Chemistry Reference
In-Depth Information
Because the colour change only takes place in the
presence of water, the reaction is used to test for the
presence of water.
These processes give a simple example of a
reversible reaction:
CuSO
4
(
s
) + 5H
2
O(
l
)
CuSO
4
.5H
2
O(
s
)
Some crystal hydrates
efloresce
, that is they lose some
or all of their water of crystallisation to the atmosphere.
For example, when colourless sodium carbonate
decahydrate crystals are left out in the air they become
coated with a white powder, which is the monohydrate
(Figure 8.22). The process is called
efflorescence
.
Na
2
CO
3
.10H
2
O(
s
) → Na
2
CO
3
.H
2
O(
s
) + 9H
2
O(
g
)
Figure 8.20
Hydrate crystals (left to right): cobalt nitrate, calcium nitrate
and nickel sulfate (top) and manganese sulfate, copper sulfate and
chromium potassium sulfate (bottom).
When many hydrates are heated the water of
crystallisation is driven away. For example, if crystals of
copper(
ii
) sulfate hydrate (blue) are heated strongly,
they lose their water of crystallisation. Anhydrous
copper(
ii
) sulfate remains as a white powder:
copper(
ii
) sulfate → anhydrous copper(ii)
ii
) + water
pentahydrate sulfate
CuSO
4
.5H
2
O(
s
) → CuSO
4
(
s
) + 5H
2
O(
g
)
When water is added to anhydrous copper(ii)
ii
) sulfate
the reverse process occurs. It turns blue and the
pentahydrate is produced (Figure 8.21). This is an
extremely exothermic process.
CuSO
4
(
s
) + 5H
2
O(
l
) → CuSO
4
.5H
2
O(
s
)
Figure 8.22
A white powder forms on the surface of sodium carbonate
decahydrate when it is left in the air.
With some substances, not necessarily salt hydrates,
the reverse of efflorescence occurs. For example,
if anhydrous calcium chloride is left in the air,
it absorbs water vapour and eventually forms a
very concentrated solution. This process is called
deliquescence
, and substances which behave like this
are said to be
deliquescent
. Solid sodium hydroxide
will deliquesce.
There are some substances which, if left out in
the atmosphere, absorb moisture but do not change
their state. For example, concentrated sulfuric acid, a
colourless, viscous liquid, absorbs water vapour from
the air and becomes a solution. Substances which do
this are said to be
hygroscopic
.
Calculation of water of crystallisation
Sometimes it is necessary to work out the
percentage, by mass, of water of crystallisation in a
hydrated salt. The method is the same as that used in
Chapter 4, p. 65, but this time the 'H
2
O' is treated
as an element in the calculation.
Figure 8.21
Anhydrous copper(
ii
) sulfate is a white powder which turns
blue when water is added to it.
