CBSE Class 6 Science Separation of Substances Exam Notes

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Separation of Substances

There are large number of materials such as water, sugar, salt, milk, tea, fruits, vegetables, air, etc. Around us, we use these materials daily. In some of these substances we take only the useful components by separating them from unuseful components. For example, we filter the tea before drinking it to separate tea leaves from the tea. Similarly, we peel the skins of many fruits and vegetables before eating them. We also take out butter from milk by churning the milk.

A pure substance is one which is made up of only one kind of a substance or substances. It has a definite, well-defined set of properties. It has the same composition throughout. All elements and compounds are examples of pure substances.

A mixture is made up of two or more substances. These substances are called the constituents or components of the mixture. The components may be mixed in any proportion. The individual constituents of a mixture retain their own properties. These components can be separated by a simple physical process.
All mixture are heterogeneous except solutions, which are homogeneous. The mixture of water and sugar is homogeneous.

The process by which unwwanted or harmful components of a mixture are removed to get a pure substance is called separation. We usually separate the components of a mixture to
1. Remove undesirable or unwanted components
2. Get a pure sample of a substance
3. Extract useful components from the mixture
4. Remove harmful components

Each constituent of a mixture has some property unique to itself and is shared by no other components. This difference in property is utilised to separate the various components from the mixture.

Various methods are used for separating the components of a mixture. The choice of the method to be used depends upon the nature and composition of the mixture. Sometimes we might use a combination of various methods. Now, let us study some methods commonly used by us for separating the components of a mixture.

Different methods are used by us at home for cleaning food. Hand picking and sieving are commonly practised by your mother to clean food. Now, let us learn about these mehtods.

This is the simplest method of separating unwanted substances from the useful ones. Here we take advantage of difference in size, shape, or colour of different substances. Pulses, wheat, and rice are quite often mixed with small stones and insects. Housewives often clean pulses and spices by this method. They remove small pebbles, dust, dirt and other unwanted materials. This method can be used only where one of the components is present in small quantity.



Threshing is the process of beating out the grain from the chaff. The harvested crop is threshed to separate the grain from its outer covering called chaff. The threshing of paddy is traditionally carried out by beating the harvested crop with sticks. On the other hand, the threshing of wheat and other crops is carried out by treading the crop with animals such as bullocks and buffaloes. These methods are limited to very small farmers. However, in case of big fields, threshing is done by using threshers. The mothod of using threshers is not only economical but also quicker and more efficient than treading with animals.

this method is generally used by farmers to separate grain from husk after threshing. The farmer allows the mixture of grain and the husk to fall from a height. The grains which are heavier fall vertically down on the ground. The husk which is lighter, is carried away by the wind and forms
a separate heap at a short distance from the heap of grains as shown. This process is called winnowing.


Sedimentation and Decantation

This method is used for separating an insoluble solid from a liquid. It is not a good method as the complete separation of solid from liquid is not possible. Why do doctors often instruct us to shake the bottle containing liquid medicine before use ? It is because some solid particle in these liquid medicine settle down at the bottom of the bottle. To learn more about these two methods let us perform.

Another simple method for removing suspended impurities from the liquid is filtration. It is the process of removing an isoluble substance from a liquid by a looowing it to pass through a filter paper or muslin cloth. The filter paper is similar to blotting paper. It contains many small holes. These holes are large enough to allow water to pass through but do not allow particles of insoluble substances like chalk powder, caly, sulphur, etc. Filtration is often used in our homes, laboratories and industries. Preparing tea or coffee involves filtration. While making cottage cheese (paneer) at home, solid paneer is separated from the liquid by filtration through a muslin cloth. Our water supply is filtered through a bed of gravel, sand, and charcoal. The sewage water is also filtered by using large metallic meshes to avoid choking of sewers.

The process in which a liquid changes into its vapour state is called evaporation. This method is used to separate solids dissolved in a liquid. This process is used on a large scale to obtain common salt from sea water. Sea water which contains dissolved salts is allowed to flow into shallow pools. The heat from the Sun eveporates the water and the solid salt is left behind.

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