2 January 2014

Separation of Substances

Methods of Separation

Separation of substances  can be carried out under two conditions
  1. Dry conditions
  2. Wet conditions

Separation of substances under dry conditions:

There are various methods for separating substances, such as handpicking, threshing, winnowing and sieving.


It involves simply picking out substances by hand and separating them from others. The substances being separated may be impurities that have to be thrown away. Or it may be that both the substances being separated are useful – such as if you separate green grapes from black ones from a mixture of the two.


This is done after harvesting of crops. Once wheat is harvested, the stalks are dried. Then, each grain of wheat is separated from the stalks to which it is attached, so that it can be ground into flour. This method of separating grains from stalks is called threshing.


Winnowing follows threshing and is used to separate heavier and lighter components of a mixture by wind or by blowing air. The husk is separated from the grains by winnowing.


The method used to separate the wheat bran from the flour is called sieving. Sieving is a method of separating substances that are of different sizes.Separation of wheat bran from flour is performed by sieving.

Grains can be separated from dirt by sedimentation and decantation. When water is added to the grains, the dust and dirt dissolve in the water, making it muddy. Thus, the dirt and grains are separated.

Methods of separation of substances under wet conditions:


The grains settle at the bottom because they are heavier. This process of the settling of the heavier substance is called sedimentation.
Now, the grains can be separated by simply pouring out the water. This process is called decantation.
  • Oil and water can also be separated by decantation and sedimentation. Water is heavier than oil, so it settles at the bottom if left alone for some time. The oil can then simply be poured out.
  • The water left behind can be further cleaned using a filter paper. While water passes through the fine pores of the filter, dirt will sift out, leaving the water clean.This process of using a filter to separate substances is called filtration
  • Salt and water from salty water can be separated by evaporation. We need to boil this water so that its temperature rises and it converts into steam. This process is called evaporation
  • The steam turns back into water when it comes in contact with a cold metal lid. This process is called condensation.

A mixture of sand and salt can be separated by combination of methods. 

The first method is sedimentation and decantation. This mixture is put in water and left for the sand to settle for some time. Then, we will decant the salty water, which will separate the sand from the mixture.
Now the salt can be separated from the water by evaporation. The water will boil away, leaving the salt behind.
So, the mixture of the sand, salt and water has been separated successfully using a combination of sedimentation, decantation, evaporation and condensation.