Melting, Boiling and Evaporation

Melting, Boiling and Evaporation

If we take the ice out of the freezer, it melts. If we heat water in a kettle, it boils.

If we place a thermometer on the melting ice, it would read 0 degrees Celsius. The reading stays at 0 while the ice melts. Only after all the ice has melted would the temperature start to go above 0.

If we place a thermometer in the steam in a boiling kettle (don't try this at home), it would read 100 degrees. It would continue to read 100, until all the water has boiled off. Only then would the the temperature go up.

During melting, we know that heat goes from the surrounding into the ice, because the surounding is warmer. So why does tempreature of melting ice or boiling water remain unchanged?

The resaon is that the heat absorbed is used to melt the ice. During melting, more and more ice turns into water. This requires energy - to overcome attraction between the molecules in ice, so the molecules can move around.

The property of ice is such that if any part of it goes above 0 degrees, that part must become water. In melting ice, there would be a mixture of ice and water. Why is the water not above 0 degree? Actually it can be. Only if we keep stirring the mixture would the temperature stay 0 until all ice has melted.

The reason is similar for steam. The difference is that it takes a lot more energy to boil water, and to melt the same mass of ice. This is because in boiling, attraction between molecules have to be completely overcome to become steam. In melting, a much smaller amount of energy is needed to separate the molecules slightly, enough for them to move around.

The energy needed to change phase is called latent heat. So the latent heat of vaporisation (boiling) is much larger than the latent heat of fusion (melting).

Apart from melting and boiling, there is also evaporation.

If you wet your hand, it feels cool. The water itself is at the same temperature as the room. Why does it feel colder on your hand? The answer is, because it evaporates.

We need to give energy to water if we boil it. When water evaporates, where does it get the energy from? If it is not heated how can it change to vapour?

We can explain these using the ideas about the motion of molecules. Consider the water on your hand. The molecules are always in random motion. There would always be a very small fraction that just happens to have enough energy to overcome the attractions between molecules completely. If they happen to be at the surface of the water, they would break free and escape into the air. This is evaporation.

Since the most energetic of the molecules have escaped, the average energy of the water falls, so temperature drops. Hence the cooling effect. To answer the above questions: the evaporation gets the energy from the thermal energy of the water itself, which is why the water cools down. It does not have to be heated. The molecules just leave randomly - there is nothing to stop them. Of course, as the water cools, the fraction of molecules with enough energy to escape also falls. So the evaporation slows down.

The Physics Notes