Class 6 Science Chapter 9: Methods of Separation in Everyday Life Notes

What is a mixture, and why do we separate it?

When two or more substances are mixed together, we get a mixture: stones in grains, bran in flour, salt in seawater. In the chapter, Malli and Valli travel across India, from Haryana to Ahmedabad, Puducherry, Bhopal and Shillong, and see mixtures being separated everywhere. We separate a mixture for two reasons: to remove a part that is not useful, or to separate two useful parts. The method depends on how the parts differ.

Handpicking and threshing

Handpicking picks out unwanted pieces by hand, like small stones and husk from wheat, using differences in size, colour and shape. It suits a small number of pieces that are easy to pick. Threshing separates the grains from harvested stalks, for example by beating the stalks on a wooden log. See handpicking, threshing and winnowing.

Winnowing

After threshing, the grains are mixed with light husk. The farmer stands on a raised platform and tilts a soop (bamboo tray) in the direction of the wind. The light husk is blown aside and the heavy grains fall straight down. This is winnowing. Machines called threshers do threshing and winnowing together.

Sieving

A sieve lets fine particles pass through its holes and holds back bigger ones. It removes bran and small stones from flour, and at construction sites it separates pebbles from sand. Sieving is used for solid–solid mixtures whose particles have different sizes.

Evaporation

Salt dissolved in water cannot be filtered out. To get it back, the water is removed as vapour: this is evaporation, the change of a liquid into its vapour. Seawater is kept in shallow pits in the sun and air; in a few days the water evaporates and salt is left behind, which is then purified. Cooling the vapour so that it condenses gives the water back too. See separating salt by evaporation.

Sedimentation, decantation and filtration

Leave a vessel of tea undisturbed and the tea leaves settle: this is sedimentation. Tilting the vessel to pour off the tea is decantation, which also separates oil from water. A few leaves still get through, so we use a strainer: filtration. Cloth and filter paper have smaller pores and can clean muddy water, leaving the mud as residue and giving clear filtrate. See sieving, sedimentation, decantation and filtration.

Churning and magnetic separation

In Bhopal, a painting shows a woman churning curd with a big mathni. The lighter butter floats to the top and buttermilk is left behind. In Shillong, a carpenter drops iron nails in sawdust; a magnet moved through the sawdust picks up all the nails. Using a magnet to separate magnetic substances like iron is magnetic separation. Recyclers use magnets fitted to cranes to pick scrap iron out of heaps of waste.

Choosing the right method

Method	Works because of	Example
Handpicking	Size, colour, shape	Stones from grains
Winnowing	Weight (light and heavy)	Husk from grains
Sieving	Particle size	Bran from flour
Sedimentation and decantation	Heavier part settles	Tea leaves, oil and water
Filtration	Pore size of the filter	Mud from water
Evaporation	Liquid evaporates, solid stays	Salt from seawater
Churning	Butter is lighter	Butter from curd
Magnetic separation	Attraction to a magnet	Iron nails from sawdust

A mixture with many parts usually needs several of these methods, one after another.