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Chemistry - Separating Mixtures

How to separate a solid from a liquid:

These four methods are commonly used to separate a solid from a liquid:

Filtering

Evaporating

Crystallizing

Centrifuging

Filtering

If a solid does nor dissolve in water, for example chalk, then separate the solid from the liquid by filtering the suspension through filter paper.

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

Evaporating

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

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

Crystallizing

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

If the solution is saturated, then when the solvent evaporates, what is left behind cannot hang on to as much of the solute, so the solute leaves the solution and forms crystals.

Centrifuging

A centrifuging 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.

How to separate a mixture of 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 left to do is o heat the salty water, allowing the water to evaporate leaving behind the salt.

How to separate two liquids

If two liquids are miscible (mix together well), they can be separated using fractional distillation.

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

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

Once the beads are 78 ° C, 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.

How to separate a mixture of coloured substances

Paper chromatography

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

Water is then dropped onto the ink.

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

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

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

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