Latent Heat
When ice melts, temperature stays at 0 degree Celsius. Yet, heat has to be absorbed from the surrounding to change ice to water. This heat is called the latent heat. The word "latent" suggests "hidden." In this case, heat absorbed does not show as an increase in temperature.
Specific latent heat of melting ice is the heat needed to melt 1 kg of the ice. This has been measured to be 334000 J/kg, or 334 kJ/kg. This is much larger than the heat needed to raise 1 kg of water by 1 degree, which is 4200 J. How do we measure the specific latent heat of melting ice?
To set up an experiment, we need:
- ice,
- a weighing scale, and
- a way to supply and measure heat.
The idea is to give the ice some heat Q, and measure the mass m that melts. The specific latent heat is then l = Q/m.
We need a way to measure the mass of the melted ice. If we put a cup of ice on a weighing scale, we would measure both the ice and the water. We really only want the mass of the water, because this is the melted ice. A simple way to separate the water from the ice is to put the ice in a funnel, and allow the water to drip into a cup below. Then we can measure the mass of just the water itself.
Next we need a way to give and measure heat. We can give heat using a heating element, and a power supply to pass electricity through the element. We also need a voltmeter and an ammeter to measure the voltage V and current I of the element. The heat transferred to the melting ice is P = VI (this will be explained on another page). If we switch on the heater for a time t, the heat given to the ice is Q = Pt.
During this time, a mass m of water would be collected below the funnel and measured. The specific latent heat can then be calculated using l = Q/m = Pt/m = VIt/m.
A similar idea can be used to measure the specific latent heat of boiling. Instead of ice in a funnel, we could have water in a kettle. Since the steam would escape on its own, we only need to measure the mass of the water before and after boiling. The difference m is the mass of the water that is changed to steam. Then we can use the same formula l = VIt/m to find the specific latent heat.
The specific latent heat of boiling water has been measured to be 2260000 J/kg, or 2260 kJ/kg. This is much greater than the 334 kJ/kg for melting ice, because a lot more energy is needed to completely separate the water molecules from each other during boiling.
The proper term for the heat needed to melt a substance at constant temperature is latent heat of fusion. It is often denoted Lf. The proper term for the heat needed to melt 1 kg of a substance at constant temperature is specific latent heat of fusion. It is often denoted lf. They are therefore related by lf = Lf/m.
The proper term for the heat needed to vaporise (or boil) a substance at constant temperature is latent heat of vaporisation. It is often denoted Lv. The proper term for the heat needed to vaporise 1 kg of a substance at constant temperature is specific latent heat of vaporisation. It is often denoted lv. They are therefore related by lv = Lv/m.
Copyright 2010 by Kai Hock. All rights reserved.
Last updated: 17 October 2010.