Boiling Water

[Amees]

As you heat water why does the rate at which it heats slow down as the water gets hotter

[buildersmate]

A variation of this question appeared very recently on this site asking the same question the other way around - why water doesn't lose as much temperature as it progressively cools down.

The answer still hasn't changed - the rate of temperature rise slows as it gets hotter because more heat is being lost by convection or conduction to the surfaces around it. The rate of heat loss is propertional to the heat differential between the water and its surroundings. So bigger temperature gap, more heat being lost and less energy left to raise the temperature of the water yet further. Will that do you?

[JudgeJ]

What buildersmate says is only a partial explanation. There is also the question of latent heat to consider.

As the water approaches boiling point it begins to vaporise. Some of the heat energy has to be used to induce a change of state (i.e. from liquid to gas) rather than to raise the temperature of the liquid. The energy is needed to separate the water’s molecules to enable it to become a gas. So the energy applied at boiling point is not required to raise the temperature, but to turn the water in to steam.

Every substance has a specific latent heat which determines how much energy is needed to convert it from liquid to gas (and indeed from solid to liquid). Water has a particularly high specific latent heat and requires about 55 times as much energy to vaporise it as it does to raise its temperature by one degree C.

So you will observe that, as the water in your kettle approaches boiling point, the rate that the water is heated tails off. This is because most of the energy is being used to convert the water in to steam and not to raise its temperature.

One useful side effect of latent heat is cooling by evaporation. When we get hot we produce sweat (which is mainly water). The heat energy from our body is used by the sweat to cause it to evaporate, thus cooling our skin.