There are various ways of classification of substances in chemistry. At this level, we classify substances based on their composition and properties. Substances are classified as pure substances and mixtures.

Mixtures

This salt solution is a mixture because of the following main reasons:

• The components, salt and water, are simply physically mixed. i.e they are not chemically combined.
• This salt solution retains many of the properties of its constituents, sodium chloride and water. In other words, the salt solution shows the properties of the salt and the water. For example, the solution is colorless like water, and also it tastes salty like the original sodium chloride salt.
• The salt solution can be separated into salt and water by physical processes like evaporation or distillation.

Separation of Mixtures and its Application

Most of the substances around us exist in the form of mixtures. However, these mixtures can be separated into pure substances using various separation techniques. The methods used to separate mixtures are mainly physical processes. This is because the substances in a mixture are physically combined.

The most important separation techniques for a given mixture depend on: the type of mixture, and which substance in a mixture we are more interested in. For example, in a mixture of salt solution, if we need to separate only the salt, evaporation can be used.

We will discuss some of the methods that are used to separate the components of solid -solid, solid-liquid and liquid-liquid mixtures.

Separation of Mixture of two Solids (solid – solid mixtures)

1. Mechanical Picking or Handpicking:
   In this method, components are separated on the basis of physical characteristics like size and shape. Example- Removing stones from grains.
2. By using a suitable solvent:
   Here, a mixture of two solids can be separated from each other by using a solvent which will make one of the substances soluble in the solvent, thus separating the two substances. E.g. A mixture of sugar and sand can be separated by using water as a solvent as sugar is soluble in water and sand is insoluble in water.
3. Separation by sublimation:
   The changing of a solid directly into vapours on heating, and of vapours into solid on cooling is called sublimation. The process of sublimation is used to separate those substances from a mixture that sublime on heating. Examples: Ammonium chloride, iodine, camphor, naphthalene etc.

Separation of Mixture of Solid and Liquid

A mixture of solid and liquid can be found in a homogeneous solution or heterogeneous mixture. The liquid in which the solid substance is mixed or gets dissolved is known as a solvent. Whereas, the dissolving solid constituent is known as solute.

Separation of Liquid – liquid mixtures

Distillation

Distillation is a fast way to separate mixtures of two or more pure liquids. Distillation is a purification method that involves vaporizing the constituents of a liquid mixture, then condensing and isolating them. The vapor condenses back into liquid after passing through a cooled tube (a condenser). The collected condensate is known as distillate.

A mixture of two miscible liquids can also be separated by simple distillation. Liquids which mix with each other to form a solution are called miscible liquids. The mixture of alcohol and water, benzene and oil are some examples of miscible liquids.

Consider a mixture of alcohol and water. Ethanol, which is an alcohol boils at 78°C, and water boils at 100°C. When the mixture is heated, the alcohol, which has the lower boiling point vaporizes more rapidly than the water. The vapor of alcohol passes through the condenser and then collected as a distillate in the receiver.