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.
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
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
The same idea in a real exam question:
Example 1 · Acids, Bases and Salts · Water of Crystallization
KMnO4 has no water of crystallization
Concept 2 of 2: Water of crystallization and 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
The same idea in a real exam question:
Example 2 · Acids, Bases and Salts · Water of Crystallization
Blue copper sulphate owes its colour to water
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
Test yourself on Acids, Bases and Salts
15 past NDA questions from this chapter, timed at 15 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.