explain why the temperature of boiling water remains constant as the water evaporates.

The temperature of boiling water stays constant because all the heat you add is used to change the state of the water, not to raise its temperature.

Quick Scoop

When water boils at 100 °C (at normal atmospheric pressure), it has reached its boiling point. From that moment on, even if you keep supplying heat (for example, by keeping the stove on), the thermometer will keep reading about 100 °C until almost all the liquid water has turned into steam.

What the heat is actually doing

• Before boiling, added heat increases the kinetic energy of water molecules, so the temperature rises.

• At the boiling point, molecules already have enough kinetic energy to escape the liquid.

• Now, extra heat goes into increasing the potential energy of the molecules, helping them break intermolecular attractions and move apart into the gas phase.

• Because their average kinetic energy (which temperature measures) does not increase further, the temperature remains constant.

A simple way to picture it:

Heating to boiling is like pushing a ball up a hill (temperature rises); once you reach the top, extra energy goes into pushing the ball over the edge into a new valley (changing state), not into making the top of the hill any higher.

Phase change and latent heat

• The energy used during boiling without temperature change is called latent heat of vaporization.

• This energy is “hidden” in the phase change: it is stored in the new arrangement of particles (liquid → gas), not seen as a temperature rise.

• Only after all (or almost all) the liquid has turned to steam can further heating increase the temperature of the steam above 100 °C.

Why this matters in real life

• It explains why steam burns are so severe: condensing steam releases a large amount of latent heat back into your skin.

• It’s the same principle used in cooling systems and in weather: huge amounts of energy move around when water evaporates or condenses, with little change in temperature during the actual phase change.

TL;DR:
At the boiling point, added heat no longer makes water hotter; instead, it is all spent on overcoming intermolecular forces and converting liquid water into steam, so the temperature of the boiling water stays constant until the phase change is essentially complete.

Information gathered from public forums or data available on the internet and portrayed here.