A Blue Crystal Turns White and Then Blue Again
Learn what water of crystallisation is and how to spot it in a formula, calculate what fraction of a crystal is really water, see the colour changes when hydrated salts are heated, and sort substances into efflorescent, deliquescent and hygroscopic.
How can a dry blue crystal be more than a third water?
Because the water is built into the crystal rather than sitting on it.
A crystal of blue vitriol feels perfectly dry and looks perfectly solid, yet about 36 per cent of its mass is water. That water is chemically bound into the crystal's structure, in a fixed number of molecules per formula unit — not a damp film you could wipe off.
Heat it and the water leaves, and the blue turns white. Add water back and the blue returns, along with a burst of heat. This page covers the third part of the ICSE Class 8 Chemistry chapter on water: water of crystallisation, what heating does to it, and the three different ways salts deal with moisture in the air.
A crystal of blue vitriol feels perfectly dry and looks perfectly solid, yet about 36 per cent of its mass is water. That water is chemically bound into the crystal's structure, in a fixed number of molecules per formula unit — not a damp film you could wipe off.
Heat it and the water leaves, and the blue turns white. Add water back and the blue returns, along with a burst of heat. This page covers the third part of the ICSE Class 8 Chemistry chapter on water: water of crystallisation, what heating does to it, and the three different ways salts deal with moisture in the air.
Formula
What is water of crystallisation, and how much of a crystal is it?
Water of crystallisation is the fixed number of water molecules chemically combined with a salt in its crystalline form. It gives the crystal its shape and often its colour.
A salt containing it is hydrated; one without it is anhydrous. In a formula, the water is written after a dot:
- Copper sulphate pentahydrate, — blue vitriol, molecules, blue
- Sodium carbonate decahydrate, — washing soda, molecules
- Calcium sulphate dihydrate, — gypsum, molecules
- Ferrous sulphate heptahydrate, — green vitriol, molecules, green
- Magnesium sulphate heptahydrate, — Epsom salt, molecules
- Zinc sulphate heptahydrate, — white vitriol
Salts with no water in the formula — , , , — are anhydrous. So reading a formula tells you at once which you have: look for the dot.
Calculating the fraction that is water:
**Worked example 1 — blue vitriol, .** With , , , :
**Worked example 2 — washing soda, .** With , :
So washing soda is more than half water by mass, while looking and feeling like a dry solid.
Why the number of molecules is fixed. The water occupies definite positions in the crystal lattice, so a hydrate has a definite composition exactly as any compound does. There is no such thing as — which is precisely why a hydrate counts as a pure substance and not as a damp mixture.
A salt containing it is hydrated; one without it is anhydrous. In a formula, the water is written after a dot:
- Copper sulphate pentahydrate, — blue vitriol, molecules, blue
- Sodium carbonate decahydrate, — washing soda, molecules
- Calcium sulphate dihydrate, — gypsum, molecules
- Ferrous sulphate heptahydrate, — green vitriol, molecules, green
- Magnesium sulphate heptahydrate, — Epsom salt, molecules
- Zinc sulphate heptahydrate, — white vitriol
Salts with no water in the formula — , , , — are anhydrous. So reading a formula tells you at once which you have: look for the dot.
Calculating the fraction that is water:
**Worked example 1 — blue vitriol, .** With , , , :
**Worked example 2 — washing soda, .** With , :
So washing soda is more than half water by mass, while looking and feeling like a dry solid.
Why the number of molecules is fixed. The water occupies definite positions in the crystal lattice, so a hydrate has a definite composition exactly as any compound does. There is no such thing as — which is precisely why a hydrate counts as a pure substance and not as a damp mixture.
What happens when a hydrated salt is heated?
The water of crystallisation is driven off, the salt becomes anhydrous, and the crystal loses its shape and usually its colour.
Copper sulphate, heated in a dry test tube:
The observations, all of which are marked:
- The blue crystals turn white as they become anhydrous copper sulphate.
- The crystals crumble to a powder, losing their shape.
- Water droplets condense on the cooler upper walls of the tube.
- There is a loss in mass.
Adding the water back. Put a few drops of water on the white powder and it turns blue again, and the tube becomes warm — the reaction is exothermic.
So the change is reversible, and this is the standard laboratory test for the presence of water: white anhydrous copper sulphate turning blue proves water is there.
Washing soda, heated:
The transparent crystals become a white powder and lose a great deal of mass — over half of it, as the previous section's calculation showed.
Green vitriol, , turns from green to a dirty white anhydrous powder on gentle heating.
What the colour change actually tells you. The colour of hydrated copper sulphate belongs to the hydrated form, not to copper as such. Remove the water and the blue goes with it. This is one of the clearest demonstrations that water of crystallisation is part of the substance rather than an impurity in it.
A boundary case. Heat some hydrates too strongly and they decompose altogether rather than simply losing water — and that change is not reversible. Gentle heating drives off water; strong heating destroys the salt, which is why the experiment specifies heat gently.
Copper sulphate, heated in a dry test tube:
The observations, all of which are marked:
- The blue crystals turn white as they become anhydrous copper sulphate.
