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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.

All Acids, Bases and Salts notesNDA Chemistry strategy

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.
SaltFormulaWater molecules
Blue vitriol (copper sulphate)CuSO4·5H2O5
Green vitriol (ferrous sulphate)FeSO4·7H2O7
Ferrous sulphate crystal carries 7 water molecules.
Washing sodaNa2CO3·10H2O10
GypsumCaSO4·2H2O2
Mohr's saltFeSO4·(NH4)2SO4·6H2O6
Potassium permanganateKMnO40 (none)
KMnO4 has NO water of crystallization — the bank's answer for 'which salt has none'.
Practice this conceptself-check · 5 quick reps

Try it yourself

Which salt does NOT have water of crystallization: potassium permanganate, blue vitriol, washing soda or Mohr's salt?

Practice — Level 1 (5 reps)

Quick reps to lock in the method. Try each, then check.

  1. 1.
    How many water molecules are in a ferrous sulphate crystal?
  2. 2.
    Water of crystallization in copper sulphate (blue vitriol)?
  3. 3.
    Water of crystallization in washing soda?
  4. 4.
    Which salt has NO water of crystallization: KMnO4, blue vitriol, washing soda or Mohr's salt?
  5. 5.
    Water of crystallization in Mohr's salt?

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.
FormColourCause
CuSO4·5H2O (hydrated)BlueWater of crystallization
The blue colour of copper sulphate crystals is due to water — heat it out and it turns white.
CuSO4 (anhydrous, after heating)WhiteWater removed
Practice this concept3 quick reps

Practice — Level 1 (3 reps)

Quick reps to lock in the method. Try each, then check.

  1. 1.
    Why are copper sulphate crystals blue?
  2. 2.
    What colour is copper sulphate after the water is driven off by heating?
  3. 3.
    What is responsible for the blue colour of CuSO4·5H2O: oxygen, nitrogen, water or hydrogen?

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
SaltFormulaWater molecules
Blue vitriol (copper sulphate)CuSO4·5H2O5
Green vitriol (ferrous sulphate)FeSO4·7H2O7
Ferrous sulphate crystal carries 7 water molecules.
Washing sodaNa2CO3·10H2O10
GypsumCaSO4·2H2O2
Mohr's saltFeSO4·(NH4)2SO4·6H2O6
Potassium permanganateKMnO40 (none)
KMnO4 has NO water of crystallization — the bank's answer for 'which salt has none'.
Water of crystallization and colour2 rows
FormColourCause
CuSO4·5H2O (hydrated)BlueWater of crystallization
The blue colour of copper sulphate crystals is due to water — heat it out and it turns white.
CuSO4 (anhydrous, after heating)WhiteWater removed

Watch out for (2)

Mastery check — 1 interleaved questions

Try each one before clicking. Questions are interleaved across the concepts above, not grouped — interleaving sharpens transfer.

Example 1Acids, Bases and SaltsMODERATE
Which one of the following salts does not possess water of crystallization?

[Q104 · Sep · 2022]

Drill every past-year question on this subtopic

3 questions from the bank — paginated, with cart and Word-export support.

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