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Separations for AQA GCSE Chemistry

Separations

This page covers the following topics:

1. Filtration
2. Crystallisation
3. Distillation

Filtration is a technique used to separate a mixture of insoluble solids and a liquid. Filtration is possible due to the size differences of solid and liquid particles. Smaller liquid molecules can pass through a filter paper in a funnel to form filtrate, while larger solid particles cannot, forming residue. Dissolved ions are small enough to pass through the holes in the filter paper.

Filtrate is a filtered liquid which has passed through a filter paper. Residue is a solid which has been filtered out of a mixture and remains on the filter paper.

Filtration

Crystallisation is the formation of crystals from a solution, it allows the separation of a dissolved solid from its solvent. Crystallisation is a physical process as no chemical reaction is happening.

To crystallise a salt from its solution, the salt solution in an evaporating basin is heated slowly using a Bunsen burner. A wire gauze placed under the solution provides a level surface which can conduct the heat and evenly distribute it to the evaporating basin. This causes the solvent to evaporate until a solid salt is left behind. This solid is the salt crystals.

Crystallisation

Distillation is a technique which separates a mixture of liquids by their boiling point. This is a physical process as no chemical reaction is happening.

The mixture of liquids is placed in a round bottom flask and placed above a hot plate. A hot plate can be warmed up to a specific temperature, allowing more control than a Bunsen burner which is important when separating liquids with similar boiling points. As the mixture is heated, the liquid with the lowest boiling point will evaporate and travel up the glassware. A stopper placed at the top of the flask adapter prevents the gaseous samples from escaping and forces them through the condenser to be collected. When it reaches the cold environment of the condenser, the gas will condense and be collected as pure liquid. This is repeated until all the liquids are separated.

There are different forms of distillation such as simple distillation and fractional distillation. Fractional distillation is used for liquids with similar boiling points while simple distillation is used for liquids with relatively different boiling points. Fractional distillation is often used to separate a larger number of liquids than simple distillation, for example, crude oil.

Distillation

1

Explain why crystallisation is a physical process.

Crystallisation is a physical process as no chemical reaction is happening.

No chemical reaction occurs.

Explain why crystallisation is a physical process.

2

Which of the following could be used in a crystallisation experiment?

โ€ข funnel
โ€ข Bunsen burner
โ€ข evaporating basin
โ€ข burette
โ€ข wire gauze

To crystallise a salt from its solution, the salt solution in an evaporating basin is heated slowly using a Bunsen burner. A wire gauze placed under the solution provides a level surface which can conduct the heat and evenly distribute it to the evaporating basin. This causes the solvent to evaporate until a solid salt is left behind. This solid is the salt crystals.

โ€ข funnel: filtration โ†’ no
โ€ข Bunsen burner: crystallisation โ†’ yes
โ€ข evaporating basin: crystallisation โ†’ yes
โ€ข burette: titration โ†’ no
โ€ข wire gauze: crystallisation โ†’ yes

Bunsen burner, evaporating basin, wire gauze

Which of the following could be used in a crystallisation experiment? 

โ€ข funnel 
โ€ข Bunsen burner 
โ€ข evaporating basin 
โ€ข burette 
โ€ข wire gauze

3

What is the purpose of a stopper in a distillation setup?

A stopper placed at the top of an adapter prevents gaseous samples from escaping and forces them through the condenser to be collected.

preventing gas from escaping

What is the purpose of a stopper in a distillation setup?

4

Suggest why a hot plate might be a better choice while heating a mixture during distillation than a Bunsen burner.

A hot plate can be warmed up to a specific temperature, allowing more control than a Bunsen burner which is important when separating liquids with similar boiling points.

Specific temperature setup on the Bunsen burner allows separating liquids with similar boiling points.

Suggest why a hot plate might be a better choice while heating a mixture during distillation than a Bunsen burner.

5

Describe how a crystallisation experiment works.

To crystallise a salt from its solution, the salt solution in an evaporating basin is heated slowly using a Bunsen burner. A wire gauze placed under the solution provides a level surface which can conduct the heat and evenly distribute it to the evaporating basin. This causes the solvent to evaporate until a solid salt is left behind. This solid is the salt crystals.

The solution in an evaporating basin over a wire gauze is heated with a Bunsen burner to evaporate the solvent and leave behind salt crystals.

Describe how a crystallisation experiment works.

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