Episode 608: Latent heat

Energy is involved in changes of phase, even though there is no change of temperature.

Lesson Summary

• Discussion: Defining specific latent heat (10 minutes)
• Demonstration: Boiling water (15 minutes)
• Student experiment: Measuring l (30 minutes)
• Student experiment: Cooling curves (30 minutes)
• Worked example: Latent and specific heat (5 minutes)
• Student questions: Involving c and l (40 minutes)

Discussion: Defining specific latent heat

The final point in this topic is to return to the original definition of (internal energy) as being a combination of both the energy stored kinetically (kinetic energy) and energy stored chemically i.e. in the motion of the particles and the bonds between them. In talking about ideal gases all the energy was assumed to be stored kinetically because there were assumed to be no bonds between the atoms. However, in a solid or liquid there are bonds and clearly some energy is needed to break those bonds. That means that, in melting a solid or boiling a liquid, a substantial amount more energy needs to be transferred which does not raise the temperature. This is the hidden heat or latent heat.

The energy you need to transfer to a mass m of a substance to melt it is given by

Δ E = m × L

Or the 'specific latent heat' is the energy you need to transfer to change the unit mass from one phase to another.

Demonstration: Boiling water

Ask your class to watch some water boiling and think about what is going on. Energy is being transferred, but the temperature is not rising. Intermolecular bonds are breaking, and, as a physicist would say, work is being done to separate the particles against intermolecular attractive forces.

The key point from these is that, for certain materials, there is a phase transition where the energy transferred no longer raises the temperature (adds to each molecule's kinetic energy) but instead breaks bonds and separates the particles. This should be made quantitative. Likewise, the reverse processes involve energy being transferred from the substance. So evaporating liquids are good coolants and freezing water to make ice is considerably more of an effort than cooling water to 0  ° C.

Episode 608-1: Examination of boiling (Word, 34 KB)

Student experiment: Measuring L

It is useful to have measured a specific latent heat – for example, that of melting ice.

Episode 608-2: The specific latent heat of fusion of ice (Word, 67 KB)

Student experiment: Cooling curves

If you have a class set of data-loggers for recording temperature, determination of the cooling curve of stearic acid, naphthalene or lauric acid is worthwhile. Even as a demonstration this is good and can be left running in the background while the students work on calculations.

Episode 608-3: Heating and cooling curves (Word, 52 KB)

Worked examples: Latent and specific heat

Scalds from water and steam

We assume that our hand is at 37  ° C, and that we put 10 g of water at 100  ° C accidentally on our hand. The water will cool to 37  ° C. Assuming that all the energy lost by the water will be gained by our hand:

Energy shifted from water = mc Δ T

Energy shifted = 1.5 kg  ×  4.2  kJ kg^-1 ° C^-1  ×  63  ° C

Energy shifted = 2 646 J.

But if the 10 g had been steam then the steam would first have to condense.

Energy shifted in condensing = mL

Energy shifted in condensing = 1.5 kg  ×  2 260 kJ kg^-1

Energy = 22 600 J

So the energy lost in 10 g of steam turning to water at 37  ° C is 25,246 J.

This is nearly ten times as much as the water alone!

The worked example is based on one from Resourceful Physics.

Student questions: Involving c and L

Practice in situations involving specific heat capacity and specific latent heat.

Episode 608-4: Questions on specific heat capacity and specific latent heat (Word, 27 KB)

Episode 608-5: Further specific and latent heat questions (Word, 30 KB)