Crystallisation and Water of Crystallisation

Introduction

Crystals of copper(II) sulfate are a beautiful deep blue. Heat them gently and something surprising happens: steam is given off and they crumble to a white powder. Add a few drops of water to the white powder and the blue returns, with the powder becoming warm. The blue colour depends on water molecules built into the crystal itself. This water, called water of crystallisation , is part of many salts that look completely dry.

Core explanation

How crystals form. Most salts are more soluble in hot water than in cold. When a hot solution is concentrated until it is saturated and then cooled, it can no longer hold all the dissolved salt. The excess comes out of solution as solid. Ions pack together in a regular, repeating arrangement, building crystals with flat faces and definite angles. Slow cooling gives the ions time to find their places, producing large, well-shaped crystals; fast cooling gives many small ones.

Water inside crystals. When some salts crystallise from water, water molecules become part of the crystal lattice in a fixed ratio to the ions. The result is a hydrated salt . Its formula shows the water after a dot:

- copper(II) sulfate pentahydrate: CuSO₄·5H₂O (blue) - magnesium sulfate heptahydrate (Epsom salts): MgSO₄·7H₂O (colourless) - sodium carbonate decahydrate (washing soda): Na₂CO₃·10H₂O (colourless) - cobalt(II) chloride hexahydrate: CoCl₂·6H₂O (pink)

The prefixes penta-, hexa-, hepta- and deca- mean 5, 6, 7 and 10. The crystals feel dry because the water is bonded into the structure, not lying on the surface.

Hydrated and anhydrous forms. Heating a hydrated salt drives off its water of crystallisation, leaving the anhydrous salt:

CuSO₄·5H₂O(s) ⇌ CuSO₄(s) + 5H₂O(l or g)

Heating in the forward direction takes in energy (endothermic). Adding water to the anhydrous salt reverses the change and releases energy (exothermic), which is why the white powder warms up as it turns blue.

A test for water. Because white anhydrous copper(II) sulfate turns blue with water, it is used to test whether a liquid contains water. Blue cobalt(II) chloride paper, which turns pink with water, is used in the same way. These tests show water is present but not that the liquid is pure water; purity is checked by boiling point (100 °C at standard pressure).

Why does water change the colour? In the hydrated crystal, water molecules are bonded to the copper(II) ions. This changes how the ions absorb light, so the crystal absorbs red-orange light and looks blue. Without the water, the copper ions absorb very little visible light, and the powder looks white.

Losing water in air. Some hydrated salts, such as washing soda, slowly lose water to dry air and become powdery on the surface. This is called efflorescence. Others, such as anhydrous calcium chloride, absorb water from the air and are used as drying agents.

Formulae

Molar mass of CuSO₄·5H₂O = 63.5 + 32.1 + (4 × 16.0) + 5 × (2 × 1.0 + 16.0) = 159.6 + 90.0 = 249.6 g/mol.

Step-by-step reasoning

To find the percentage of water in a hydrated salt:

1. Calculate the molar mass of the whole hydrated formula. 2. Calculate the mass of the water part (number of H₂O × 18.0). 3. Divide the water mass by the total molar mass. 4. Multiply by 100.

Visual explanation

Picture a blue crystal as a tidy stack of copper ions and sulfate ions with water molecules tucked neatly around each copper ion. Heat shakes the water molecules loose; they escape as steam and the stack collapses into a white powder with gaps where the water used to be.

Real-world analogy

A hydrated crystal is like a sponge that feels dry but still holds water deep inside its structure. Squeezing it — or heating a crystal — forces the hidden water out and changes the sponge's shape and feel.

Real-world example

Plaster of Paris is a partially hydrated calcium sulfate. When mixed with water it takes water back into its structure to become gypsum, CaSO₄·2H₂O, setting into a hard solid. This is how plaster casts for broken bones and decorative mouldings harden.

Why?

Why does adding water to anhydrous copper(II) sulfate release heat? Bonds form between water molecules and copper ions as the hydrated structure is rebuilt. Forming bonds releases energy — the same energy that had to be supplied to break them when the crystals were heated.

Common misconception

"Blue copper(II) sulfate crystals are wet." The crystals are dry to the touch. The water is chemically combined in a fixed ratio, which is why the formula CuSO₄·5H₂O is exact and not a variable amount of dampness.

Worked example

Question: Calculate the percentage by mass of water in CuSO₄·5H₂O (molar mass 249.6 g/mol).

Reasoning: Mass of water = 5 × 18.0 = 90.0 g/mol. Percentage = 90.0 ÷ 249.6 × 100.

Answer: About 36.1% of the mass of the blue crystals is water.

Quick check

1. What colour change is seen when water is added to anhydrous copper(II) sulfate? Answer: It changes from white to blue.

Exam focus

Learn the reversible reaction CuSO₄·5H₂O ⇌ CuSO₄ + 5H₂O, the colour change white to blue as a test for water, and that heating is endothermic while rehydrating is exothermic. Be able to include water of crystallisation in molar mass calculations.

Advanced insight

The number of water molecules in a hydrate can be found experimentally by weighing a sample, heating it to constant mass and weighing again. The mass lost is water; converting both masses to moles gives the ratio of salt to water, and hence the value of x in a formula such as MgSO₄·xH₂O.

Summary

Crystals form when a saturated solution cools and the salt comes out of solution. Many salts crystallise with water of crystallisation bound in a fixed ratio, such as blue CuSO₄·5H₂O. Heating removes this water to give the anhydrous salt; adding water reverses the change and releases heat. White anhydrous copper(II) sulfate turning blue is a test for water.

Practice questions

1. What does the "·5H₂O" in CuSO₄·5H₂O mean? Answer: Each formula unit of copper(II) sulfate is combined with five water molecules in the crystal. 2. Is dehydrating blue copper(II) sulfate endothermic or exothermic? Answer: Endothermic, because energy must be supplied to break the bonds holding the water in the crystal. 3. Why does slow cooling of a saturated solution give larger crystals? Answer: The ions have more time to arrange themselves on existing crystals rather than forming many new small ones. 4. Calculate the molar mass of MgSO₄·7H₂O (Mg = 24.3, S = 32.1, O = 16.0, H = 1.0). Answer: 24.3 + 32.1 + 64.0 + 7 × 18.0 = 246.4 g/mol. 5. Why does a positive result with anhydrous copper(II) sulfate not prove a liquid is pure water? Answer: It shows only that water is present; other substances could also be dissolved or mixed in.