An open beaker of water in equilibrium with water vapour is in a sealed container. When few grams of

Q: Why doesn't the rate of condensation decrease immediately too?

The rate of condensation depends on the number of vapour molecules already present in the space above the liquid. Immediately after adding glucose, the vapour concentration hasn't changed yet, so the condensation rate remains momentarily the same. It will eventually decrease as the system reaches a new equilibrium with a lower vapour pressure.

Q: Does glucose evaporate at all?

For all practical purposes in standard chemistry problems, glucose is considered completely non-volatile. Its vapour pressure is essentially zero at room temperature.

Q: Would adding a volatile solute like ethanol have the same effect?

No! A volatile solute would also evaporate. While it might reduce the rate at which *water* leaves the solution, the *total* rate of molecules leaving the solution would include both water and ethanol, leading to a different overall vapour pressure.

Q: Does the volume of the container matter here?

The volume of the container determines how many moles of vapour are needed to reach the equilibrium vapour pressure, but it does not change the fundamental *rate* at which molecules leave the surface per unit area.

Q: Is this related to boiling point elevation?

Absolutely! Because the vapour pressure is lowered (fewer molecules leaving the solution), you have to heat the solution to a higher temperature to make its vapour pressure equal to the atmospheric pressure. This is why boiling point elevates!

Question

An open beaker of water in equilibrium with water vapour is in a sealed container. When few grams of glucose are added to the beaker of water, the rate at which water molecules

Options:

(A) leaves the solution decreases

(B) leaves the solution increases

(C) leaves the vapour increases

(D) leaves the vapour decreases

Sigma Percentile · Accepted Answer

### Analyzing the Setup

Imagine a sealed container holding a beaker of pure water. At any given temperature, the water molecules possess kinetic energy. Some molecules at the surface have enough energy to break free from the intermolecular forces and escape into the empty space above as vapour. This process is called **evaporation**.

Simultaneously, the vapour molecules bounce around in the sealed container, and some strike the water's surface, re-entering the liquid phase. This is **condensation**. Eventually, a dynamic equilibrium is established where the rate of evaporation perfectly equals the rate of condensation:

$$R_{	ext{evap}} = R_{	ext{cond}}$$

### The Disturbance: Adding a Non-Volatile Solute

Now, we introduce a twist: we add a few grams of glucose to the water. Glucose is a **non-volatile solute**, meaning it has a negligible tendency to evaporate. When it dissolves, it forms a homogeneous mixture with the water.

To understand what happens next, we must zoom in on the surface of the liquid. Evaporation is strictly a surface phenomenon. Only the molecules at the very top layer can escape into the vapour phase.

### The Master Principle: Surface Blockage

In pure water, $100\%$ of the surface is occupied by volatile water molecules. However, when glucose is added, its bulky molecules take up some of those prime surface spots.

Because the glucose molecules are non-volatile, they act like roadblocks. They do not evaporate themselves, and they physically prevent water molecules from occupying those surface positions. Consequently, the fraction of the surface covered by water molecules, denoted as $X_{	ext{water}}^{	ext{surface}}$, is significantly reduced.

### Final Consequence

Since the rate of evaporation is directly proportional to the number of water molecules at the surface:

$$R_{	ext{evap}} \propto X_{	ext{water}}^{	ext{surface}}$$

A decrease in the surface fraction of water leads to an immediate drop in the rate of evaporation. In the language of the question, the rate at which water molecules **leave the solution decreases**.

This beautiful microscopic mechanism is the exact physical reason behind **Raoult's Law** and the colligative property known as the **Relative Lowering of Vapour Pressure**.

Animated Solution for Chemistry - Solutions: An open beaker of water in equilibrium with water vapour is in a sealed container. When few grams of glucose are added to the beaker of water, the rate at which water molecules

Select Answer:

Visualized Solution

Initial Equilibrium

Adding Non-Volatile Solute

Surface Blockage

Reduced Evaporation Rate

Final Conclusion

The Sigma Insight: Henry's Law and Raoult's Law

Analyzing the Setup

The Disturbance

The Master Principle

Final Consequence

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