Methods of Separation in Everyday Life

When two or more substances are mixed together without forming a new substance, we get a mixture — for example, rice with small stones, or tea leaves in tea. In daily life we often need to separate the things in a mixture. We do this to remove harmful or useless parts (stones from rice), to get a useful part (cream from milk), or to remove impurities (mud from water).

The method we choose depends on the properties of the things being separated — their size, weight, whether they dissolve, and so on. Two simple hand methods are:

• Hand-picking — simply picking out the unwanted bits by hand. It works when the pieces are large enough to see and few enough to remove, and look different — like taking stones, husk or insects out of rice, wheat or pulses.
• Winnowing — separating heavier grains from lighter husk using wind or by blowing air. The mixture is allowed to fall from a height; the wind carries the light husk away to one side, while the heavier grain falls straight down into a heap. Farmers use winnowing to separate grain from chaff after harvesting.

Some mixtures are of a solid with a liquid, where the solid does not dissolve — like muddy water. To separate these we use settling and straining methods.

• Sedimentation — when a mixture of an insoluble solid and a liquid is left undisturbed, the heavier solid particles slowly settle to the bottom. The settled solid is called sediment. For example, mud settles at the bottom of a glass of muddy water.
• Decantation — after the solid has settled, the clear liquid on top is gently poured off into another container without disturbing the sediment. Sedimentation is almost always followed by decantation.
• Filtration — passing a mixture through a filter (such as filter paper, a cloth or a sieve) that has tiny holes. The liquid and very fine particles pass through, while the larger insoluble solid is trapped on the filter. The clear liquid that passes through is called the filtrate, and the solid left behind is the residue.

We use these methods every day: tea is poured through a strainer to hold back the leaves (filtration); and at home, muddy water can be cleaned by letting the mud settle (sedimentation), pouring off the clear water (decantation) and then filtering it.

The methods so far separate insoluble solids from liquids. But what if a solid is dissolved in a liquid, like salt in water? You cannot filter it out, because the dissolved salt passes straight through the filter with the water. For this we use evaporation.

In evaporation, the solution (for example salt water) is heated gently or left in the Sun. The water turns into vapour and escapes into the air, leaving the solid behind. This is exactly how common salt is obtained from sea water: sea water is collected in shallow pans, the Sun evaporates the water, and the salt is left behind to be collected. This is called solar evaporation.

Often we use more than one method together to fully separate a mixture. For example, to get clean salt and clean water from a mixture of sand, salt and water: first filter to remove the insoluble sand (the residue), then evaporate the salt water to obtain the salt. Choosing the right method — or combination of methods — for each property of a mixture is the key skill of this chapter.