Ch 2Solutions — Class 12 Chemistry, concept-first.

You are already familiar with mixtures. A mixture is a combination of two or more substances: air is a mixture of gases, rock is a mixture of two or more minerals, and so forth.

We generally think that a solution is either a solid dissolved in a liquid or a mixture of two liquids.

Chemists need to know the capacity of solutions to dissolve a solute. Suppose that a solute is added to a solvent. It dissolves readily at first.

The solubility of a solute is its amount per unit volume of saturated solution at a specific temperature.

The extent to which a substance dissolves in a solvent varies greatly with different substances. It depends on the nature of solute and solvent, temperature and pressure.

Consider a binary solution of two volatile liquids and . When the solution is placed in a closed container, both the liquids vaporize.

Raoult's law states that the partial vapour pressure of any volatile component of a solution is equal to the vapour pressure of the pure component multiplied by its mole fraction in the solution.

1. Ideal solutions : Solutions with the following properties are called ideal solutions.

The physical properties of solutions that depend on the number of solute particles in solution and not on their nature are called colligative properties. These are:

When a liquid in a closed container is in equilibrium with its vapours, the pressure exerted by the vapour on the liquid is its vapour pressure.

We saw in section 2.5.1 that Raoult's law expresses the quantitative relationship between the vapour pressure of a solution and the vapour pressure of the solvent.

The ratio of the vapour pressure lowering of the solvent divided by the vapour pressure of the pure solvent is called the relative lowering of vapour pressure. Thus,

We studied that the relative lowering of vapour pressure is equal to the mole fraction of the solute in the solution: from Eq. (2.6) it follows that (Eq. 2.8).

Recall that the boiling point of a liquid is the temperature at which its vapour pressure equals the applied pressure. For liquids in open containers, the applied pressure is atmospheric pressure.

To understand the elevation of boiling point, let us compare the vapour pressures of the solution and those of the pure solvent.

The boiling point elevation is directly proportional to the molality of the solution. Thus,

The Eq. (2.13) is .

Recall that the freezing point of a liquid is the temperature at which liquid and solid are in equilibrium and the two phases have the same vapour pressure.

The effect of dissolution of a nonvolatile solute on the freezing point of a solvent can be understood in terms of the vapour pressure lowering.

As verified experimentally, for a dilute solution the freezing point depression () is directly proportional to the molality of the solution. Thus,

Refer to Eq. (2.17), . The molality of the solution is given by Eq. (2.14) as

Besides the boiling point elevation and freezing point depression, the osmotic pressure is associated with vapour pressure lowering, and it can be used to determine molar masses of dissolved solutes.

When a solution and pure solvent, or two solutions of different concentrations, are separated by a semipermeable membrane, the solvent molecules pass through the membrane.

Osmosis can be demonstrated with the experimental set-up shown in Fig. 2.9. A semipermeable membrane is firmly fastened across the mouth of a thistle tube.

i. Isotonic solutions : Two or more solutions having the same osmotic pressure are said to be isotonic solutions.

For very dilute solutions, the osmotic pressure follows the equation

Consider Eq. (2.19), . If the mass of the solute in litres of solution is and its molar mass is , then . With this value of , Eq. (2.19) becomes

As mentioned earlier, osmosis is a flow of solvent through a semipermeable membrane into the solution. The direction of osmosis can be reversed by applying a pressure larger than the osmotic pressure.

Solutions of nonelectrolytes in water exhibit the colligative properties described in the preceding sections.

To obtain the colligative properties of electrolyte solutions by using the relations for nonelectrolytes, van't Hoff suggested a factor .

The expressions of colligative properties mentioned earlier for nonelectrolytes are to be modified so as to make them applicable for electrolyte solutions. The modified equations are:

The discussion of colligative properties of electrolytes in the preceding sections is based on the fact that the electrolytes are completely dissociated in their aqueous solutions.

Sample papers and previous-year board questions for this subject.