Specific latent heat

The hotter the material becomes the faster the atoms vibrate. Eventually the bonds between the atoms break. This is when the solid melts.

The breaking of the bonds requires a substantial amount of energy, this energy is called the latent heat of melting. However the kinetic energy of the atoms does not increase so at the change from solid to liquid, there is no change in temperature.

As the liquid is heated the molecules move faster and faster. They do not expand into a very much larger space because they are mutually attracted.

Eventually molecules gain so much kinetic energy that they have enough to pull away from the others against this attraction. This is evaporation.

At the energy level (temperature) where almost all have enough kinetic energy to escape this mutual attraction, the liquid is boiling.

The energy needed to escape the mutual attractive forces and to push the atmosphere away with the change in volume, is very large. This is called the latent heat of vaporisation.

The energy changes that take place when a solid melts or a liquid boils are perfectly reversible. For water to freeze to ice a large amount of heat energy has to be removed. When a vapour condenses the change is even greater. A large amount of energy from the bonds forming between the molecules and the volume reducing is changed to heat. That is the reason why steam is often used to produce heat rapidly, for example in a coffee Espresso machine.


Example calculation

The average power provided by the kettle over the minute it was switched on was 2640 watts.

The energy provided in 60 seconds  is 60 x 2640 = 158400 joules

The mass of water changed to steam in this time was 1.716 - 1.656 = 0.060 Kg

The energy to vaporize 1 Kg is therefore 158400/0.060  =  2.64 x 106 Jkg-1

 which is nearly 20% higher than the generally accepted value of 2.26 x 106 Jkg-1

but we made no attempt to reduce energy losses by other means so I suspect that some of the energy from the kettle was transferred to the surroundings by conduction and convection of warm air above the kettle.

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