Cooling curve for a pure substance

The most accurate way of finding the melting point of a solid is by taking the liquid and allowing it to cool. It is interesting to plot a graph of temperature against time in such an experiment. The resulting graph is known as a cooling cum.

The naphthalene is heated to some way above its melting point, making sure all the solid has liquefied. Then the tube is allowed to cool slowly and the temperature taken at a fixed time interval (e.g. every two minutes). The general trend for the temperature is as follows: first it decreases steadily, then it rises briefly before remaining constant, and finally it decreases again until room temperature is reached. The explanation for this series of events is as follows.

Initially, the whole sample is liquid, and as the sample loses heat to the surroundings the average kinetic energy of the molecules falls. But until the kinetic energy drops to a low enough level, the molecules continue to bounce off one another. Eventually, the molecules lose sufficient energy and the intermolecular forces take over: the molecules begin to stick together and crystals appear.

That is, the intermolecular forces are making bonds between the molecules. Making bonds is an exothermic process, and it is the heat given out when crystals are made that stops the temperature falling. The heat released also has the effect of raising the temperature of the liquid. This prevents more crystals appearing until some of the heat is dissipated into the surroundings; then more crystals appear, giving out more heat, which stops the temperature falling - and so on. Eventually, all the liquid crystallizes, and the crystals cool to room temperature.

Often there is a dip in the cooling curve just before crystals appear. This dip in temperature is known as supercooling. It can happen that the particles in the liquid fail to join up to make crystals. If this happens, we say that the liquid has entered a metastable state. A metastable state is easily disturbed, for example by the liquid being shaken, or a speck of dust entering it. When the first few crystals are made, the energy released causes the temperature to rise rapidly to the normal melting point.

SAQ

The legend says that the melting point of naphthalene is 80.4 °C at 100 kPa pressure.

a Why should melting points always be stated at a given pressure?

b What phases should be in equilibrium when the cooling curve becomes horizontal?

с Why is it important that the sample is allowed to cool slowly?

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