Chapter 16.3, Problem 30LC

Interpretation Introduction

Interpretation: The reason for depression in the freezing point and elevation in the boiling point needs to be defined and described.

Concept introduction: The properties of a solution that depends on the number of solutes and not on the nature of the solute are known as colligative properties. The colligative properties are important to understand the properties of real solutions because the ideal solution does not exist.

Answer to Problem 30LC

Depression at the freezing point occurs because more kinetic energy needs to be taken out from the solution in order to solidify the solution and elevation at the boiling point occurs. After all, more kinetic energy needs to be provided to the solution to increasing the vapor pressure.

Explanation of Solution

Depression in the freezing point and the elevation in the boiling point are the colligative properties of the solution and the reason for this deviation from the ideal solution is described as follows:

1. Depression in freezing point: The freezing point is the temperature when the liquid starts freezing and gets converted to a solid state. When a liquid solidifies, the molecules form an ordered pattern and this pattern gets disrupts when a solute is added to the solvent. Hence, the solution now freezes at less freezing point than before and this change in freezing point is known as depression in freezing point. The depression is caused because more kinetic energy needs to be withdrawn from the solution to solidify the solution.
2. Elevation in boiling point: The boiling point is the temperature when the liquid starts boiling and gets converted to the vapor state. This happens when the atmospheric pressure becomes equal to the vapor pressure of the liquid. When a non-volatile solute is added to the solvent then, the boiling point of the solution increases. This is because a non-volatile solute decreases the vapor pressure as, fewer molecules go to its vapor state, and hence, more kinetic energy needs to be given to the solution to increase its vapor pressure therefore, the boiling point of the solution becomes more than that of the pure solvent. This temperature change is known as elevation in boiling point.

Conclusion

Depression at the freezing point occurs because more kinetic energy needs to be taken out from the solution to solidify the solution and elevation at the boiling point occurs. After all, more kinetic energy needs to be provided to the solution to increase the vapor pressure.