When the air pressure is at a standard level of 1 atm, the boiling point of water will be 100oC, and you will not be able to heat the water beyond that temperature no matter how long you boil it.
If you observe a thermometer while boiling water, you will see that the temperature of the water rises until it begins to boil. Once the water starts boiling, the temperature reaches that point and remains constant, even if you continue to boil it.
So, why does no temperature change occur even if we keep boiling the water?
When initially heated, the temperature of the water keeps rising.
Heat is a form of energy that increases the temperature of a substance and leads to a change in its physical state. The temperature of the water remains constant when it begins to boil because the heat released from the boiling water is used to convert the water into steam.
To boil water, simply heating it to 100oC is not enough; you must also provide it with a significant amount of latent heat to change its phase, which means converting it into steam.
When we boil water in a pot or kettle, as it reaches a boil, the temperature of the water will hit 100oC. At this point, part of the boiling water will turn into steam, which will rise and break through the surface, evaporating into the air, creating the phenomenon of boiling. Boiling water marks the transition from the liquid state to the gaseous state.
At the beginning of boiling, the temperature of the water will continuously rise. However, when the temperature reaches 100oC, the moment the water starts to boil, the temperature is maintained because the heat is used to change the state of the water, not to increase its temperature.
This means that the heat source we use to boil the water in the container has a temperature of 100oC, and it can only raise the water in the container to 100oC.
When water reaches 100oC, it changes state. (Image: MysTown.com).
At 100oC, water is a mixture of both liquid and gaseous states; this is the equilibrium point between the liquid and gaseous states. As long as the water is in the liquid state, increasing the heat or continuing to maintain the temperature will cause the water to gradually transition to the gaseous state. Whether in a gaseous or liquid state, the boiling temperature cannot exceed this limit.
This can be simply understood as follows:
- From the moment we start boiling until the water boils, the temperature gradually increases.
- When the water reaches 100oC, it will change state (evaporate).
- Thus, the thermal energy provided by the flame to the water will entirely convert into the kinetic energy of the water molecules.
- Therefore, if the water is already boiling and we increase the temperature with the intention of making it hotter, we will only result in the water evaporating faster!
This is why no matter how long we boil water, its temperature does not rise.
