Hydrated salts: a colour change you can reverse

Blue to white and back

Like chemical changes, physical changes can be reversible. Copper sulphate pentahydrate, CuSO₄·5H₂O, is a blue salt with five water molecules held in each formula unit.

Heat it strongly and the water is driven off: it becomes white anhydrous copper sulphate, CuSO₄. Leave the white powder in the air and it takes the water back and turns blue again.

Cobalt chloride hexahydrate, CoCl₂·6H₂O, is pink. Heating turns it into blue anhydrous CoCl₂ and water vapour.

Going back, blue CoCl₂ with only a little moisture first becomes the purple dihydrate, CoCl₂·2H₂O. With more moisture it becomes the pink hexahydrate again.

Heat pushes the water out; moisture brings it back. The colour tells you which way it went.

Key terms

Hydrated salt

The coloured starting form.

Anhydrous salt

What heating leaves behind.

Pentahydrate / hexahydrate

Naming how much water is held.

Dihydrate

The purple in-between cobalt form.

Short questions with model answers

1. Q1. Blue copper sulphate is heated gently, then strongly, then cooled in air. Describe what you see. (Activity, p. 85)

   • Heat: blue → white, water vapour leaves
   • Cool in air: white → blue

   CuSO₄·5H₂O ⇌ CuSO₄ + 5H₂O

2. Q2. Write the three cobalt chloride forms in order as moisture is added to the anhydrous salt, with colours. (p. 85)

   • CoCl₂: blue
   • CoCl₂·2H₂O: purple
   • CoCl₂·6H₂O: pink

   Blue → purple → pink

Common mistakes

• ✗ Anhydrous copper sulphate is blue.

  ✓ Hydrated is blue; anhydrous is white.

• ✗ Cobalt chloride turns pink when heated.

  ✓ Pink hexahydrate turns blue when heated.

• ✗ Blue CoCl₂ goes straight to pink with a little moisture.

  ✓ With limited moisture it first forms the purple dihydrate.

MCQs

1. 1. Heating blue CuSO₄·5H₂O strongly gives:

   1. (a) White CuSO₄
   2. (b) Pink CuSO₄
   3. (c) Purple CuSO₄·2H₂O
   4. (d) No change
   Show answer

   (a) p. 84.

2. 2. Anhydrous cobalt chloride is:

   1. (a) Pink
   2. (b) Blue
   3. (c) White
   4. (d) Purple
   Show answer

   (b) p. 84.

3. 3. CoCl₂·2H₂O is formed when anhydrous CoCl₂ gets:

   1. (a) Strong heat
   2. (b) Limited moisture
   3. (c) Excess water
   4. (d) Copper sulphate
   Show answer

   (b) p. 85: limited moisture gives the purple dihydrate.

4. 4. How many water molecules are in one formula unit of copper sulphate pentahydrate?

   1. (a) 2
   2. (b) 5
   3. (c) 6
   4. (d) 10
   Show answer

   (b) Penta means five: CuSO₄·5H₂O.

5. 5. White anhydrous CuSO₄ left in moist air will:

   1. (a) Stay white
   2. (b) Turn blue
   3. (c) Turn pink
   4. (d) Melt
   Show answer

   (b) p. 84: it turns blue again.

Quick revision

• Physical changes can be reversible too.
• CuSO₄·5H₂O (blue) ⇌ CuSO₄ (white) + 5H₂O: heat one way, moisture the other.
• Cobalt chloride: pink hexahydrate, purple dihydrate, blue anhydrous.