The capacity of double-effect evaporator is less than half of the capacity of two single effects, each of which is operating over same terminal temperature difference, when the

ANS:A - solution has an elevation of boiling point.

The term "elevation of boiling point" refers to the increase in the boiling point of a solution compared to the boiling point of the pure solvent. This elevation occurs when a non-volatile solute, such as a dissolved salt or sugar, is added to the solvent. Here's an explanation of why adding solute causes the boiling point to rise:

1. Raoult's Law:
   • According to Raoult's law, the vapor pressure of a solution is directly proportional to the mole fraction of the solvent in the solution.
   • When a non-volatile solute is added to a solvent, the mole fraction of the solvent decreases because the solute molecules occupy some of the space in the solution.
   • As a result, the vapor pressure of the solution decreases compared to the pure solvent.
   • Since boiling occurs when the vapor pressure of the liquid equals the external pressure, the boiling point of the solution increases to achieve this equilibrium.
2. Elevation of Boiling Point Formula:
   • The elevation of boiling point (ΔTb​) can be calculated using the formula: ΔTb​=i×Kb​×m
   • Where:
     • i is the van't Hoff factor, which represents the number of particles formed when the solute dissolves in the solvent.
     • Kb​ is the ebullioscopic constant, a property of the solvent.
     • m is the molality of the solution, the number of moles of solute per kilogram of solvent.
3. Significance in Evaporation:
   • In the context of evaporation processes, such as in evaporators or distillation units, understanding the elevation of boiling point is crucial for controlling the operating conditions.
   • As the boiling point of the solution increases, more heat energy is required to vaporize the solvent and separate it from the solute.
   • This elevation of boiling point affects the design and operation of evaporators, as it influences the temperature at which the solution must be heated to achieve evaporation.

In summary, the elevation of boiling point occurs when a non-volatile solute is added to a solvent, resulting in an increase in the boiling point of the solution compared to the pure solvent. This phenomenon has implications for various processes, including evaporation, where controlling the operating conditions is essential for efficient separation and processing.