They conduct electricity when they are melted (molten) because the ions are free to move and so charge can flow between electrodes.
2.7 +
–
+
heat
–
+
– –
+ +
+
–
–
heat solid
liquid + – +
Figure 2.18 Energy is needed to overcome the strong attraction between positive and negative ions of ionic compounds, for the solid to become a liquid when melting.
Ionic compounds such as sodium chloride can dissolve in water. When it is dissolved in water it makes a solution. Sodium chloride solution can conduct electricity as the ions move and split up from each other. Two electrodes are put into the solution to allow the electricity to pass through. 2
Substance D has a high melting point and conducts electricity in solid and liquid state. Explain whether D is ionic.
3
Explain why sodium chloride does not conduct electricity when it is a solid.
– + – + – + – + –
Figure 2.19 Ionic solid melting to produce ions that are free to move.
Comparing NaCl with other ionic compounds electrodes
Sodium chloride NaCl is a typical ionic compound. It dissolves in water, does not conduct electricity when solid, but does when melted, and it has a high melting point (804 °C). Magnesium oxide MgO and potassium chloride KCl are other ionic solids. Look at the data table to see which solid will require the most energy to overcome the forces between its ions. Melting point ºC
Conducts electricity when solid
Conducts electricity when molten
sodium chloride
804
No
Yes
magnesium oxide
2800
No
Yes
potassium chloride
770
No
Yes
These melting points are high because in all the compounds the ions form giant lattice structures that need a lot of energy to break down. A lot more energy is needed to overcome the forces in MgO than in NaCl and KCl. Can we explain why this should be? 4
Suggest why magnesium oxide has a higher melting point than sodium chloride or potassium chloride.
d.c. power supply
+
– electrode
electrode
– –
– –
+ ion – ion
Figure 2.20 Sodium chloride solution can conduct electricity as the ions are free to move.
KEY INFORMATION You only need to know the details about sodium chloride. In this example we are using evidence to make a reasoned suggestion. You do not need to know the details of magnesium oxide or potassium chloride.
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