Hydrated and Anhydrous Salts

Introduction

Crystals of copper(II) sulfate are a beautiful deep blue. Heat them and they crumble into a white powder while steam rises. Add a few drops of water to the white powder and it turns blue again and gets warm. This striking colour change is one of the classic reversible reactions in chemistry, and it gives a simple, reliable test for water. This page explains what is happening.

Core explanation

Hydrated salts. Many salts form crystals that contain water molecules locked into their structure. This water is called water of crystallisation , and the crystals are described as hydrated . The water is not simply wetness on the surface: the crystals feel completely dry. The water molecules are held in fixed positions, bonded to the metal ions.

Blue copper(II) sulfate crystals have the formula CuSO₄·5H₂O, meaning there are five water molecules for every copper(II) sulfate unit. The dot in the formula shows that the water is part of the crystal.

Anhydrous salts. "Anhydrous" means "without water". When hydrated copper(II) sulfate is heated, the water of crystallisation is driven off as steam, and white anhydrous copper(II) sulfate, CuSO₄, is left.

hydrated copper(II) sulfate ⇌ anhydrous copper(II) sulfate + water CuSO₄·5H₂O ⇌ CuSO₄ + 5H₂O (blue) (white)

Why the colour changes. The blue colour depends on water molecules being bonded around each copper(II) ion. Without them, the copper(II) ions absorb light differently, and the solid looks white. The colour change is therefore evidence that the substance has changed.

Energy changes. Removing the water needs energy: the forward reaction is endothermic , which is why heating is needed. Adding water to the anhydrous salt is exothermic : the powder gets noticeably warm, sometimes enough to hiss as water touches it. The energy released equals the energy taken in during heating.

A reversible chemical change. A new substance with different properties forms in each direction, so this is chemical, yet the starting material is easily recovered, so it is reversible.

A test for water. Because anhydrous copper(II) sulfate turns from white to blue in the presence of water, it is used to detect water. A positive result shows water is present but does not prove the liquid is pure; pure water is confirmed by boiling at 100 °C and freezing at 0 °C.

Salt Hydrated colour Anhydrous colour --- --- --- Copper(II) sulfate Blue White Cobalt(II) chloride Pink Blue Iron(II) sulfate Pale green White

Step-by-step reasoning

To explain the copper(II) sulfate change:

1. Start with blue hydrated crystals containing water of crystallisation. 2. Heating supplies energy that breaks the bonds holding the water in the crystal. 3. The water escapes as steam and white anhydrous copper(II) sulfate is left. 4. Adding water rebonds water molecules to the copper(II) ions, releasing energy. 5. The blue hydrated salt forms again.

Visual explanation

Picture a copper(II) ion drawn as a central sphere, surrounded by small bent water molecules like petals around a flower centre, coloured blue. An arrow labelled "heat" leads to the same copper(II) ion without its water petals, coloured white. A return arrow labelled "add water" restores the petals and the blue colour.

Real-world analogy

Hydrated crystals are like a sponge cake soaked in syrup, where the syrup is part of the cake's structure rather than just sitting on top. Baking it dry changes its colour and texture. Unlike the cake, though, copper(II) sulfate can take the water back and return exactly to its original form.

Real-world example

Small packets of silica gel protect electronics and shoes from damp. Some types contain an indicator, often cobalt(II) chloride, or safer modern alternatives, that changes colour as the gel absorbs water. When the colour shows the gel is saturated, it can be dried in a warm oven and reused, the same hydrated-to-anhydrous cycle described on this page.

Why?

Why do the crystals feel dry if they contain so much water? About 36% of the mass of blue copper(II) sulfate crystals is water, yet the water molecules are bonded into fixed positions in the crystal lattice. They are not free to move or wet other surfaces, so the crystals behave as a dry solid.

Common misconception

"Hydrated crystals are just wet crystals." Water of crystallisation is chemically bound in the structure in a fixed ratio. Wet crystals have loose water on the surface that can be dabbed away; hydrated crystals are dry to the touch.

Worked example

Question: A student adds a colourless liquid to white anhydrous copper(II) sulfate. It turns blue and warms up. What can be concluded, and what cannot?

Reasoning: The white to blue change shows that water is present, and the warming fits the exothermic hydration. However, many solutions contain water, so the test does not show that the liquid is pure.

Answer: The liquid contains water, but it may not be pure water; boiling point and freezing point measurements are needed to confirm purity.

Quick check

1. What colour is anhydrous copper(II) sulfate, and what colour does it turn with water? Answer: It is white and turns blue when water is added.

Exam focus

Learn the colours (blue hydrated, white anhydrous), the reversible equation with ⇌, and the energy changes (heating is endothermic, adding water is exothermic). A frequent question asks how to test for water: add anhydrous copper(II) sulfate, which turns from white to blue. Remember that this tests for water, not for purity.

Advanced insight

The number of water molecules in a hydrated salt can be found by heating a weighed sample until its mass stops changing, then weighing again. The mass lost is the water. For copper(II) sulfate, the results show five water molecules per formula unit, confirming the formula CuSO₄·5H₂O.

Summary

Hydrated salts contain water of crystallisation bound within their crystals; anhydrous salts contain none. Heating blue hydrated copper(II) sulfate removes the water and leaves white anhydrous copper(II) sulfate, an endothermic change. Adding water reverses it, releasing heat and restoring the blue colour. The white-to-blue change is a standard test for the presence of water.

Practice questions

1. What is meant by water of crystallisation? Answer: Water molecules that are chemically bound within the crystal structure of a salt. 2. Write the word equation, with the correct symbol, for heating hydrated copper(II) sulfate. Answer: hydrated copper(II) sulfate ⇌ anhydrous copper(II) sulfate + water. 3. Why does the mixture get warm when water is added to anhydrous copper(II) sulfate? Answer: The hydration reaction is exothermic, releasing the same energy that was taken in when the water was removed. 4. Describe how anhydrous copper(II) sulfate is used to test for water. Answer: Add the liquid to the white powder; if water is present, the powder turns blue. 5. Is the change from blue to white copper(II) sulfate physical or chemical? Explain. Answer: Chemical, because a new substance with different properties forms, even though the change is reversible.