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Chemistry — IGCSE

Mole calculations are the highest-value topic on the paper and the most commonly skipped. Start there.

Syllabus code 0620 · checked against the board on 2026-09-13 · 8 questions available on this subject's topics (17 marks)

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Practice available now (8 questions)

A bottle of ammonia is opened at one end of a classroom. Explain, in terms of particles, why the smell is noticed at the other end of the room several seconds later.

3 marks · Explain · Particles, states and separation techniques

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Answer: Ammonia particles move randomly and spread out (diffuse) from where there are many to where there are few, colliding with air particles on the way.

  1. Particles of ammonia are in constant random motion
  2. They move from a region of higher concentration to lower concentration (diffusion)
  3. Repeated collisions with air particles spread them through the room

"Gas spreads" earns nothing. Name the particles, say they move randomly, and say the direction of movement is from high to low concentration.

A neutral atom is written as ³⁵₁₇Cl. How many protons, neutrons and electrons does it contain?

1 mark · State · Atomic structure, bonding and formulae

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Answer: 17 protons, 18 neutrons, 17 electrons

  1. Protons = atomic number = 17
  2. Electrons = protons in a neutral atom = 17
  3. Neutrons = mass number − protons = 35 − 17 = 18

One rule does all the work: bottom number protons, top minus bottom gives neutrons, and neutral means equal protons and electrons.

Balance the equation __Fe + __O₂ → __Fe₂O₃ and state whether the reaction is oxidation or reduction.

2 marks · Balance · Equations, moles and reacting masses

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Answer: 4Fe + 3O₂ → 2Fe₂O₃; oxidation (iron gains oxygen)

  1. Oxygen atoms need to match: 3 O₂ gives 6 oxygen atoms, so 2Fe₂O₃
  2. Iron then needs 4 atoms on the left
  3. Gaining oxygen is oxidation

Balance the odd element last. Starting with iron leaves you adjusting it twice, which is where the second mistake usually happens.

25.0 cm³ of 0.100 mol/dm³ sodium hydroxide solution is exactly neutralised by 20.0 cm³ of hydrochloric acid. Calculate the concentration of the hydrochloric acid in mol/dm³.

3 marks · Calculate · Acids, bases, salts and titration calculations

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Answer: 0.125 mol/dm³

  1. Moles of NaOH = 0.100 × (25.0 ÷ 1000) = 0.0025 mol
  2. NaOH and HCl react 1:1, so moles of HCl = 0.0025 mol
  3. Concentration = 0.0025 ÷ (20.0 ÷ 1000) = 0.125 mol/dm³

Two conversions to dm³, one ratio, one division. Write the volume conversion as its own step; it is the step learners skip and markers reward.

Which is the general formula of the alkanes?

1 mark · State · Organic chemistry, fuels and polymers

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Answer: CₙH₂ₙ₊₂

  1. Alkanes are saturated hydrocarbons
  2. Each carbon has four single bonds
  3. General formula CₙH₂ₙ₊₂

Test a formula by its simplest member: methane is CH₄, and n = 1 gives CH₄ from CₙH₂ₙ₊₂.

Define the term isotope, and explain why two isotopes of the same element have identical chemical properties but different relative atomic masses.

3 marks · Define · Atomic structure, bonding and formulae

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Answer: Atoms of the same element with the same number of protons but different numbers of neutrons; chemical properties depend on electrons, and both isotopes have the same electron arrangement.

  1. Same number of protons (same atomic number) (1 mark)
  2. Different number of neutrons, so a different mass number (1 mark)
  3. Chemical properties are set by the electrons, and the electron arrangement is the same in both (1 mark)

A definition question is marked on the words you choose. "Same element, different mass" is not a definition; it is a description, and it usually stops at one mark.

Which instrument is most suitable for measuring 25.0 cm³ of dilute acid to the nearest 0.1 cm³?

1 mark · Choose · Units, measuring instruments and significant figures

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Answer: Burette

  1. Precision needed: 0.1 cm³
  2. Burette reads to 0.05 cm³, so it meets the precision requirement

Instrument questions are answered in two steps: what precision is demanded, then which instrument provides at least that precision.

A learner of mass 45 kg climbs a 4.0 m staircase in 6.0 s. Take g = 10 N/kg. Calculate the power she develops.

3 marks · Calculate · Work, energy, power and cost of electricity

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Answer: 300 W

  1. Work done = mgh = 45 × 10 × 4.0 = 1800 J
  2. Power = work ÷ time = 1800 ÷ 6.0
  3. Power = 300 W

Energy first, power second. Answers that go straight to a power formula without showing the 1800 J normally lose the first mark.