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IGCSE · Practice questions · Elements, compounds and mixtures

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Six original Cambridge-style questions on elements, compounds and mixtures: definitions, the iron and sulfur experiment, classifying substances, particle pictures and a fixed-ratio calculation.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Count the atom types, then check for bonds.

01
Recall
[3 marks]

Define the terms element, compound and mixture.

  • Element: a substance made of only one type of atom. ✓
  • Compound: a substance made of two or more elements chemically combined in a fixed ratio. ✓
  • Mixture: two or more substances that are not chemically combined. ✓
02
Explain
[3 marks]

A mixture of iron filings and sulfur is heated until it glows. Describe how a magnet could be used to show that a new substance has formed, and explain the result.

  • Before heating, a magnet attracts the iron and pulls it out of the mixture. ✓
  • After heating, the black product is not attracted to the magnet (assuming no iron is left over). ✓
  • The iron has chemically combined with the sulfur to form a compound, iron(II) sulfide, FeS, which has different properties from iron. ✓
03
Analysis
[3 marks]

Classify each substance as an element, a compound or a mixture: copper, carbon dioxide, air, sodium chloride, sea water, nitrogen.

  • Elements: copper and nitrogen. ✓
  • Compounds: carbon dioxide and sodium chloride. ✓
  • Mixtures: air and sea water. ✓
04
Analysis
[2 marks]

A box contains four particles each made of two identical atoms joined together, and three particles each made of one atom of one element joined to one atom of a different element. Classify the contents of the box and explain your answer.

  • The identical-atom particles are molecules of an element; the others are molecules of a compound. ✓
  • The two substances are not joined to each other, so the box contains a mixture of an element and a compound. ✓
05
Calculation
[3 marks]

Iron and sulfur always combine in the mass ratio 56 : 32. A student heats 14 g of iron with 10 g of sulfur. Calculate the mass of iron(II) sulfide formed and the mass of the substance left over. (Ar: Fe 56, S 32)

  • 14 g of iron needs 14 × 32 ÷ 56 = 8 g of sulfur. ✓
  • Mass of FeS formed = 14 + 8 = 22 g. ✓
  • Sulfur is in excess: 10 − 8 = 2 g of sulfur is left over. ✓
06
Explain
[2 marks]

A student says that oxygen gas, O2, is a compound because each molecule contains two atoms. Explain why the student is wrong.

  • Both atoms in an O2 molecule are oxygen atoms, so there is only one type of atom. ✓
  • A compound must contain two or more different elements chemically combined, so O2 is an element. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.