In addition, the impurity can also reduce the ‘sharpness’ of the melting or boiling point. An impure substance sometimes melts or boils over a range of temperatures, not at a particular point. The presence of an impurity in a substance: ◆ lowers the melting point, and ◆ raises the boiling point of the substance.
Study tip Remember that pure substances have definite, sharp melting and boiling points. The presence of an impurity means that these changes will be spread over a range of temperatures in each case.
Figure 2.6
An electrical melting-point apparatus.
is usually seen as a solid, a liquid or a gas. For example, if the m.p. is below 20 °C and the b.p. is above 20 °C, the substance will be a liquid at room temperature.
Study tip Remember to practise using melting and boiling point data to decide whether a particular substance is a solid, a liquid or a gas at room temperature. These are quite common questions. Be careful with temperatures below 0 °C; −100 °C is a higher temperature than −150 °C.
Heating and cooling curves The melting point of a solid can also be measured using the apparatus shown in Figure 2.7. A powdered solid is put in a narrow melting-point tube so that it can be heated easily. An oil bath is used so that melting points above 100 °C can be measured. We can follow the temperature of the sample before and after melting. These results can then be used to produce a heating curve (Figure 2.8, overleaf). Similar apparatus can be used to produce a heating curve but the thermometer must be placed in a test tube containing the solid being studied. Figure 2.8 shows how the temperature changes when a sample of solid naphthalene (a single pure substance)
thermometer stirrer melting-point tube rubber band oil or water
The effect of impurities Seawater is impure water. You can show this if you put some seawater in an evaporating dish and boil away the water, because a solid residue of salt is left behind in the dish. Seawater freezes at a temperature well below the freezing point of pure water (0 °C) and boils at a temperature above the boiling point of pure water (100 °C). Other impure substances show similar differences.
solid
heat Figure 2.7 Apparatus for measuring the melting point of a solid. A water bath can be used for melting points below 100 °C and an oil bath for those above 100 °C.
Original material © Cambridge University Press 2014
Chapter 2: The nature of matter
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