Introduction

Introduction

You have in an earlier chapter about pure substances and mixtures. Pure substances are either elements or compounds. They are rarely found on their own in nature. They usually mix to from mixture. For example, air is a mixture of gases like oxygen, nitrogen, carbon dioxide and water. Tap water is a mixture–it contains water and many dissolved salts. Milk is mixture–it contains water, cream and other things. Wood is a mixture containing mainly cellulose and water.

Thus, mixture contains molecules of two or more substances. The pure substance that make up a mixture are called components or the constituents of the mixture.

Man has made many mixture that are more useful than pure substances. For example during making jewellery, gold is mixture with copper and solver to make it more stronger and durable. Medicine, soaps detergents, fertilizers, pesticides etc are also mixtures.

Types of Mixtures

Mixture are usually classified on the basis of the state on the constituents. If two or more solids are mixed, the resulting mixture is called a solid mixture. Example mixture of sand and salt. Similarly mixture of two or more liquids is called liquid mixture and that of two or more gases is known as gaseous mixture. Besides, we can also have solid liquid; solid gas and liquid mixtures.

These mixture are further broadly classified into two categories:

1.         Homogeneous mixture

2.         Heterogeneous mixture

In homogeneous mixture, the individual components or constituents are not visible. For example sugar dissolved in water or milk in water.

Whereas in heterogeneous mixture, the constituents doe not gets mixture up completely and hence they are clearly visible. For example, sugar crystals mixed with sand particles; chalk powder mixed with water. Fruit salad is a common example of heterogeneous mixture.

Table Types of Mixture

Illustration 1:

What is an alloy?

Sol:      Alloy is a homogeneous mixture of two or more metals or a metal and non metals e.g. Brass (Copper + Zinc).

Illustration 2:

What is a mist?

Sol.      A mist is a heterogeneous mixture of droplets of water   with air.

Properties of Mixtures

1.         The constituents of a mixture may be in any ratio

2.         They retain their individual properties

3.         The constituents can be separated by simple methods.

Need For Separation

The constituents of mixture need to be separated for the following reasons:

1.         To remove the undesirable constituents: For example, we need to separate impurities from ordinary water to make it potable (safe for drinking). We must remove husk, Stones and dirt from food grains before we cook them.

(i)   Small stones present in rice can be harmful to our teeth and body. It is necessary to remove them before cooking the rice. Tea leaves are separated from tea by using a strainer.

Sometimes, harmful substance are present in river water. They must be removed before supplying that water to our homes. This is done at water purification plants.

(ii)  Butter can be obtained from milk or curd by changing it. Crude oil or petroleum ia mixture of several useful constituents, petrol, kerosene, diesel, tar etc are obtained by separating them for crude oil.

(iii) Also, sometimes substances in their pure from are needed for use at home or for research work and other purposes.

2.         To obtain desirable substance: For example, we obtain common salt from sea water, which contains many substances including salt.

3.         To obtain highly pure substances: For example, from ordinary water, we obtain highly pure samples of water for medicinal purposes.

Illustration 4:

Why is separation of mixtures necessary ?

Sol.      (a) Remove undesirable constituents

(b) Obtain useful constituents

(c) Obtain pure substances