question_answer 26)

What is meant by positive and negative deviations from Raoult's law and how is the sign of  related to positive and negative deviations from Raoult's law?

Answer:

If a pure liquid is covered with a bell-jar, a part of the liquid evaporates and fills the available space with the vapours. At a particular temperature an equilibrium is established between the vapours and the liquid. The pressure exerted by the vapours in equilibrium with its liquid at a given temperature is known as vapour pressure of the liquid at that temperature. Vapour pressure depends upon: (i) The nature of the liquid and (ii) Temperature. When a non-volatile solute is dissolved in a solvent, then some molecules of the molecules on the surface are replaced by the molecules of the solute. Consequently, the number of solvent particles on the surface decreases and this is the cause of the lowering of vapor pressure of the solution.