Chemistry Reference
In-Depth Information
Methods of preparing soluble salts
There are four general methods of preparing soluble
salts:
The excess magnesium is removed by filtration
(Figure 8.11).
The magnesium nitrate solution is evaporated
slowly to form a saturated solution of the salt
(Figure 8.12).
The hot concentrated magnesium nitrate solution
produced is tested by dipping a cold glass rod into it.
If salt crystals form at the end of the rod the solution
is ready to crystallise and is left to cool. Any crystals
produced on cooling are filtered and dried between
clean tissues.
Acid
+
metal
This method can only be used with the less reactive
metals. It would be very dangerous to use a reactive
metal such as sodium in this type of reaction. The
metals usually used in this method of salt preparation
are the
MAZIT
metals, that is,
m
agnesium,
a
luminium,
z
inc,
i
ron and
t
in. A typical experimental
method is given below.
Excess magnesium ribbon is added to dilute nitric
acid. During this addition an effervescence is observed
due to the production of hydrogen gas. In this reaction
the hydrogen ions from the nitric acid gain electrons
from the metal atoms as the reaction proceeds.
hydrogen ions + electrons → hydrogen gas
(from metal)
2H
+
+ 2e
−
→ H
2
(
g
)
How would you test the gas to show that it was
hydrogen? What would be the name and formula of the
compound produced during the test you suggested?
magnesium +
nitric → magnesium + hydrogen
acid
nitrate
Mg(
s
)
+ 2HNO
3
(
aq
) → Mg(NO
3
)
2
(
aq
) + H
2
(
g
)
Figure 8.12
The solution of magnesium nitrate is concentrated by
slow evaporation.
Acid
+
carbonate
This method can be used with any metal carbonate
and any acid, providing the salt produced is soluble.
The typical experimental procedure is similar to that
carried out for an acid and a metal. For example,
copper(
ii
) carbonate would be added in excess to
dilute nitric acid. Effervescence would be observed
due to the production of carbon dioxide.
How would you test the gas to show it was carbon
dioxide? Write an equation to help you explain what
is happening during the test you have chosen.
copper(
ii
) + nitric → copper(ii)
ii
) + carbon + water
carbonate acid nitrate dioxide
CuCO
3
(s) + 2HNO
3
(aq) → Cu(NO
3
)
2
(aq) + CO
2
(g) + H
2
O(l)
