C2.6 5c
Ionic substances
FH
Ionic substances have high melting and boiling points because there is a strong attraction between
positive and negative ions. Ionic substances conduct electricity when they are melted and if they
dissolve in water. This is because the ions can move. They do not conduct electricity as solids because
their ions cannot move around. The formula of an ionic compound represents the ratio of ions in the
structure.
1 Why do ionic compounds have high melting and boiling points?
2 Why do ionic compounds conduct electricity when melted or dissolved in water?
3 Why do ionic compounds not conduct electricity as solids?
4 Look at the following table of data and give the letters of the substances which are ionic.
Substance Melting
Boiling
point (°C) point (°C)
A
B
C
D
E
F
650
114
801
2040
119
1610
Does it conduct electricity?
Solid
yes
no
no
no
no
no
1107
184
1467
2980
445
2230
Liquid
conducts
no
yes
yes
no
no
Solution (aq)
insoluble
no
yes
insoluble
insoluble
insoluble
5 Write the formula of the following ionic compounds using the table of ions.
aluminium
Positive ions
Al3+ lead
ammonium NH4+
lithium
Pb2+
bromide
Li+
carbonate
Negative ions
iodide
Br–
I–
CO32–
nitrate
NO­3–
barium
Ba2+
magnesium Mg2+ chloride
Cl–
oxide
O2–
calcium
Ca2+
potassium
K+
fluoride
F–
sulphate
SO42–
copper
Cu2+
silver
Ag+
sulphide
S2–
hydrogen
H+
sodium
Na+
hydrogencarbonate HCO3–
hydroxide
OH– iron(ii)
Fe2+
zinc
Zn2+
iron(iii)
Fe3+
a calcium bromide
b aluminium fluoride
c potassium sulphide
d magnesium nitrate
e silver nitrate
f ammonium chloride
g copper carbonate
h iron(ii) sulphate
i iron(iii) sulphate
j copper oxide
k zinc sulphate
l ammonium carbonate
© Edexcel Limited 2007
GCSE Additional Science Copymaster File
191