States of matter and mixtures
The three states of matter
| State | Particle arrangement | Energy | Shape | Volume |
|---|---|---|---|---|
| Solid | Regular lattice, tightly packed | Lowest | Fixed | Fixed |
| Liquid | Random, close together | Medium | No fixed shape | Fixed |
| Gas | Random, far apart, fast-moving | Highest | No fixed shape | Fills container |
Changes of state
Melting (solid → liquid): particles gain enough energy to overcome their fixed lattice positions. Boiling/evaporation (liquid → gas): particles break free entirely. Condensation (gas → liquid): particles lose energy, intermolecular forces pull them together. Freezing (liquid → solid): particles lose energy, return to fixed positions. Sublimation (solid → gas directly): e.g. iodine, dry ice (CO₂).
Edexcel Core Practical CP5 — Paper chromatography
Paper chromatography separates mixtures of soluble substances (e.g. food dyes, inks). Method:
- Draw a pencil baseline on chromatography paper (pencil is insoluble).
- Spot the sample onto the baseline.
- Stand paper in solvent (e.g. water, ethanol) below the baseline.
- Allow solvent to run up by capillary action.
- Remove and mark solvent front immediately.
- Calculate Rꜰ values.
Rꜰ = distance travelled by substance ÷ distance travelled by solvent front.
Rꜰ values are always between 0 and 1. A substance with low solubility in the solvent has a low Rꜰ; high solubility → high Rꜰ. Rꜰ values are fixed for a given substance in a given solvent — used for identification.
Separating mixtures
The technique chosen depends on the nature of the mixture:
Filtration
Separates an insoluble solid from a liquid. Filter paper in a funnel; the residue stays on the paper; the filtrate passes through.
Crystallisation
Separates a soluble solid from a solution. Evaporate the solvent until the solution is saturated, then allow to cool slowly. Crystals form because solubility decreases at lower temperatures.
Simple distillation
Separates a solvent from a dissolved solid (or two liquids with very different boiling points). The solution is heated; the liquid with the lower boiling point evaporates first; vapour passes through a condenser and is collected.
Fractional distillation
Separates a mixture of liquids with close boiling points (e.g. ethanol/water mixture, or crude oil fractions). A fractionating column maintains a temperature gradient; each component condenses at its own boiling point.
Edexcel exam note: choosing the method
- Salt from salt water → evaporation / crystallisation
- Ink colours → chromatography
- Insoluble sand from salt water → filter (then evaporate to get salt)
- Ethanol from ethanol/water mixture → fractional distillation
Pure substances vs mixtures
An element contains only one type of atom. A compound contains two or more elements chemically bonded. A mixture contains two or more substances NOT chemically bonded — they retain their individual properties.
A pure substance (element or compound) has a sharp melting point. A mixture melts over a range of temperatures. Edexcel Paper 1 often asks you to interpret melting point data to decide if a substance is pure.
⚠Common mistakes
- Confusing filtration and centrifugation: filtration is gravity + filter paper; centrifugation spins the mixture (used in biology).
- Rꜰ > 1: this cannot happen. Check your measurements.
- Pencil vs pen on the baseline: always pencil — ink runs in the solvent.
- Simple vs fractional distillation: if boiling points are similar, you need fractional; if very different, simple works.
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