GCSE Chemistry revision

Soluble salts

Reactions of acids

AQA 4.4.2.3 Foundation & Higher

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Revision summary

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A mixture consists of two or more different substances that are mixed together but are not chemically joined.
Mixtures can be separated using physical techniques because the substances are not chemically bonded.

An insoluble solid does not dissolve in a liquid and simply sinks to the bottom, such as sand in water.
A solution is formed when a solute (e.g. salt) dissolves in a solvent (e.g. water) to produce a solution (e.g. salt water).
The key difference between these mixture types determines which separation technique should be used.

2. Types of Mixtures AQA

An insoluble solid does not dissolve in a liquid and simply sinks to the bottom, such as sand in water.
A solution is formed when a solute (e.g. salt) dissolves in a solvent (e.g. water) to produce a solution (e.g. salt water).
The key difference between these mixture types determines which separation technique should be used.

Filtration is used to separate insoluble solids from liquids.
Filter paper contains tiny holes that allow liquid to pass through but retain solid particles.
The filter paper is placed inside a filter funnel so the mixture can be poured through easily, leaving the solid behind on the paper.

Evaporation is used to separate a soluble solid from a solution when the solid is not susceptible to thermal decomposition.
The solution is placed in an evaporating dish or crucible on a tripod and heated with a Bunsen burner.
As the solvent evaporates, the solution becomes more concentrated until dry crystals of the solid remain.
The advantage of evaporation is that it is a relatively quick and straightforward technique.

4. Evaporation AQA

Evaporation is used to separate a soluble solid from a solution when the solid is not susceptible to thermal decomposition.
The solution is placed in an evaporating dish or crucible on a tripod and heated with a Bunsen burner.
As the solvent evaporates, the solution becomes more concentrated until dry crystals of the solid remain.
The advantage of evaporation is that it is a relatively quick and straightforward technique.

Thermal decomposition occurs when a substance breaks down into other substances upon being heated.
If a solid is susceptible to thermal decomposition, evaporation cannot be used as it would destroy the solid, making crystallisation necessary instead.

5. Thermal Decomposition — A Limitation of Evaporation AQA

Thermal decomposition occurs when a substance breaks down into other substances upon being heated.
If a solid is susceptible to thermal decomposition, evaporation cannot be used as it would destroy the solid, making crystallisation necessary instead.

Crystallisation is used to separate soluble solids that would decompose if heated too strongly.
The solution is heated gently, often using a water bath rather than a Bunsen burner, to evaporate some of the solvent.
Heating is stopped once crystals begin to form, and the solution is left to cool.
As the solution cools, more crystals form because solids are less soluble at lower temperatures.
The crystals are then filtered out from the remaining solution using filter paper and a funnel.
Finally, the crystals are dried by leaving them in a warm place or placing them in a drying oven.

6. Crystallisation AQA

Crystallisation is used to separate soluble solids that would decompose if heated too strongly.
The solution is heated gently, often using a water bath rather than a Bunsen burner, to evaporate some of the solvent.
Heating is stopped once crystals begin to form, and the solution is left to cool.
As the solution cools, more crystals form because solids are less soluble at lower temperatures.
The crystals are then filtered out from the remaining solution using filter paper and a funnel.
Finally, the crystals are dried by leaving them in a warm place or placing them in a drying oven.

Filtration is the correct technique when separating an insoluble solid from a liquid.
Evaporation is suitable for separating a soluble solid from a solution when the solid is stable under heat.
Crystallisation is preferred when the soluble solid would undergo thermal decomposition if heated too strongly.

7. Choosing the Right Separation Technique AQA

Filtration is the correct technique when separating an insoluble solid from a liquid.
Evaporation is suitable for separating a soluble solid from a solution when the solid is stable under heat.
Crystallisation is preferred when the soluble solid would undergo thermal decomposition if heated too strongly.