Separating Mixtures

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Four methods are commonly used to separate a solid from a liquid:

1. Filtering

2. Evaporating

3. Crystallizing

4. Centrifuging

Filtering

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If your solid does not dissolve in water, for example chalk, then you can separate the solid from the liquid by filtering the suspension through filter paper.

In this example the chalk that remains on the filter paper is called the residue. The water (the substance) that passes through the filter paper is called the filtrate.

Evaporating

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If your mixture is a solution, such as salty water, then filtering will not separate the salt from the water.

Instead, by heating the soluton the solvent (water) evaporates leaving the solid (salt) behind. This is possible since the two substances have different boiling points.

Centrifuging

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A centrifuge is used to separate small amounts of solid held in suspension from the liquid. For example, chalk from water.

The centrifuge contains test-tubes that are spun around at high speed that causes the solid to sink to the bottom of the tube. The liquid is the decanted (poured off) leaving the solid behind.

Crystallising

You can separate many solids contained in saturated solutions by leaving them to form crystals. This process is called crystallisation.

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If the solution is saturated, then when the solvent evaporates, what's left behind can't hang on to as much of the solute. So the solute leaves the solution and forms crystals.

By dissolving one of the two solids

If you have a mixture of salt and sand, then by placing the mixture in water you will find that the salt dissolves but the sand remains.

If this new mixture is then filtered, the salt in the salty water solution passes through the filter paper to form the filtrate and the sand remains as the residue.

All that is now left to do is to heat the salty water, allowing the water to evaporate leaving behind the salt.

How to separate the solvent from solution

Simple distillation

For example: to obtain pure water from saltwater, this apparatus would be used.

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The solution is heated in the round-bottomed flask. As it boils, steam rises into the condenser (this cools the steam back to water). Eventually the salt (solute) is left behind. The water collected in the beaker is called distilled water.

Fractional distillation

If two liquids are miscible (i.e. they mix together well), they can be separated using this apparatus.

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For example: to separate a mixture of ethanol and water.

Since, ethanol boils at 78oC and water at 100oC, then by gradually heating the mixture, ethanol and water vapour rises up the column making the glass beads hot as they condense on them.

Once the beads are 78oC, the ethanol vapour is forced into the condenser, whilst the water vapour continues to condense and drip back into the flask. Mean while the ethanol, now in the condenser, condenses and drips into the beaker as liquid ethanol.

Separating funnel

If two liquids are immiscible, then a separating funnel is used.

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For example: If you pour a mixture of oil and water into the funnel, the oil floats on top of the water. All that is left to do is for the tap to be opened to allow the water to pour through. The tap is closed once all the water has passed.

Paper chromatography

For example: separating the coloured substances in black ink - the apparatus below could be used.

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1. A small drop of black ink is placed into the centre of the filter paper.

2. Water is then dropped onto the ink.

3. The ink slowly spreads out, separating into rings of different colours.

4. The filter paper with its coloured rings is called a chromatogram.

5. The coloured substance furthest from the original black ink spot is the most soluble.

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