Chemistry Reference
In-Depth Information
In a similar way, a bromide and an iodide will react to
produce either a cream precipitate of silver bromide
(AgBr) or a yellow precipitate of silver iodide (AgI)
(Figure 8.19).
Questions
1
Complete the word equations and write balanced chemical
equations for the following soluble salt preparations:
a
magnesium
+
sulfuric acid
→
b
calcium carbonate
+
hydrochloric acid
→
c
zinc oxide
+
hydrochloric acid
→
d
potassium hydroxide
+
nitric acid
→
Also write ionic equations for each of the reactions.
2
Lead carbonate and lead iodide are insoluble. Which two
soluble salts could you use in the preparation of each
substance? Write
a
a word equation
b
a symbol equation
c
an ionic equation
to represent the reactions taking place.
3
An analytical chemist working for an environmental health
organisation has been given a sample of water which
is thought to have been contaminated by a sulfate, a
carbonate and a chloride.
a
Describe how she could confi rm the presence of these
three types of salt by simple chemical tests.
b
Write ionic equations to help you explain what is
happening during the testing process.
Figure 8.19
AgCl, a white precipitate, AgBr, a cream precipitate, and
AgI, a yellow precipitate.
An alternative test for iodide ions is the addition of
lead nitrate solution to the iodide which results in a
bright yellow precipitate of lead iodide, PbI
2
.
●
Crystal hydrates
Some salts, such as sodium chloride, copper
carbonate and sodium nitrate, crystallise in their
anhydrous forms (without water). However, many
salts produce
hydrates
when they crystallise from
solution. A hydrate is a salt which incorporates
water into its crystal structure. This water is referred
to as
water of crystallisation
. The shape of the
crystal hydrate is very much dependent on the
presence of water of crystallisation. Some examples
of crystal hydrates are given in Table 8.6 and shown
in Figure 8.20.
Testing for a carbonate
If a small amount of an acid is added to some of the
suspected carbonate (either solid or in solution) then
effervescence occurs. If it is a carbonate then carbon
dioxide gas is produced, which will turn limewater
'milky' (a cloudy white precipitate of calcium
carbonate forms, see Chapter 13, p. 215).
carbonate + hydrogen → carbon + water
ions ions dioxide
CO
3
2−
(
aq
) + 2H
+
(
aq
) → CO
2
(
g
) + H
2
O(
l
)
Testing for a nitrate
By using Devarda's alloy (45% Al, 5% Zn, 50% Cu)
in alkaline solution, nitrates are reduced to ammonia.
The ammonia can be identifi ed using damp indicator
paper, which turns blue.
Table 8.6
Examples of crystal hydrates.
Salt hydrate
Formula
Cobalt(ii)
ii
) chloride hexahydrate
CoCl
2
.6H
2
O
Copper(
ii
) sulfate pentahydrate
CuSO
4
.5H
2
O
3NO
3
−
(
aq
) + 8Al(
s
) + 5OH
−
(
aq
) + 18H
2
O(
l
) → 3NH
3
(
g
) + 8[Al(OH)
4
]
−
(
aq
)
In the reaction the nitrate ion is reduced, as oxygen
is removed from the nitrogen atom, and it gains
hydrogen to form ammonia, NH
3
. The gain of
hydrogen is also a defi nition of reduction.
Iron(
ii
) sulfate heptahydrate
FeSO
4
.7H
2
O
Magnesium sulfate heptahydrate
MgSO
4
.7H
2
O
Sodium carbonate decahydrate
Na
2
CO
3
.10H
2
O
Sodium hydrogensulfate monohydrate
NaHSO
4
.H
2
O
Sodium sulfate decahydrate
Na
2
SO
4
.10H
2
O
