Separating mixtures

In a nutshell

There are different ways to separate mixtures, including filtration, evaporation, chromatography and distillation.

Filtration

Definition

Filtration is when you separate an insoluble solid from a liquid.

Example

Mixture of sand and water. This mixture can be separated using filtration because sand is insoluble in water.

Science; Pure and impure substances; KS3 Year 7; Separating mixtures

procedure

1.	Pour the beaker containing the sand and water mixture into the filter funnel.
2.	Water (liquid) particles are small enough to go through the pores of the filter paper and into the beaker. Sand (solid) particles are too large to pass through so remain on the filter paper.

Evaporation

Evaporation is used to separate a soluble solid from a liquid.

Example

Magnesium sulfate crystals are soluble in water. You can use evaporation to separate them because water evaporates leaving the magnesium sulfate crystals behind.

Science; Pure and impure substances; KS3 Year 7; Separating mixtures

1.	Solution
2.	Evaporating basin
3.	Bunsen burner
4.	Concentrated solution
5.	Crystallised solute

procedure

1.	Place a gauze on the tripod.
2.	Place a solution in an evaporating basin and heat using a Bunsen burner.
3.	Volume of solution will decrease and crystals will become visible at the bottom of the evaporating basin.
4.	Remove the evaporating basin with a pair of tongs. Caution: hot!

Paper chromatography

Paper chromatography is used to separate dissolved substances from one another.

Example

Paper chromatography can be used to separate dyes in ink.

Science; Pure and impure substances; KS3 Year 7; Separating mixtures

1.	$1\ cm$ height
2.	Base line
3.	Beaker
4.	Solvent
5.	Watch glass

PROCEDURE

1.	Draw a pencil line $1 \, cm$ above the bottom of the chromatography paper.
2.	Put spots of inks along the line ensuring they are not too close to each other.
3.	Place chromatography paper inside a beaker containing solvent.
4.	Cover beaker with a watch glass.
5.	Solvent will seep up the chromatography paper taking some the dye with it.
6.	Different dyes in the ink will travel up with the solvent at different speeds so will be placed at different points on the chromatography paper.
7.	Compare the pattern of spots of your unknown dyes with known references to identify which dyes are present

Simple distillation

Simple distillation is used to separate liquids from solids.

Example

Separating salt water.

Science; Pure and impure substances; KS3 Year 7; Separating mixtures

1.	Flame
2.	Mixture of liquids
3.	Flask
4.	Thermometer
5.	Cooling water going out
6.	Condenser
7.	Cooling water going in
8.	Distilled water in the beaker

PROCEDURE

1.	Heat mixture in a round bottom flask. The water will boil, forming steam.
2.	The steam will become cool and turns back to a liquid inside the condenser.
3.	Liquid water is collected in a beaker or vial.
4.	Salt is left behind in the flask.

Checking for purity

Pure chemical substances have fixed melting and boiling points. Substances can be identified by finding out the boiling or melting point and comparing it to reference values. If impurities are present it will change the melting and boiling point, so you can easily identify whether impurities are present or not.

Example

Pure water boils at $100\degree C$ . If you want to see if you have separated a mixture and obtained pure water measure the boiling point. If it boils at $100\degree C$ , it is pure water. If not, there are impurities present in the water.