At the boiling point of a pure liquid under constant external pressure, what happens to its temperature while boiling continues?

Introduction / Context:
Boiling is a common phase change where a liquid turns into vapour throughout its volume. At the boiling point, important energy exchanges occur without a rise in temperature. Understanding this behaviour is essential for thermodynamics and practical tasks such as cooking and industrial heating. This question checks whether you know that during boiling at a fixed pressure, the temperature of the liquid stays constant even though heat continues to be supplied.

Given Data / Assumptions:

• The liquid is pure and is boiling under constant external pressure, such as one atmosphere.
• The liquid has reached its boiling point at that pressure.
• Heat is continuously supplied during the boiling process.
• A steady state of boiling is assumed, with bubbles forming throughout the liquid.
• No significant change in external conditions occurs during the observation.

Concept / Approach:
At the boiling point, the vapour pressure of the liquid equals the external pressure. When heat is supplied at this point under constant pressure, the energy goes into breaking intermolecular bonds and converting liquid into vapour, not into raising the kinetic energy of molecules further. This energy absorbed without temperature rise is called latent heat of vaporisation. As long as both liquid and vapour are present and pressure remains constant, the temperature of the boiling liquid and its vapour stays practically constant.

Step-by-Step Solution:
Step 1: Recognise that the boiling point is defined for a given external pressure as the temperature where the vapour pressure of the liquid equals that external pressure.Step 2: Understand that at this temperature, bubbles of vapour can form inside the liquid and rise to the surface.Step 3: When more heat is supplied at constant pressure, the extra energy is used to overcome intermolecular forces and convert more liquid into vapour.Step 4: Because the supplied energy is used as latent heat rather than increasing kinetic energy, the temperature remains effectively constant during boiling.

Verification / Alternative check:
Practical observations support this theory. If you measure the temperature of boiling water at sea level, it will stay close to 100 degree Celsius as long as water is still present, even if you continue heating. Only after all liquid water has converted to steam can further heating raise the temperature of the steam. Similar behaviour is observed for other pure liquids, each with its own boiling point at a given pressure. This confirms that temperature remains constant during boiling at constant external pressure.

Why Other Options Are Wrong:
Option a is wrong because the temperature does not continue to rise during boiling under constant pressure; it stays at the boiling point. Option b is incorrect since external atmospheric pressure does not automatically increase during boiling; it is usually fixed by the environment. Option d is wrong because the vapour pressure at the boiling point matches external pressure and does not decrease as boiling progresses. Option e is incorrect as density generally decreases when a liquid changes into vapour; vapour is much less dense than the liquid phase.

Common Pitfalls:
Students may confuse heating a liquid below its boiling point, where temperature rises, with heating at the boiling point, where temperature remains constant. Another misunderstanding is to think that any added heat must always cause a temperature increase. It is important to remember the distinction between sensible heat, which changes temperature, and latent heat, which changes phase at constant temperature. Watching a heating curve of a substance plotted against time helps reinforce the idea that plateaus correspond to phase changes at fixed temperature.

Final Answer:
Its temperature remains constant while latent heat of vaporisation is supplied.