When vapour pressure of a liquid is equal to the atmospheric pressure, what happens to the liquid ?

Question

Accepted Answer

When the vapour pressure of a liquid becomes equal to the atmospheric pressure, the liquid reaches its boiling point and begins to boil. This is because boiling occurs when the pressure exerted by the vapour of the liquid equals the pressure exerted by the surrounding environment, which is typically the atmospheric pressure. At this point, bubbles of vapour form within the liquid and rise to the surface, leading to the rapid transition of the liquid into its gaseous state.

1. **Explanation for Option 3 (Correct Answer):**  
   - **Boiling Process:** Boiling is a phase transition from the liquid state to the gaseous state, occurring when a liquid's vapour pressure equals the surrounding atmospheric pressure. This is a fundamental principle in thermodynamics and fluid mechanics. When this equilibrium is reached, the liquid molecules have enough energy to overcome atmospheric pressure and form bubbles of vapour within the liquid, which is the hallmark of boiling.

2. **Explanation for why other options are incorrect:**

- **Option 1 (The liquid freezes):**  
     Freezing is the process where a liquid turns into a solid. This occurs when the temperature of the liquid is lowered to its freezing point, not when the vapour pressure equals atmospheric pressure. Therefore, freezing is unrelated to the condition described in the question.

- **Option 2 (The liquid becomes viscous):**  
     Viscosity refers to the thickness or resistance to flow in a liquid. While temperature changes can affect viscosity, the condition of vapour pressure equaling atmospheric pressure does not inherently cause a liquid to become more viscous. Instead, it leads to boiling, not a change in viscosity.

- **Option 4 (The liquid does not undergo any change):**  
     When the vapour pressure of a liquid equals the atmospheric pressure, a significant change occurs: the liquid boils. Therefore, it is incorrect to say that the liquid does not undergo any change. Boiling is a major physical change, marking the transition from liquid to gas.

In conclusion, Option 3 is the correct answer because it accurately describes the physical phenomenon that occurs when a liquid's vapour pressure equals the atmospheric pressure, leading to boiling.