8.10Chapter 8: Pure Substances and Solutions

Chapter 8 Review: Pure Substances and Solutions — Alhussan Chemistry - Stage 8 Revision Notes & Topical Questions

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Lesson summary

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

Pure substance it contains only one type of element or compound and melts and boils at one fixed temperature.

Mixture

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

Purity test this is why melting point is used to test purity.

Cooling curve

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

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

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

Soluble and insoluble a substance that dissolves is soluble and one that does not is insoluble.

Solubility

Solubility the maximum mass of solute that dissolves in 100 g of solvent at a stated temperature.

Saturated solution

Saturated solution no more solute will dissolve.

Temperature

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

Concentrated

Concentrated a large amount of solute in a given volume, with the solute particles close together.

Dilute

Dilute a small amount of solute in a given volume.

Formula

Formula concentration (g/dm³) = mass of solute (g) ÷ volume of solution (dm³).

Part 4 of 5Investigating solubility and temperature

Independent variable

Independent variable water temperature.

Dependent variable

Dependent variable the mass of solute that dissolves.

Control variables

Control variables the volume of water, the type of solute and the stirring method.

Prediction

Prediction hotter 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

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

Choosing a method it depends on the property that differs between the substances.

Key terms

Term	Meaning
Solute	a substance that dissolves in a solvent.
Solvent	the liquid that dissolves the solute.
Saturated solution	a solution in which no more solute will dissolve at that temperature.
Solubility	the maximum mass of solute that dissolves in 100 g of solvent.
Filtrate	the liquid that passes through filter paper.
Anomalous result	a measurement that does not fit the pattern of the other results.

Avoid these mistakes

Saying that a clear liquid must be pure — a clear solution can contain dissolved substances and is a mixture.

Confusing dissolving with melting — dissolving needs a solvent, whereas melting is a change of state.

Putting the chromatography start line below the solvent level — it must be above so the dyes are not washed away.

Treating an anomalous point as part of the trend — identify it and leave it out of the line of best fit.

Last updated: 13 October 2026