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?
- (a)Oxygen
- (b)Nitrogen
- (c)Water
- (d)Hydrogen
Answer
Why
Correct — C, Water. Blue copper sulphate crystals are hydrated, CuSO4.5H2O, and it is the water of crystallisation — the water molecules bound in the crystal — that gives them their blue colour. Careful heating drives off this water, leaving white anhydrous CuSO4, which proves that water is responsible for the blue colour.
Why the others are wrong
- (a)Oxygen — Oxygen is part of the sulphate ion but is not what colours the crystal; removing only the water turns it white while the sulphate stays in place.
- (b)Nitrogen — Copper sulphate contains no nitrogen at all, so nitrogen cannot be responsible for the colour.
- (d)Hydrogen — Hydrogen is present only as part of the water molecules; it is the water of crystallisation as a whole, not free hydrogen, that gives the blue colour.
Concept
Many salts crystallise with a fixed number of water molecules locked into the crystal, called the water of crystallisation. Copper sulphate crystallises as the blue pentahydrate, CuSO4.5H2O; heating removes the water to give white anhydrous copper sulphate, and adding water restores the blue colour.
The colour change on heating is the clue. Since only water leaves the crystal when it turns white, water must be the cause of the blue colour. Oxygen and hydrogen are ruled out because they are merely atoms inside the sulphate or the water, and copper sulphate has no nitrogen.
Key facts
- Blue copper sulphate is the pentahydrate, CuSO4.5H2O.
- Its five water molecules make up the water of crystallisation.
- Heating removes the water to give white anhydrous CuSO4, and adding water restores the blue.
- This reversible colour change is a classic laboratory test for the presence of water.
Study next
Common traps
- Attributing the colour to oxygen or hydrogen atoms rather than to water.
- Forgetting that the white-to-blue change reverses when water is added back.
NDA/UPSC ask what gives hydrated salts their colour or what happens on heating them — link the colour to the water of crystallisation.
Related PYQs
Number of molecules of water of crystallization in copper sulphate, sodium carbonate and Gypsum are
- (a) 5, 10 and 2 respectively
- (b) 10, 2 and 5 respectively
- (c) 5, 2 and 10 respectively
- (d) 2, 5 and 10 respectively
Answer(a) 5, 10 and 2 respectively
NDA GAT 2020 tested the water of crystallisation in copper sulphate directly (5 molecules, CuSO4.5H2O) — the very water that gives the blue crystals their colour and is lost on heating.
Practice
- practice — not a real PYQ
The number of water molecules of crystallisation in blue copper sulphate is
- (a)2
- (b)5
- (c)7
- (d)10
Answer(b) 5 — blue copper sulphate is the pentahydrate, CuSO4.5H2O. - practice — not a real PYQ
White anhydrous copper sulphate turns blue when
- (a)it is heated strongly
- (b)water is added to it
- (c)it is exposed to nitrogen
- (d)it is dissolved in acid
Answer(b) water is added to it — the return of water of crystallisation restores the blue colour, a test for water.