Vapour pressure is the pressure exerted by vapours

Options

A. In equilibrium with liquid
B. In any condition
C. In an open system
D. In atmospheric conditions

Answer

**Step-by-step Solution:**

1. **Understanding Vapour Pressure:**
- Vapour pressure is defined as the pressure exerted by the vapours of a substance when it is in equilibrium with its liquid or solid phase.

2. **Definition of Vapour Pressure:**
- Vapour pressure can be defined as the pressure exerted by gas particles (vapours) in a closed container at a specific temperature while in equilibrium with the liquid or solid phase.

3. **Conditions for Vapour Pressure:**
- The vapour pressure is only relevant in a closed container, meaning that no gas can escape and the system can reach equilibrium.
- It is also important to note that vapour pressure is dependent on temperature; it varies with changes in temperature.

4. **Analyzing the Options:**
- **Option A: In equilibrium with liquid.**
- This option is correct because vapour pressure is indeed the pressure exerted by vapours in equilibrium with the liquid.
- **Option B: At any condition.**
- This option is incorrect. Vapour pressure is not applicable at any condition; it is specific to equilibrium conditions.
- **Option C: An open system.**
- This option is incorrect. Vapour pressure is measured in a closed system, not an open one.
- **Option D: In atmospheric condition.**
- This option is also incorrect. Vapour pressure is measured in a closed container, not under atmospheric conditions.

5. **Conclusion:**
- The correct definition of vapour pressure is that it is the pressure exerted by vapours in equilibrium with the liquid in a closed container at a given temperature. Therefore, the correct option is A.

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