During a change of state of a pure substance, for example from solid to liquid or from liquid to vapour, the temperature of the substance remains constant.
The temperature at which a solid becomes liquid at standard pressure is called the melting point of the solid. When the liquid is heated further, boiling takes place – bubbles of vapour are formed within the liquid. Throughout boiling, the temperature remains constant. The temperature at which boiling takes place at standard pressure is called the boiling point of the liquid. After all the liquid has been converted to vapour, the temperature of the vapour rises as heat is added to it.
When the liquid is cooled, the temperature again becomes steady during the change from vapour back to liquid. The change from vapour back to liquid is called condensation. As the liquid is further cooled, the temperature again becomes steady during the change from liquid back to solid. The change from liquid to solid is called freezing.
Figure 11.11 shows the heating of candle wax in a water bath. Since the vapour of candle wax is fl ammable, it is not recommended that this activity be done in school.
melting point ❯
A graph of temperature against time shows that the temperature of the solid rises and then becomes steady (figure 11.12 (a)). It is at this steady temperature that a change of state occurs (from solid to liquid, in this case). When all the solid has been converted into liquid, the temperature of the wax, now liquid, starts to rise again until the temperature of the water bath is reached. Temperature Time solid solid + liquid liquid (a) Temperature Time solid solid + liquid liquid (b)
Figure 11.12 Solid–liquid cooling curves: graphs of temperature against time for candle wax
during: (a) heating, and (b) cooling.
During a change of state, the average separation distance of the particles within a substance changes. In the case of the wax, as the solid is heated, the average distance between the wax molecules increases. The heat energy supplied causes molecules of solid wax to increase their speed and amplitude of vibration. This results in an increase in the overall energy of the molecules, which is both kinetic and potential (stored) energy. However, the energy supplied during the change of state does not increase the kinetic energy of the molecules. Rather, it only does work in increasing the separation of the molecules. That is why the temperature is constant during the change from solid to liquid (i.e. during melting).
When the liquid wax is cooled, the temperature again becomes steady during the change from liquid back to solid (called ‘freezing’). The freezing temperature, or freezing point, is the same as the melting point of the wax, at a given external pressure. During the change from liquid to solid, the average separation distance between molecules decreases and energy (latent heat) is given up to the surroundings, while the temperature still remains constant. At a certain temperature, which is higher than the boiling point of water, liquid wax begins to change into vapour by boiling. (Caution! Don’t try it!) Throughout boiling, the temperature remains constant. At this temperature, the heat energy being supplied does work in effecting a very large increase in average separation distance between molecules as liquid wax changes to vapour. (The reverse of the process of boiling occurs during condensation, when the vapour becomes liquid upon cooling, and gives up heat without its temperature changing.) When all the liquid has changed to vapour, the temperature of the vapour rises as heat energy continues to be supplied.
Evaporation
The change from liquid to vapour that takes place at the surface of a liquid is called evaporation.
The Kinetic Theory explains evaporation as follows. In a liquid, the particles are moving about with a variety of speeds, and therefore with a variety of kinetic energies, even though the liquid as a whole is at a uniform temperature. Some of the faster particles, when present at the surface, have enough energy to enable them to break loose from the liquid and move independently above the liquid as a vapour. Thus evaporation can take place
ITQ4
(a) How do you explain the horizontal shape of the graph (figure 11.12) as solid candle wax changes to liquid? (b) What if there was no distinct
horizontal shape in the graph during the change from solid to liquid – how would you explain that?
change of state ❯
Latent heat
When heat is applied to a pure substance undergoing a change of state, a temperature change of the substance does not manifest itself. The heat involved is called ‘latent (hidden) heat’ since its application is not manifested by a change in temperature. The topics of latent heats and changes of state are taken up in more detail in chapter 13.
Volatile liquids
A liquid is said to be volatile if it evaporates readily at or below room temperature (e.g. chloroform, Freon, methanol).
thermometer test tube beaker candle wax water heat
Figure 11.11 Heating crushed candle wax
temperature increases, since, at high temperatures, more of the particles have the energy needed to break free from the liquid.