Q.Assertion: When methyl alcohol is added to water, boiling point of water increases. Reason: When a volatile solute is added to a volatile solvent elevation in boiling point is observed. Choose the correct option:

The key idea is that adding a volatile solute (methyl alcohol) to a volatile solvent (water) does not always raise the boiling point — it depends on the relative volatilities. Henry’s Law governs the partial pressures of both components in the vapour phase.

Reasoning:

1. Methyl alcohol (methanol) is more volatile than water (lower boiling point, higher vapour pressure at a given temperature).
2. When added to water, the vapour pressure of the solution is not simply lowered — it becomes a mixture of both vapours. The total vapour pressure may actually be higher than that of pure water at the same temperature. …

The core idea: Henry’s Law and vapour pressure

Boiling happens when the vapour pressure of the liquid equals the external (atmospheric) pressure.

Adding a non-volatile solute (like salt or sugar) to a solvent lowers the solvent’s vapour pressure — so you need a higher temperature to reach atmospheric pressure. That’s the familiar elevation in boiling point.

But here, methyl alcohol (methanol) is volatile — it has its own significant vapour pressure. When you add a volatile solute to a volatile solvent, the total vapour pressure above the mixture is the sum of the partial pressures of both components (Raoult’s law for ideal mixtures). That sum can be higher than the vapour pressure of pure water, not lower.

Step-by-step reasoning

1. What happens to the boiling point when you add methanol to water?

   Methanol has a much higher vapour pressure than water at any given temperature (it boils at $65^\circ\text{C}$, water at $100^\circ\text{C}$). Adding methanol to water increases the total vapour pressure of the mixture above that of pure water.

   Since the mixture now reaches atmospheric pressure at a lower temperature, the boiling point decreases.

   So the assertion — “boiling point of water increases” — is false.

2. Is the reason statement correct?

   The reason says: “When a volatile solute is added to a volatile solvent, elevation in boiling point is observed.” …

Concept: Colligative Properties & Volatile Solutes

The relevant concept is Raoult’s Law for volatile solutes and how it affects boiling point. A colligative property depends only on the number of solute particles, not their identity — but this holds strictly for non-volatile solutes. When the solute is volatile, both solute and solvent contribute to the vapour pressure, changing the boiling point behaviour.

Method: Conceptual Analysis of Boiling Point Elevation with Volatile Solute

Step 1: Understand the Assertion

“When methyl alcohol is added to water, boiling point of water increases.”

• Methyl alcohol (methanol, $\ce{CH3OH}$) is volatile (it boils at $64.7^\circ\text{C}$).
• Water boils at $100^\circ\text{C}$.
• When methanol is added, the vapour pressure of the solution is the sum of partial pressures of both components (Raoult’s law for ideal solutions).
• Methanol has a higher vapour pressure than water at a given temperature.
• So, the solution’s total vapour pressure is higher than pure water’s.
• A higher vapour pressure means the solution boils at a lower temperature, not higher.

Conclusion: Assertion is incorrect.

Step 2: Understand the Reason

“When a volatile solute is added to a volatile solvent, elevation in boiling point is observed.”

• For a non-volatile solute, boiling point elevation occurs (vapour pressure decreases).
• For a volatile solute, the vapour pressure of the solution may increase or decrease depending on the relative volatilities.
• In general, adding a volatile solute does not guarantee boiling point elevation — it can cause depression of boiling point. …

Here’s a breakdown of the common mistakes students make with this question, and how to avoid each.

The Core Concept: Colligative Properties & Volatile Solutes

This question tests your understanding of colligative properties — properties that depend on the number of solute particles, not their identity. The key twist here is that both the solute and solvent are volatile.

• Normal boiling point elevation: Adding a non-volatile solute (like salt) to a volatile solvent (like water) raises the boiling point. The solute particles block solvent molecules from escaping, so you need a higher temperature to boil.
• Volatile solute + volatile solvent: If the solute is also volatile (like methyl alcohol), it lowers the partial pressure of the solvent, but it adds its own vapor pressure. The net effect on the boiling point depends on the relative volatilities. In the case of methyl alcohol (more volatile than water) added to water, the boiling point of the solution decreases (or remains nearly the same, but definitely does not increase).

Common Mistake #1: Assuming all solutes raise the boiling point

The Mistake: Students blindly apply the rule “adding a solute raises the boiling point” without checking if the solute is volatile. They assume the Assertion is correct.

Why it happens: The standard textbook example is salt in water. Students memorize the result without understanding the mechanism (vapor pressure lowering).

How to Avoid:

• Always ask: “Is the solute volatile?” If yes, the colligative property rule for boiling point elevation does not apply in the same way.
• Remember the mechanism: Boiling point elevation happens because the solute reduces the vapor pressure of the solvent. A volatile solute adds its own vapor pressure, which can increase the total vapor pressure, thus lowering the boiling point.
• Key fact: Methyl alcohol (methanol) is more volatile than water. Adding it to water makes the solution easier to boil (lower boiling point).

Correct understanding: The Assertion is wrong. The boiling point of water decreases when methyl alcohol is added.

Common Mistake #2: Confusing “volatile” with “non-volatile” in the Reason

The Mistake: Students think the Reason is correct because it sounds like a standard textbook statement. They don’t notice the critical word “volatile” in the Reason.

Why it happens: The Reason says: “When a volatile solute is added to a volatile solvent elevation in boiling point is observed.” This is a false statement. Students often read it quickly and mentally replace “volatile” with “non-volatile.”

How to Avoid:

• Read every word carefully. The Reason explicitly says “volatile solute” and “volatile solvent.” That combination does not give boiling point elevation.
• Create a mental checklist:
  • Non-volatile solute + volatile solvent → Elevation in boiling point.
  • Volatile solute + volatile solvent → Depression or no change in boiling point (depends on relative volatilities).
• Practice with examples: Sugar (non-volatile) in water → elevation. Alcohol (volatile) in water → depression.

Correct understanding: The Reason is wrong because it states the opposite of the correct behavior.

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