Problem 24 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following has high vapour pressure at temperature below its melting point? (a) benzoic acid (b) citric acid (c) salicylic acid (d) all

Step-by-Step Solution

Answer

Benzoic acid has high vapor pressure below its melting point.

Step 1: Understand Vapor Pressure

Vapor pressure is the pressure exerted by a vapor that is in equilibrium with its liquid or solid phase at a given temperature. Substances with higher vapor pressures evaporate more quickly and readily from the solid or liquid phase.

Step 2: Recognize Sublimable Substances

A substance with a high vapor pressure at temperatures below its melting point is often capable of sublimation. Sublimation is the phase transition from solid to gas without passing through the liquid phase. Substances that sublime tend to have high vapor pressures at lower temperatures.

Step 3: Identify Characteristics of Each Substance

Benzoic acid is known to sublime, which means it transitions from solid to gas with relative ease, indicating high vapor pressure below its melting point. Citric acid and salicylic acid have higher melting points and typically do not sublime as easily as benzoic acid.

Step 4: Conclude the Suitable Substance

Based on the characteristic of sublimation and known chemical properties, benzoic acid has a relatively high vapor pressure below its melting point compared to the others.

Key Concepts

Sublimation

Sublimation is a fascinating process where a substance transitions directly from solid to gas. This happens without going through the liquid phase. In other words, the solid particles gain enough energy to break free directly into the gas phase. This is typically observed when substances have high vapor pressures below their melting points. Such substances, like benzoic acid, can easily transition to the gas state when the temperature changes, as they don't need to first become liquid.

For sublimation to occur, certain conditions need to be met. The pressure and temperature must be arranged so that they allow the transition directly into vapor. A classic everyday example of sublimation is dry ice (solid carbon dioxide), which becomes a gas without first liquefying. Other examples include iodinine and naphthalene, which are also known to sublimate under particular conditions.

Phase Transition

Phase transitions are the changes between different states of matter—solid, liquid, and gas. This includes not only melting, freezing, and boiling, but also sublimation.

If you think about water, it can move between solid (ice), liquid (water), and gas (steam) at certain temperatures and pressures. Similarly, substances like benzoic acid undergo phase transitions from solid directly to gas, thanks to sublimation.

• Solid to Liquid: Happens during melting.
• Liquid to Gas: Occurs when boiling or evaporating.
• Solid to Gas: This is sublimation.

Understanding these transitions is crucial in predicting the behavior of substances under different conditions.

Melting Point

The melting point of a substance is the temperature at which it changes from solid to liquid. It's a key property that helps us understand how a material behaves when heated. Substances with lower melting points tend to convert into liquid and gas states more readily than those with higher melting points.

In the case of substances like benzoic acid, the melting point helps determine how easily it might also sublimate. While benzoic acid does have a higher melting point than room temperature, its ability to sublime indicates that vapor pressure is still a crucial factor. Compared to citric acid and salicylic acid, which have higher melting points, benzoic acid's behavior makes it unique.

Equilibrium

Equilibrium in the context of vapor pressure involves a balance between phases. When a substance is in equilibrium, its rate of evaporating from a solid or liquid to a gas is equal to its rate of condensation from a gas to solid or liquid state.

• Dynamic Process: Even though the process is balanced, it's dynamic—molecules are constantly moving between phases.
• Vapor Pressure: At a particular temperature, the vapor pressure remains constant in equilibrium state, as seen with benzoic acid when it sublimates.

This dynamic and balanced condition is crucial in understanding how substances behave under various atmospheric conditions, temperatures, and pressures. In scientific terms, reaching equilibrium means achieving a stable stage without net change in the phases involved.