Water of crystallization

Water locked inside crystals

1. Iron ores and extraction of iron

2. Dry crystals, wet tube

Heat dry blue copper(II) sulphate crystals in a test tube. They turn white. Water droplets collect near the cool top of the tube.

Pick the right answer

Predict: where did the water come from?

A. The crystals were wet on the outside
B. Water was locked inside the crystals
C. The heat made water from copper and sulphur

الشرح

The water was locked inside the crystals as water of crystallization. Heating drove it out, so the blue crystals turned white; it cannot come from copper and sulphur, because

CuSO4 CuSO X_{4}

contains no hydrogen.

3. The water in the formula

Water of crystallization

A fixed number of water molecules held inside the crystal of a solid.

CuSO4.5H2O CuSO X_{4} \cdot 5 H X_{2} O

Blue copper(II) sulphate crystals

The dot joins the salt and its water.

5H2O 5 H X_{2} O

means five water molecules for every one

CuSO4 CuSO X_{4}

The number is fixed for each substance.

4. Hydrated, then heated

Hydrated

Describes a substance that contains water of crystallization.

CuSO4.5H2O(s) \to[Δ] CuSO4(s) + 5H2O(v) CuSO X_{4} \cdot 5 H X_{2} O (s) Δ CuSO X_{4} (s) + 5 H X_{2} O (v)

Heating drives the water off as vapour

The blue hydrated crystals become a white powder. The solid left has lost all its water of crystallization.

5. A number in front

Sometimes a number stands in front of the first part too.

2CaSO4.H2O 2 CaSO X_{4} \cdot H X_{2} O

Plaster of Paris

Read it as: one

H2O H X_{2} O

for every two

CaSO4 CaSO X_{4}

. Each number counts the part right after it.

Match the pairs

Your turn: match each hydrate to the water it holds.

$Na2CO3.10H2O Na X_{2} CO X_{3} \cdot 10 H X_{2} O$
$CoCl2.6H2O CoCl X_{2} \cdot 6 H X_{2} O$
$CaSO4.2H2O CaSO X_{4} \cdot 2 H X_{2} O$
$2CaSO4.H2O 2 CaSO X_{4} \cdot H X_{2} O$

6. Read these hydrates

Fill in the blanks

2Fe2O3.3H2O 2 Fe X_{2} O X_{3} \cdot 3 H X_{2} O

, there are _____ water molecules for every _____ units of

Fe2O3 Fe X_{2} O X_{3}

الشرح

2Fe2O3.3H2O 2 Fe X_{2} O X_{3} \cdot 3 H X_{2} O

there are 3 water molecules for every 2 units of

Fe2O3 Fe X_{2} O X_{3}

: the 3 counts the

H2O H X_{2} O

part and the 2 in front counts the

Fe2O3 Fe X_{2} O X_{3}

part.

Pick the right answer

Which of these iron compounds is hydrated?

A. $Fe2O3 Fe X_{2} O X_{3}$
B. $Fe(OH)3 Fe (OH) X_{3}$
C. $FeSO4.7H2O FeSO X_{4} \cdot 7 H X_{2} O$
D. $FeCO3 FeCO X_{3}$

الشرح

FeSO4.7H2O FeSO X_{4} \cdot 7 H X_{2} O

is the hydrated compound: after the dot it holds seven water molecules for every

FeSO4 FeSO X_{4}

. The others have no water part after a dot;

Fe(OH)3 Fe (OH) X_{3}

has H and O as OH groups, not water molecules.

7. Summary

Water of crystallization: a fixed number of water molecules inside a crystal.
It is written after a dot: $CuSO4.5H2O CuSO X_{4} \cdot 5 H X_{2} O$ . A number in front counts the part right after it.
Hydrated = contains water of crystallization. Heating drives this water off.

Can you explain why heated blue crystals turn white and leave droplets?

Reveal

Heating drives the water of crystallization off as vapour:

CuSO4.5H2O(s) \to[Δ] CuSO4(s) + 5H2O(v) CuSO X_{4} \cdot 5 H X_{2} O (s) Δ CuSO X_{4} (s) + 5 H X_{2} O (v)

. The white powder left has lost all its water, and the water collects as droplets near the cool top of the tube.

Next: one iron ore, limonite, is a hydrated oxide.

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