Chemistry 3 Marks Question

The freezing point depression occurs when a solute is added to a solvent is of a very small amount that cannot be measured by an ordinary thermometer because its minimum measurement is more than 0.01°C, whereas Beckmann made a special thermometer for this work which is very sensitive in which the minimum measurement is 0.01°C, due to which even a small amount of feezing point depression can be measured, hence the freezing point depression method, this special thermometer (Beckmann thermometer) is used instead of the ordinary thermometer.

In case of negative deviation in the mixtures of two liquids A and B, the force of attraction between A-A an B-B is weaker than that between A-B. Due to this vapour pressure decreases.
Example: Mixture of phenol and aniline. In this situation the intermolecular hydrogen bond between the phenolic proton and the lone pair of electron of the nitrogen atom of aniline is stronger than the hydrogen bond between similar molecules.
Similarly, the mixture of chloroform and acetone also shows negative deviation from Raoult's law. This is because the chloroform forms hydrogen bond with acetone.

When the is a positive deviation in the mixture of two liquids A and B, the A-B attraction is weaker than the attraction between A-A and B-B, that is, in this situation the intermolecular force between the solute solvent molecules is less than that of the solute-solute and solvent-solvent molecules. Therefore, form such solutions, molecules of A and B can escape more easily than the pure component, due to whih the vapour pressure increases.
Example: Mixture of pure ethanol and acetone, there are hydrogen bonds between the molecules. When acetone is added to pure ethanol, its molecules comes between the hydrogen bonds of ethanol molecule and breaks them. Due to this intermolecular forces of attraction, this mixture shows positive deviation from Raoult's law.