Water of Crystallisation
The Sponge Matrix — how crystals trap water in their very structure
🧽 The Sponge Matrix
Imagine a dry sponge — it has empty pockets waiting to be filled. Now imagine a salt crystal is like an atomic sponge. When it crystallises from water, it doesn't just let the water drain away. It grabs water molecules and locks them chemically into its lattice structure.
The result? A hydrated salt — vivid in colour, perfectly crystalline, and hiding water inside every unit cell. Heat it up, and the water escapes as steam, leaving behind a dull, collapsed anhydrous powder.
Hydrated
Contains water molecules
CuSO₄·5H₂O — Blue
Anhydrous
No water molecules
CuSO₄ — White
🔬 Inside the Crystal Lattice
Each CuSO₄ unit has exactly 5 water molecules locked between the ions.
5 H₂O molecules per formula unit · CuSO₄·5H₂O
🔥 The Heating Cycle — Watch the Water Escape
Heat the hydrated crystals to drive off the water of crystallisation.
Before Heating
Blue crystals
Heating
Water escaping
After Heating
White powder
🧮 Mr Calculation — Step by Step
Calculate the relative formula mass of CuSO₄·5H₂O
Examiner Trap!
The · (dot) in the formula means ADD the Mr values together — it does NOT mean multiply! Many students confuse this.
Step 1: Start with the formula
CuSO₄ · 5H₂O
Step 2: Find Mr of CuSO₄
Click to reveal
Step 3: Find Mr of 5H₂O
Click to reveal
Step 4: ADD them together
Click to reveal
📝 Definition Builder
Assemble the Cambridge definition for Water of Crystallisation in the correct order.
Definition so far:
🎨 Colour Chemistry Recap
Link to Topic 6.3 — colour changes reveal hydration state.
Copper(II) Sulfate
Blue crystals → White powder
CuSO₄·5H₂O → CuSO₄
Cobalt(II) Chloride
Pink crystals → Blue powder
CoCl₂·6H₂O → CoCl₂
Sodium Sulfate
White crystals → White powder
Na₂SO₄·10H₂O → Na₂SO₄
🔬 The CoCl₂ Test — Paper 6 Classic
Anhydrous cobalt(II) chloride paper turns from blue → pink in the presence of water. This is the standard test for water in IGCSE chemistry.
📌 Syllabus Definitions — Commit These
Water molecules that are present in hydrated crystals
The dot · in the formula = water molecules are chemically combined
A substance that is chemically combined with water
e.g. CuSO₄·5H₂O — blue crystals
A substance that contains no water
e.g. CuSO₄ — white powder
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