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Chemistry · Topic

Particles, states and separation techniques

Explanations must name the particles and what they do — not just "because it is hot".

2 original questions in the bank, worth 4 marks · above the two-per-topic floor

Drill this topic All particles, states and separation techniques questions

The questions, with the examiner reasoning

Explain3 marksIGCSE

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.

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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

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

Choose1 markBGCSE

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

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Answer: Crystallisation

  1. The salt is dissolved, so it is not removed by filtering
  2. Slow evaporation, then cooling, gives crystals
  • A. Crystallisation — Correct: the water is allowed to evaporate slowly so crystals form.
  • B. Filtration — Filtration separates an insoluble solid from a liquid. The copper(II) sulfate is dissolved, so it passes straight through the paper.
  • C. Decanting — Decanting pours off liquid — it does not recover a dissolved salt.
  • D. Distillation — This would recover the water as the distillate, leaving a dry residue at the bottom of the flask rather than crystals.

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