NDA Chemistry · Acids, Bases and Salts

Water of Crystallization

The fixed number of water molecules locked into a salt crystal's structure, written after a dot in the formula — and the salts that have none.

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Why this matters

Three PYQs, all single-fact recall: the water count of a named hydrate, the salt that has no water of crystallization, and why hydrated copper sulphate is blue. Memorise the handful of counts and these are free marks.

Concept 1 of 2

Water of crystallization of common salts

Intuition

Water of crystallization is the fixed number of water molecules built into a crystal, written after a dot (for example CuSO4·5H2O). The bank asks for the count of a specific salt, or which salt has none. Learn the numbers.

Definition

The hydration numbers the bank tests:

Blue vitriol (copper sulphate) = CuSO4·5H2O → 5.
Green vitriol (ferrous sulphate) = FeSO4·7H2O → 7.
Washing soda (sodium carbonate) = Na2CO3·10H2O → 10.
Gypsum (calcium sulphate) = CaSO4·2H2O → 2.
Mohr's salt = FeSO4·(NH4)2SO4·6H2O → 6.
Potassium permanganate (KMnO4) has NO water of crystallization.

Salt	Formula	Water molecules
Blue vitriol (copper sulphate)	CuSO4·5H2O	5
Green vitriol (ferrous sulphate)	FeSO4·7H2O	7 Ferrous sulphate crystal carries 7 water molecules.
Washing soda	Na2CO3·10H2O	10
Gypsum	CaSO4·2H2O	2
Mohr's salt	FeSO4·(NH4)2SO4·6H2O	6
Potassium permanganate	KMnO4	0 (none) KMnO4 has NO water of crystallization — the bank's answer for 'which salt has none'.

Practice this conceptself-check · 5 quick reps

From the bank · past-year question

Example 1Acids, Bases and SaltsEASY

What is the number of water molecules present in a Ferrous Sulphate crystal?

[Q103 · Sep · 2021]

KMnO4 has no water of crystallization

Blue vitriol (5), washing soda (10) and Mohr's salt (6) are all hydrates. Potassium permanganate (KMnO4) is the one with no water of crystallization — there is no dot-water term in its formula.

Drill 1 more on water of crystallization of common salts

Concept 2 of 2

Water of crystallization and colour

Intuition

The water locked into a crystal can give it its colour. Blue copper sulphate crystals owe their blue to that water; heat it out and the crystal turns white. Add water back and the blue returns. The water of crystallization is the colour.

Definition

The colour-from-water facts:

Hydrated copper sulphate CuSO4·5H2O is blue; the blue colour is due to the water of crystallization.
On careful heating the water is driven off, leaving anhydrous CuSO4, which is white.
Adding water back turns it blue again — a standard test for the presence of water.

Form	Colour	Cause
CuSO4·5H2O (hydrated)	Blue	Water of crystallization The blue colour of copper sulphate crystals is due to water — heat it out and it turns white.
CuSO4 (anhydrous, after heating)	White	Water removed

Practice this concept3 quick reps

From the bank · past-year question

Example 2Acids, Bases and SaltsEASY

Copper sulphate crystals available in the market are blue coloured crystals. By careful heating, they turn to white colour. Which one of the following is responsible for the blue colour?

[Q113 · Apr · 2023]

Blue copper sulphate owes its colour to water

The blue of copper sulphate crystals is caused by the water of crystallization, not by oxygen, nitrogen or hydrogen. Heating drives the water off and the solid turns white.

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Reference tables (2)

Water of crystallization of common salts6 rows

Salt	Formula	Water molecules
Blue vitriol (copper sulphate)	CuSO4·5H2O	5
Green vitriol (ferrous sulphate)	FeSO4·7H2O	7 Ferrous sulphate crystal carries 7 water molecules.
Washing soda	Na2CO3·10H2O	10
Gypsum	CaSO4·2H2O	2
Mohr's salt	FeSO4·(NH4)2SO4·6H2O	6
Potassium permanganate	KMnO4	0 (none) KMnO4 has NO water of crystallization — the bank's answer for 'which salt has none'.

Water of crystallization and colour2 rows

Form	Colour	Cause
CuSO4·5H2O (hydrated)	Blue	Water of crystallization The blue colour of copper sulphate crystals is due to water — heat it out and it turns white.
CuSO4 (anhydrous, after heating)	White	Water removed

Watch out for (2)

KMnO4 has no water of crystallization
→ Water of crystallization of common salts
Blue copper sulphate owes its colour to water
→ Water of crystallization and colour

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