- The crystals crumble to a powder, losing their shape.
- Water droplets condense on the cooler upper walls of the tube.
- There is a loss in mass.
Adding the water back. Put a few drops of water on the white powder and it turns blue again, and the tube becomes warm — the reaction is exothermic.
So the change is reversible, and this is the standard laboratory test for the presence of water: white anhydrous copper sulphate turning blue proves water is there.
Washing soda, heated:
The transparent crystals become a white powder and lose a great deal of mass — over half of it, as the previous section's calculation showed.
Green vitriol, , turns from green to a dirty white anhydrous powder on gentle heating.
What the colour change actually tells you. The colour of hydrated copper sulphate belongs to the hydrated form, not to copper as such. Remove the water and the blue goes with it. This is one of the clearest demonstrations that water of crystallisation is part of the substance rather than an impurity in it.
A boundary case. Heat some hydrates too strongly and they decompose altogether rather than simply losing water — and that change is not reversible. Gentle heating drives off water; strong heating destroys the salt, which is why the experiment specifies heat gently.
What is the difference between efflorescence, deliquescence and hygroscopy?
All three concern a solid and the moisture in the air, but they go in different directions.
Efflorescence — a hydrated salt loses its water of crystallisation to the air on its own, becoming a powder. The salt gives water out.
Washing soda left on a plate becomes a dull white powder within days. Others: Glauber's salt , green vitriol , and blue vitriol slowly.
Deliquescence — a substance absorbs so much moisture from the air that it dissolves in it, forming a solution. The substance takes water in, and changes state from solid to liquid.
Examples: sodium hydroxide, potassium hydroxide, calcium chloride, magnesium chloride, ferric chloride. A stick of sodium hydroxide left out becomes a puddle.
This is also why common salt turns damp and cakes in the monsoon — pure sodium chloride is not deliquescent, but the magnesium chloride present as an impurity is.
Hygroscopy — a substance absorbs moisture from the air but does not dissolve in it and does not change state. It stays solid, or stays a liquid if it was one.
Examples: concentrated sulphuric acid, quicklime (), phosphorus pentoxide, silica gel, and anhydrous copper sulphate.
The two tests that separate them.
- Does the substance gain or lose water? Losing means efflorescent. Gaining means deliquescent or hygroscopic.
- Does it dissolve in the water it absorbed? Dissolving into a solution means deliquescent. Staying solid means hygroscopic.
So deliquescence and hygroscopy differ only in whether the substance dissolves, and that is the distinction examiners test.
Where these properties are put to work. Hygroscopic substances are used as drying agents — concentrated sulphuric acid and calcium chloride dry gases in the laboratory, and a sachet of silica gel keeps medicines and electronics dry. Deliquescent substances must be stored in airtight bottles, and efflorescent ones too, for the opposite reason.
And a caution about quicklime. It absorbs water and also reacts with it to form slaked lime, so it is doing more than hygroscopy alone. Substances that merely hold water can give it back on heating; quicklime cannot, because a chemical change has occurred.
Efflorescence — a hydrated salt loses its water of crystallisation to the air on its own, becoming a powder. The salt gives water out.
Washing soda left on a plate becomes a dull white powder within days. Others: Glauber's salt , green vitriol , and blue vitriol slowly.
Deliquescence — a substance absorbs so much moisture from the air that it dissolves in it, forming a solution. The substance takes water in, and changes state from solid to liquid.
Examples: sodium hydroxide, potassium hydroxide, calcium chloride, magnesium chloride, ferric chloride. A stick of sodium hydroxide left out becomes a puddle.
This is also why common salt turns damp and cakes in the monsoon — pure sodium chloride is not deliquescent, but the magnesium chloride present as an impurity is.
Hygroscopy — a substance absorbs moisture from the air but does not dissolve in it and does not change state. It stays solid, or stays a liquid if it was one.
Examples: concentrated sulphuric acid, quicklime (), phosphorus pentoxide, silica gel, and anhydrous copper sulphate.
The two tests that separate them.
- Does the substance gain or lose water? Losing means efflorescent. Gaining means deliquescent or hygroscopic.
- Does it dissolve in the water it absorbed? Dissolving into a solution means deliquescent. Staying solid means hygroscopic.
So deliquescence and hygroscopy differ only in whether the substance dissolves, and that is the distinction examiners test.
Where these properties are put to work. Hygroscopic substances are used as drying agents — concentrated sulphuric acid and calcium chloride dry gases in the laboratory, and a sachet of silica gel keeps medicines and electronics dry. Deliquescent substances must be stored in airtight bottles, and efflorescent ones too, for the opposite reason.
And a caution about quicklime. It absorbs water and also reacts with it to form slaked lime, so it is doing more than hygroscopy alone. Substances that merely hold water can give it back on heating; quicklime cannot, because a chemical change has occurred.
Exam tip
Exam tip: separate gaining water from dissolving in it
Answer any classification here with two facts: the direction of water movement and whether the substance dissolves.
- Efflorescent — loses water of crystallisation to the air, becoming a powder.
