where the gas condenses to a liquid, and when the liquid freezes. These experiments show that heat energy is needed to change a solid into a liquid, or a liquid into a gas. During the reverse processes, heat energy is given out.
90
naphthalene
Temperature / ºC
80
When a solid is melted, or a liquid is boiled, the temperature stays constant until the process is complete. The same is true in reverse when a gas condenses or a liquid freezes.
70 wax
60
Activity 2.1 Plotting a cooling curve
50 0
2
4 6 Time / minutes
8
10
Figure 2.8 The heating curves for naphthalene (a pure substance) and wax (a mixture of substances).
is heated steadily. The solid melts at precisely 80 °C. Notice that, while the solid is melting, the temperature stops rising. It will only begin to rise again when all the naphthalene has melted. Generally, the heating curve for a pure solid stops rising at its melting point. The heating curve for wax, which is a mixture of substances, shows the solid wax melting over a range of temperatures. It is possible to heat a liquid in the same apparatus until its boiling point is reached. Again, the temperature stays the same until all the liquid has boiled. The reverse processes can be shown if a sample of gas is allowed to cool. This produces a cooling curve (Figure 2.9). The level portions of the curve occur
Temperature / ºC
gas 78
liquid 0 –15 solid Time
Figure 2.9 The cooling curve for a substance. The temperature stays constant while the gas condenses, and while the liquid freezes. A cooling mixture of ice and salt could be used to lower the temperature below 0 °C.
26
Cambridge IGCSE Chemistry
In this experiment, you will plot cooling curves for two different substances. A worksheet, with a self-assessment checklist, is included on the accompanying CD-ROM. Adaptations of this experiment and details of the use of it in assessing practical skills AO3.3 and AO4.4 are given in the Notes on Activities for teachers/technicians.
Types of mixture Our world is very complex, owing to the vast range of pure substances available and to the variety of ways in which these pure substances can mix with each other. In everyday life, we do not ‘handle’ pure substances very often. The air we breathe is not a single, pure substance – and we could not live in it if it were! Water would be rather tasteless if we drank it pure (distilled). Each mixture must be made from at least two parts, which may be solid, liquid or gas. There are a number of different ways in which the three states can be combined. In some, the states are completely mixed to become one single state or phase – ‘you cannot see the join’. Technically, the term solution is used for this type of mixture. Solid salt dissolves in liquid water to produce a liquid mixture – a salt solution (Figure 2.10). In general terms, the solid that dissolves in the liquid is called the solute. The liquid in which the solid dissolves is called the solvent. In other types of mixture, the states remain separate. One phase is
Original material © Cambridge University Press 2014