Question

Explain why, on addition of 1 mol of KCl to 1 litre of water, the boiling point of water increases, while the addition of 1 mol of methyl alcohol to 1 litre of water decreases the boiling point.

Hint:

Non-volatile solutes elevate the boiling point of a solvent, whereas volatile solutes may lower the boiling point if they increase the total vapour pressure of the solution.

Answer 1

Concept: The boiling point of a liquid depends upon its vapour pressure. A liquid boils when its vapour pressure becomes equal to the atmospheric pressure. According to the colligative properties of solutions, the addition of a non-volatile solute lowers the vapour pressure of the solvent. As a consequence, a higher temperature is required to make the vapour pressure equal to the atmospheric pressure, resulting in an elevation of boiling point. However, when a volatile substance is added to a solvent, the total vapour pressure of the solution may increase, causing the boiling point to decrease. Thus, the effect on boiling point depends upon whether the added substance is volatile or non-volatile.

Step 1: Effect of adding KCl to water. Potassium chloride (KCl) is an ionic compound and is non-volatile in nature. When KCl is dissolved in water, it dissociates into ions: \[ KCl \rightarrow K^+ + Cl^- \] These ions occupy positions among water molecules and reduce the tendency of water molecules to escape from the liquid surface. As a result, the vapour pressure of water decreases. According to Raoult's law: \[ P_{\text{solution}} \lt P^\circ_{\text{water}} \] where \[ P^\circ_{\text{water}} \] is the vapour pressure of pure water. Since the vapour pressure becomes lower, a higher temperature is needed for the solution to boil. Therefore, the boiling point increases. This phenomenon is called

elevation of boiling point. Mathematically, \[ \Delta T_b = iK_bm \] where \[ i = \text{van't Hoff factor} \] For KCl: \[ i \approx 2 \] because it produces two ions in solution. Hence the increase in boiling point is even more significant.

Step 2: Effect of adding methyl alcohol to water. Methyl alcohol (methanol), represented as \[ CH_3OH \] is a volatile liquid. Unlike KCl, methanol itself possesses appreciable vapour pressure. When methanol is added to water, both water molecules and methanol molecules contribute to the vapour phase. Therefore, the total vapour pressure of the solution becomes: \[ P_{\text{total}} = P_{\text{water}} + P_{\text{methanol}} \] The presence of volatile methanol increases the total vapour pressure of the solution. Since the vapour pressure becomes higher, the solution can attain atmospheric pressure at a lower temperature. Consequently, the boiling point decreases.

Step 3: Comparison of the two cases. For KCl: Non-volatile solute \[ \Downarrow \] Vapour pressure decreases \[ \Downarrow \] Boiling point increases For methyl alcohol: Volatile solute \[ \Downarrow \] Total vapour pressure increases \[ \Downarrow \] Boiling point decreases

Conclusion: The addition of KCl increases the boiling point because KCl is a non-volatile electrolyte that lowers the vapour pressure of water. On the other hand, methyl alcohol is a volatile liquid that increases the total vapour pressure of the solution, thereby decreasing its boiling point.