Particles & Separation

IGCSE Edexcel Chemistry
1.1–1.10 Particle model, states of matter, and separation techniques
Key Concepts: The particle model explains the properties of solids, liquids and gases. State changes involve energy transfer. Mixtures can be separated using physical techniques such as filtration, distillation, and chromatography.

Section A — The Particle Model

1. Complete the table describing particles in each state of matter. [6]
Property Solid Liquid Gas
Arrangement of particles
Movement of particles
Energy of particles (relative)
2. Explain why gases are compressible but solids are not. [2]
3. Explain why a liquid can flow but a solid cannot. [2]

Section B — Changes of State

4. Complete the table naming each change of state and stating whether it is endothermic or exothermic. [8]
Change Name Endothermic or Exothermic?
Solid → Liquid
Liquid → Gas
Gas → Liquid
Liquid → Solid
5. A heating curve shows temperature against time as ice is heated to steam. Explain why the temperature remains constant during melting and boiling. [2]
6. State two differences between evaporation and boiling. [2]
7. Define diffusion. Explain why gases diffuse faster than liquids. [3]

Section C — Separation Techniques

8. State the most suitable separation technique for each mixture. Choose from: filtration, simple distillation, fractional distillation, crystallisation, paper chromatography. [5]
MixtureTechnique
Sand and water
Salt solution (to obtain pure water)
Ethanol and water
Salt solution (to obtain salt crystals)
Ink (to identify the dyes present)
9. Describe the process of paper chromatography. Explain what an Rf value tells you. [5]
10. Explain why fractional distillation must be used to separate ethanol and water rather than simple distillation. [2]
11. A student separates sand from sea water and then obtains salt crystals from the filtrate. Describe the steps involved in order. [4]

Total marks: 41

Mark Scheme

1. Solid: regular/close-packed, vibrate about fixed positions, lowest; Liquid: close but irregular, slide past each other, medium; Gas: far apart/random, move rapidly in all directions, highest [6]
2. Gases have large gaps/spaces between particles [1]; these can be compressed by an external force; solids have particles tightly packed with almost no gaps [1] [2]
3. Liquid particles are not in fixed positions and can slide past each other [1]; solid particles are held in fixed positions in a lattice structure [1] [2]
4. Solid→Liquid: melting, endothermic; Liquid→Gas: boiling/evaporation, endothermic; Gas→Liquid: condensation, exothermic; Liquid→Solid: freezing/solidifying, exothermic [8 — 2 per row]
5. Energy (heat) is absorbed but goes into breaking the intermolecular bonds/forces between particles [1]; not into increasing kinetic energy/temperature, so the temperature stays constant until the change is complete [1] [2]
6. Any two: evaporation occurs at any temperature/only at the surface; boiling occurs at a fixed temperature throughout the liquid; boiling is faster/more vigorous [2]
7. Net movement of particles from high to low concentration [1]; gas particles move faster than liquid particles (more kinetic energy/greater spacing) [1]; so collisions and movement are more frequent in gases [1] [3]
8. Sand and water: filtration; salt solution (water): simple distillation; ethanol and water: fractional distillation; salt solution (crystals): crystallisation; ink: paper chromatography [5]
9. Draw a pencil baseline on chromatography paper [1]; spot the ink/mixture on the baseline [1]; dip into solvent (not above baseline) and allow solvent to travel up the paper [1]; remove and mark solvent front; Rf = distance travelled by spot ÷ distance travelled by solvent front [1]; Rf value is characteristic of a substance in a given solvent — used to identify unknown substances [1] [5]
10. Ethanol and water have similar boiling points (78°C and 100°C) [1]; a fractionating column is needed to separate them by repeated evaporation and condensation; simple distillation cannot separate liquids with close boiling points efficiently [1] [2]
11. Filter the sand-sea water mixture through filter paper in a funnel [1]; the sand stays on the filter paper (residue), the salt solution passes through (filtrate) [1]; heat the salt solution (filtrate) to evaporate most of the water [1]; allow to cool so salt crystals form; filter off crystals and dry [1] [4]