CHEMISTRY · Stage 8 · Chapter 8: Pure Substances and Solutions

Chemistry 8.10 Chapter 8 Review: Pure Substances and Solutions

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8.10 Chapter 8 Review: Pure Substances and Solutions — Alhussan International Academy — Chemistry (Stages 7–9) notes: Revise pure substances, mixtures, solutions, solubility and separation methods.Pure substance — it contains only one type of element or compound and melts and boi…

Purity test
Cooling curve
Anomalous result
Soluble and insoluble
Saturated solution
Temperature

Part 1 of 5Pure substances, mixtures and melting points

Pure substance — it contains only one type of element or compound and melts and boils at one fixed temperature.
Mixture — it contains two or more substances that are not chemically joined, so each keeps its own properties.
- A mixture melts over a range of temperatures, usually lower than the melting point of the pure substance.
Purity test — this is why melting point is used to test purity.
Cooling curve — when a liquid such as salol is cooled and its temperature recorded every minute, the temperature falls, stays constant while it freezes, then falls again.
- The flat section is the freezing point.
Anomalous result — a single result that does not fit the pattern should be spotted on the graph, then repeated or ignored when drawing the line of best fit.

Part 2 of 5Solutions and solubility

Solution — it forms when a solute dissolves in a solvent; in salt water, salt is the solute and water is the solvent.
- Solute particles spread out between solvent particles, so the mixture looks clear and does not settle.
Soluble and insoluble — a substance that dissolves is soluble and one that does not is insoluble.
Solubility = the maximum mass of solute that dissolves in 100 g of solvent at a stated temperature.
Saturated solution — no more solute will dissolve.
Temperature — for most solids solubility increases as temperature rises, so the curve of mass dissolved per 100 g of water rises.

Part 3 of 5Concentration

Concentrateda large amount of solute in a given volume, with the solute particles close together.
Dilutea small amount of solute in a given volume.
Formulaconcentration (g/dm³) = mass of solute (g) ÷ volume of solution (dm³).

Part 4 of 5Investigating solubility and temperature

Independent variablewater temperature.
Dependent variablethe mass of solute that dissolves.
Control variablesthe volume of water, the type of solute and the stirring method.
Predictionhotter water dissolves more of most solids, such as potassium chloride, which is a sensible, testable prediction.

Part 5 of 5Separation methods

Method	Separates	Property used
Filtration	Insoluble solid from liquid	Particle size
Evaporation	Dissolved solid from solution	Solvent evaporates, solid does not
Crystallisation	Pure crystals of a soluble solid	Lower solubility when cold
Simple distillation	Solvent from solution	Boiling point
Fractional distillation	Mixture of liquids	Different boiling points
Chromatography	Dissolved coloured substances	Different solubility in the solvent

Chromatography — the pencil start line must be above the solvent level, otherwise the dyes dissolve into the beaker.
- A pure substance gives one spot and a mixture gives several.
Choosing a method — it depends on the property that differs between the substances.

Last updated: 13 October 2026