💡 What is Vapour Pressure?

 

💡 What is Vapour Pressure?

(Class 12 Chemistry – NCERT Chapter 1: Solutions)

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Vapour pressure is the pressure exerted by the vapour of a liquid (or solid) when it is in dynamic equilibrium with its liquid (or solid) phase at a given temperature.

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🔁 Dynamic Equilibrium Explained:

In a closed container:

• Some liquid molecules evaporate into vapour.

• At the same time, some vapour molecules condense back into the liquid.

When the rate of evaporation = rate of condensation, the system reaches equilibrium, and the pressure exerted by the vapour at that point is called the vapour pressure.

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🧪 Factors Affecting Vapour Pressure:

1. Nature of the Liquid

   • More volatile liquids (e.g., acetone) → higher vapour pressure

   • Less volatile liquids (e.g., water, glycerin) → lower vapour pressure

2. Temperature

   • As temperature ↑ → kinetic energy ↑ → vapour pressure ↑

   • Vapour pressure increases exponentially with temperature

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📘 Example:

• At 25°C, the vapour pressure of water is about 23.8 mm Hg.

• That means in a sealed container at 25°C, water vapour exerts a pressure of 23.8 mm Hg over its liquid.

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📝 CBSE Board Tip:

Define vapour pressure clearly, mention dynamic equilibrium, and include the effect of temperature and volatility in your answer.

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Would you like a graph of vapour pressure vs temperature or a comparison of different liquids next?