are physical changes
  4. D
    melting is a physical change and burning is a chemical change
Show answer

Answer: D. Molten wax is still wax, so melting is physical. Burning produces carbon dioxide and water, which are new substances.

2. A 2.4 g piece of magnesium ribbon is burnt completely in air. The white ash formed has a mass of 4.0 g. What does this show?

  1. A
    a physical change, because the ash is still magnesium, just a different colour
  2. B
    a physical change, because a mass change of 1.6 g is too small to be a chemical change
  3. C
    a chemical change, because 1.6 g of magnesium has been lost from the ribbon as a gas
  4. D
    a chemical change, because the magnesium has combined with 1.6 g of oxygen from the air
Show answer

Answer: D. The mass has increased because magnesium combined with oxygen to make a new substance, magnesium oxide: 2.4 g of Mg (0.1 mol) forms 4.0 g of MgO, so 1.6 g of oxygen was gained. Making a new substance means a chemical change.

3. A student heats 5.0 g of a green solid in an open dish. A colourless gas that turns limewater milky is given off and 3.2 g of a black solid is left. What does this show?

  1. A
    a physical change, because the solid has only changed colour when heated
  2. B
    a chemical change, because a new gas and a new black solid have formed
  3. C
    a physical change, because heating only changes the state of a substance without making new ones
  4. D
    a physical change, because the loss in mass shows that some of the solid has evaporated
Show answer

Answer: B. A new gas that turns limewater milky (carbon dioxide) and a new solid with different properties have formed, so this is a chemical change (thermal decomposition). The loss of mass is the mass of gas that escaped.

More questions on this topic

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