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Solid Changes

(or to state the obvious)

Solidification

When materials change temperature they can change state, this is called a phase change. The obvious examples are melting, vaporisation, condensing and freezing (solidifying). There are also the less obvious subliming and resubliming (changing directly from solid to gas and back again) together with phase changes that can occur in a solid at different temperatures and pressures (there are for example more than a dozen different types of frozen water, although only two can exist outside a laboratory on Earth).

Whenever there is a phase change (change of state) in a material, the atoms or molecules involved are rearranged. This usually involves a change of energy which can be seen as the latent heat (of fusion or vaporisation etc.). After the phase change the rearranged atoms will respond to a change in energy differently and so the specific heat capacity is also likely to change. In addition the rearrangement may well mean a change in the way the atoms are packed together and so a change in density is also very common.

Cooling Curves and Phase Changes

If you used the worksheet “Chilling Tales” you would have looked at cooling curves whilst the material is in a single phase. If a phase change is involved then the graph will be a different shape. This is the shape of the curve if the material is a pure substance such as water, ethanol or mercury.

 Cooling Curve: Pure 

And if the substance is a mixture, such as candle wax the curve will look slightly different.

 Cooling Curve: Impure 

The energy that is lost from the object during solidification is called the Latent Heat of Fusion. If the object has a unit mass (usually a kilogram) then the term Specific Latent Heart of Fusion is used. Latent heat (both of fusion and its liquid to gas equivalent the Latent Heat of Vaporization) are important in many situations:

Measuring Specific Latent Heat

There are many methods that can be used to do this and the best one for the purpose will depend in part on the materials being tested. The method for measuring the latent heat of fusion of ice would not work very well for iron or aluminium. Here are two different specific latent heat experiments for you to carry out.


Method 1

This method works well for ice as well as other chemicals with a melting point at a little below room temperature.

 Latent Heat Experiment Setup Power Circuit

Experimental Setup with Power Circuit

The two set ups should be identical other than the fact that the control is not connected to a power supply.:

Analysis 1

Method 2

This method works well for substances with a melting point at a modest distance above room temperature. Chemicals such as stearic or lauric acid are ideal.

 Latent Heat Experiment Setup Power Circuit

Experimental Setup with Power Circuit

Analysis 2

 Latent Heat Experiment Graph 

The latent heat of fusion DeltaQ of the sample is given by:

 

Where L is the specific latent heat of fusion and delta m is the mass of the sample. If we divide by time we have the power P sub L, that is going solely into melting the sample.

Equation

But the total power supplied is divided between this and the rate of heat loss to the surroundings P sub H:

Writing this out in full we can put the equation into straight line equation form:

Equation

So if we draw a graph of total power against the inverse of the melting time we should produce a straight line graph with a y intercept of the rate of heat loss to the surroundings and a gradient of the specific heat capacity multiplied by the mass of the sample.

 Latent Heat Experiment Graph 

There is one additional point that can be added to the graph as the x intercept corresponds to zero total power and the relevant time is therefore the time it took the sample to solidify when the power had been turned off.

In Practice

In practice methods 1 & 2 would be too slow and prone to error. A typical commercial laboratory would carry out many tests on similar materials and so would require a method that was both quick, automatic and reliable. Ideally a dedicated piece of equipment would be purchased so that results would be easily repeated and measurements calibrated. A (relatively) simple method has been developed that lends itself to this automation; it is called ‘differential scanning calorimetry’.

The technique works as follows:

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