The solutions which obey Raoult's law over the entire range of concentration are known as ideal solutions. For an ideal solution $\Delta V_{\text {mix }}=O$ and $\Delta V_{\text {mix }}=O$. The ideal behaviour of the solutions can be explained by considering two components $A$ and $B$.

In pure components, the intermolecular attractive interactions will be of $A-A$ type and $B-B$ type, whereas in the binary solutions in addition to these two, $A-B$ type of interaction will also be present. If $A-A$ and $B-B$ intermolecular forces are nearly equal to those between $A-B$, this leads to the formation of ideal solution e.g., solution of $n$-hexane and $n$-heptane. When a solution does not obey-Raoult's law over the entire range of concentration, then it is called non-ideal solution. The vapour pressure of such a solution is either higher or lower, than that predicted by Raoult's law.

If it is higher, the solution exhibits positive deviation and if it is lower it exhibits negative deviation from Raoult's law. In case of positive deviation, $A-B$ interactions are weaker than those between $A-A$ or $B$ - B. i.e., the attractive forces between solute solvent molecules are weaker than those between solute-solute and solvent-solvent molecules e.g., mixture of ethanol and acetone.

For such solutions $\quad \Delta H_{\text {mixing }}=+$ ve and $\Delta V_{\text {mixing }}=+$ ve

On the other hand, in case of negative deviation the intermolecular attractive forces between $A-A$ and $B-B$ are weaker than those between $A-B$ molecules. Thus, the escaping tendency of $A$ and $B$ types of molecules from the solution becomes less than from the pure liquids i.e., mixture of chloroform and acetone.

For such solution $$\quad \Delta H_{\text {mix }}=-\mathrm{ve} \text { and } \Delta V_{\text {mix }}=-\mathrm{ve}$$

The solution or mixture having same composition in liquid as well as in vapour phase and boils at a constant temperature is known as azeotropes. Due to constant composition it can't be separated by fractional distillation. There are two types of azeotropes

(i) Minimum boiling azeotropes Solutions which show large positive deviation from Raoult's law form minimum boiling azeotropes at a specific composition. e.g., ethanol -water mixture

(ii) Maximum boiling azeotropes Solutions which show large negative deviation from Raoult's law form maximum boiling azeotropes. e.g., solution having composition 68$\%$ $\mathrm{HNO}_3$ and $32 \%$ water by mass.

This phenomenon is called endo osmosis, i.e., movement of water inside the raisin and shown with the help of diagram as

The process of osmosis is of immense biological as well as industrial important. It is evident from the following examples.

(i) Movement of water from soil into plant roots and subsequently into upper portion of the plant is partly due to osmosis.

(ii) Preservation of meat against bacterial action by addition of salt.

(iii) Preservation of fruits against bacterial action by adding sugar. Bacterium in canned fruit loses water through the process of osmosis and become inactive.

(iv) Reverse-osmosis is used in desalination of water.

(i) In animals, water moves into different parts of the body under the effect of the process of osmosis.

(ii) Stretching of leaves, flower, etc., is also controlled by osmosis.

(iii) Osmosis helps in rapid growth of the plants and germination of seeds.

(iv) Different movements of plants such as opening and closing of flowers etc, are controlled by osmosis.

When egg is placed in mineral acid solution outershell of egg dissolves. Egg is now removed and placed in hypertonic solution. Size of egg get reduced and egg shrivels due to osmosis. Egg is now placed in a bottle with narrow neck. Finally on adding hypotonic solution egg regain its shape due to osmosis.

Diagramatically it can be represented as