Question

Which of the following statements is wrong? (1) Evaporation is a spontaneous process (2) Evaporation is a surface phenomenon (3) Vapour pressure decreases with increases of temperature (4) Vapour pressure of a solution is less than the vapour pressure of a pure liquid

Short answer

Statement (3) is incorrect.

Step-by-step solution

- Understand the statements

Read each of the given statements carefully to understand their meaning. Identify if they describe characteristics of evaporation or vapor pressure.

- Analyze Statement 1

Statement (1) says 'Evaporation is a spontaneous process'. Review the concept that evaporation occurs naturally without needing external energy, making this statement true.

- Analyze Statement 2

Statement (2) says 'Evaporation is a surface phenomenon'. Understand that evaporation involves molecules at the surface escaping into the vapor phase, making this statement true.

- Analyze Statement 3

Statement (3) says 'Vapour pressure decreases with increases of temperature'. Recall the principle that vapor pressure increases with temperature because higher temperature means more molecules have sufficient energy to escape into the vapor phase. Hence, this statement is false.

- Analyze Statement 4

Statement (4) says 'Vapour pressure of a solution is less than the vapour pressure of a pure liquid'. This is true because the presence of a solute lowers the vapor pressure by the Raoult's law (colligative properties).

- Identify the wrong statement

Based on the individual analysis, identify that statement (3) is incorrect. The correct claim is that vapor pressure increases with an increase in temperature.

Key concepts

Spontaneous Process

Evaporation is classified as a spontaneous process. This means it happens naturally and does not require any external energy input. The driving force behind evaporation is the kinetic energy of the molecules at the liquid surface.

Even at room temperature, some molecules have enough energy to break free from the liquid phase and enter the vapor phase.

This spontaneous transition occurs as the system naturally moves towards a state of higher entropy, meaning more disorder.

Because no external energy is required, evaporation is a classic example of a spontaneous process.

Surface Phenomenon

Evaporation is specifically a surface phenomenon. This means that it involves only the molecules at the surface of a liquid.

These surface molecules are the ones that have the highest kinetic energy, allowing some to escape into the air as vapor.

Unlike boiling, which occurs throughout the entire liquid, evaporation takes place solely at the surface.

This is why you might observe water slowly disappearing from an open container but not see it boiling, as evaporation silently works at the molecular level.

Conditions like increased surface area and air flow can speed up this surface phenomenon.

Vapor Pressure

Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid phase at a given temperature.

It is essential to understand that vapor pressure increases as temperature increases.

This is because higher temperatures provide more energy to the molecules, allowing more of them to escape into the vapor phase.

Therefore, the statement 'Vapor pressure decreases with increase of temperature' is incorrect.

Additionally, the vapor pressure of a solution is typically lower than that of a pure liquid due to the presence of solutes, which is explained by Raoult's law.

Temperature Relationship

The relationship between temperature and evaporation is critical to understand. As temperature increases:

• More molecules gain sufficient kinetic energy to escape the liquid
• The rate of evaporation increases
• The vapor pressure also increases

In simple terms, higher temperatures mean faster movement of molecules, which translates to a higher rate of evaporation and an increase in vapor pressure.

This direct relationship emphasizes why boiling occurs when the vapor pressure equals the atmospheric pressure.

Understanding these relationships helps in various practical applications, such as weather forecasting and industrial processes.