Page 42 - ICSE Chemistry 6

• To obtain pure sample of a substance: We need many substances in their purest form to make
medicines, in laboratories, etc. For these purposes, all the impuri es need to be removed. For
example, all the impuri es need to be removed from water as water in its purest form is required
to prepare medicines, in laboratories and car ba eries.

Principle of Separation

The principle of separa on depends upon the type of mixture and the proper es of components of mixture.
In some mixtures such as a mixture of sugar crystals and sand par cles, components can be dis nguished
on the basis of visual observa on. In some other cases, the components of a mixture cannot be seen. For
example, if sugar is dissolved in water, a solu on is formed. In such a mixture, the two cannot be seen
separately. The method of separa on in all these examples diff ers and depends upon the components of
the mixture. The size of the component par cles is also an important factor in the separa on of mixtures.

Mixtures are also separated on the basis of some property that one component has and the other does
not have. For example, to separate a mixture of iron fi lings and sulphur, we can use a magnet as only iron
gets a racted towards the magnet.

Sieving

You must have no ced that your mother si s fl our using a sieve to
remove bran. A sieve consists of a mesh placed over a frame.

Sieving helps to separate components of a mixture that diff er in size.
While sieving, the fi ne par cles pass through the sieve whereas the
coarse par cles are le behind on the sieve. Depending on the size
and type of the components to be separated, sieves with diff erent
size of holes are used. For example, the sieve used at a construc on
site has larger holes in the mesh because it is used to separate Fig. 2.7: Sieving
stones and pebbles from sand.

Sedimentation and Decantation

When a mixture of insoluble substance and water is kept
undisturbed for some me, then the suspended insoluble Soluble
par cles se le down at the bo om under the infl uence of A substance which can be dissolved is
gravity. This process is called sedimenta on. The insoluble called soluble.
solid substance that se les down is called sediment and
Insoluble
the clear liquid over the sediment is called supernatant
A substance which cannot be dissolved
liquid. The clear liquid is then poured into another beaker, is called insoluble.
leaving the sediments undisturbed. This process is called
decanta on.

Sedimenta on and decanta on are used to separate a mixture of a solid and a liquid where the solid
component is heavier than the liquid component, and is insoluble, for example, a mixture of sand and
water.

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