Botswana General Certificate of Secondary EducationQueued — topic map only

Chemistry — BGCSE

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BGCSE subjects carry no four-digit code · no verification date claimed yet · 7 questions available on this subject's topics (14 marks)

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Particles, states and separation techniquesExplanations must name the particles and what they do — not just "because it is hot".
Atomic structure, bonding and formulaeIonic formulae come from charge balance; show the charges you balanced.
Equations, moles and reacting massesBalance first, then use moles. An unbalanced equation corrupts every later step.
Acids, bases, salts and titration calculationsConvert cm³ to dm³ by dividing by 1000 before multiplying by concentration.
Rate of reaction and collision theoryExplain with collision frequency and energy, then link back to the specific change in the question.
Organic chemistry, fuels and polymersName the homologous series and the functional group; general formulae must be exact.

Practice available now (7 questions)

Which method is used to obtain solid copper(II) sulfate from copper(II) sulfate solution?

1 mark · Choose · Particles, states and separation techniques

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Answer: Crystallisation

The salt is dissolved, so it is not removed by filtering
Slow evaporation, then cooling, gives crystals

Ask what state the wanted substance is in. Dissolved means filtration cannot help, whatever the apparatus looks like.

Zinc reacts with hydrochloric acid: Zn + 2HCl → ZnCl₂ + H₂. 6.5 g of zinc reacts completely. The relative atomic mass of zinc is 65 and the molar volume of a gas at room temperature is 24 dm³. Calculate the volume of hydrogen produced.

3 marks · Calculate · Equations, moles and reacting masses

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Answer: 2.4 dm³

Moles of zinc = 6.5 ÷ 65 = 0.10 mol
The equation shows 1 mol Zn gives 1 mol H₂, so 0.10 mol H₂
Volume = 0.10 × 24 = 2.4 dm³

Balance, moles, ratio, then convert. Four short lines that earn marks even if one number goes wrong — the ratio line alone is usually worth one.

The same mass of marble reacts with the same volume and concentration of hydrochloric acid twice: once as large chips and once as a fine powder. Explain why the powder reacts faster.

3 marks · Explain · Rate of reaction and collision theory

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Answer: The powder has a much larger surface area, so acid particles collide with the marble more frequently per second, producing a faster rate.

Powder has a greater surface area than chips for the same mass
More frequent collisions between acid particles and the surface
Rate is proportional to collision frequency, so the powder is faster

Rate answers need the words "more frequent collisions". "More surface area" on its own explains the collision count and stops one mark short.

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

Protons = atomic number = 17
Electrons = protons in a neutral atom = 17
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)

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

Which is the general formula of the alkanes?

1 mark · State · Organic chemistry, fuels and polymers

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Answer: CₙH₂ₙ₊₂

Alkanes are saturated hydrocarbons
Each carbon has four single bonds
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.

Same number of protons (same atomic number) (1 mark)
Different number of neutrons, so a different mass number (1 mark)
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.