Separation of Substances: Methods for Separating Mixtures

Why separate substances?

Most things around us are mixtures: two or more substances mixed together. We separate them for three main reasons:

1. to remove unwanted or harmful parts (stones from rice, mud from water),

2. to get a useful part (grain from stalks, salt from seawater),

3. to obtain a pure substance.

The right method depends on how the parts of the mixture differ: in size, weight, colour, or whether they dissolve in water.

Handpicking

Small amounts of bigger, easily visible impurities, such as stones, husk or insects in rice or dal, can simply be picked out by hand. It works when the unwanted particles are few and look different from the rest.

Threshing

After harvesting, grains are attached to their stalks. Threshing separates them by beating the stalks. Farmers do this by hand, with bullocks, or with threshing machines.

Winnowing

Winnowing separates heavier and lighter parts with the help of wind. The mixture of grain and husk is dropped from a height. The wind carries the light husk away to form a separate heap, while the heavier grain falls straight down.

Sieving

A sieve has holes of a particular size. Fine particles pass through and bigger ones stay behind. Sieving is used to remove bran from flour at home, and pebbles from sand at construction sites.

Sedimentation and decantation

When muddy water is left to stand, the heavy mud settles at the bottom. This settling is called sedimentation, and the settled layer is the sediment.

The clearer water on top can then be gently poured off into another container without disturbing the sediment. This is decantation. The same idea is used when washing rice: the dust and husk float or stay suspended and are poured away.

Filtration

Decanted water may still contain very fine particles. Filtration removes them by passing the liquid through a filter that lets water through but stops solid particles.

In the laboratory, filter paper is folded into a cone and placed in a funnel. The solid left on the paper is the residue, and the clear liquid that passes through is the filtrate. A muslin cloth used to strain tea leaves or paneer works on the same principle.

Evaporation

Some substances dissolve completely in water, so filtering cannot separate them. Salt in water is an example. When the solution is heated, or left in the sun, the water turns into vapour (evaporation) and the salt is left behind.

This is how salt is obtained from seawater. Seawater is let into shallow pits, the sun's heat evaporates the water over days, and the salt is collected and purified.

Using more than one method

Real mixtures often need several methods one after another.

Example: separating a mixture of sand and salt.

1. Add water and stir. The salt dissolves; the sand does not.

2. Filter. The sand stays on the filter paper as the residue.

3. Evaporate the filtrate. The salt is left behind.

How much can water dissolve?

A solution that cannot dissolve any more of a substance at a given temperature is saturated. Heating usually allows more of a solid to dissolve. If a hot saturated solution cools, the extra substance separates out again.

Summary