- Deliquescent — absorbs moisture and dissolves in it, becoming a solution.
- Hygroscopic — absorbs moisture and does not dissolve, staying solid.
Writing absorbs moisture alone cannot distinguish the last two, and that is exactly where marks are lost.
For the heating of copper sulphate, give all four observations — blue to white, crystals crumble, water droplets on the cooler walls, and loss in mass.
Name the test for water: white anhydrous copper sulphate turns blue, and the mixture warms.
Say the change is reversible on adding water, and specify gentle heating — strong heating decomposes the salt irreversibly.
For the percentage calculation, set out the two masses separately before dividing: , , total , so .
Spot a hydrate in a formula by the dot, and quote the number of water molecules.
And remember pure common salt is not deliquescent — it is the magnesium chloride impurity that makes it damp.
- Efflorescent — loses water of crystallisation to the air, becoming a powder.
- Deliquescent — absorbs moisture and dissolves in it, becoming a solution.
- Hygroscopic — absorbs moisture and does not dissolve, staying solid.
Writing absorbs moisture alone cannot distinguish the last two, and that is exactly where marks are lost.
For the heating of copper sulphate, give all four observations — blue to white, crystals crumble, water droplets on the cooler walls, and loss in mass.
Name the test for water: white anhydrous copper sulphate turns blue, and the mixture warms.
Say the change is reversible on adding water, and specify gentle heating — strong heating decomposes the salt irreversibly.
For the percentage calculation, set out the two masses separately before dividing: , , total , so .
Spot a hydrate in a formula by the dot, and quote the number of water molecules.
And remember pure common salt is not deliquescent — it is the magnesium chloride impurity that makes it damp.
Did you know
Why does a sachet of beads keep a medicine bottle dry?
Open a bottle of tablets or a box of electronics and you often find a small paper sachet of hard translucent beads, marked do not eat.
Those beads are silica gel, and they are doing one job: absorbing the water vapour in the sealed air before it can reach whatever is being protected. They are hygroscopic, so they hold the moisture without dissolving and without turning into a puddle at the bottom of the bottle — which is precisely why silica gel is chosen rather than something deliquescent like calcium chloride.
The beads also declare when they are full. Many are made with an indicator that changes colour once they have taken up about as much water as they can hold, and warming them in an oven drives the water off again so they can be reused.
That reusability is the giveaway that nothing chemical happened. The water had merely been held, not combined — which is the same reason anhydrous copper sulphate can go from white to blue and back to white indefinitely.
Those beads are silica gel, and they are doing one job: absorbing the water vapour in the sealed air before it can reach whatever is being protected. They are hygroscopic, so they hold the moisture without dissolving and without turning into a puddle at the bottom of the bottle — which is precisely why silica gel is chosen rather than something deliquescent like calcium chloride.
The beads also declare when they are full. Many are made with an indicator that changes colour once they have taken up about as much water as they can hold, and warming them in an oven drives the water off again so they can be reused.
That reusability is the giveaway that nothing chemical happened. The water had merely been held, not combined — which is the same reason anhydrous copper sulphate can go from white to blue and back to white indefinitely.
Key takeaways
Water of crystallisation and moisture behaviour: quick revision
- Water of crystallisation is a fixed number of water molecules chemically combined in a crystal, written after a dot in the formula.
- Hydrated salts: (blue vitriol), (washing soda), (gypsum), (green vitriol), (Epsom salt). Anhydrous salts have no dot.
- . Blue vitriol: , so . Washing soda: , so — more than half water.
- On heating: — blue to white, crystals crumble, water droplets on cooler walls, loss in mass.
- Adding water reverses it, turning the powder blue and giving out heat — the standard test for water.
- Washing soda gives a white powder; green vitriol goes green to dirty white. Strong heating decomposes the salt and is not reversible.
- Efflorescence — loses water of crystallisation to the air: washing soda, Glauber's salt, green vitriol.
- Deliquescence — absorbs moisture and dissolves in it: sodium hydroxide, potassium hydroxide, calcium chloride, magnesium chloride, ferric chloride.
- Hygroscopy — absorbs moisture without dissolving: concentrated sulphuric acid, quicklime, phosphorus pentoxide, silica gel, anhydrous copper sulphate.
- The two tests: gains or loses water, and dissolves or not. Deliquescence and hygroscopy differ only on the second.
- Hygroscopic substances are used as drying agents; deliquescent and efflorescent ones need airtight storage. Pure common salt is not deliquescent — its magnesium chloride impurity is.
Sort a list of a dozen substances into the three categories, giving both facts for each — that pair of facts is the whole answer the examiner wants.
- Hydrated salts: (blue vitriol), (washing soda), (gypsum), (green vitriol), (Epsom salt). Anhydrous salts have no dot.
- . Blue vitriol: , so . Washing soda: , so — more than half water.
- On heating: — blue to white, crystals crumble, water droplets on cooler walls, loss in mass.
- Adding water reverses it, turning the powder blue and giving out heat — the standard test for water.
- Washing soda gives a white powder; green vitriol goes green to dirty white. Strong heating decomposes the salt and is not reversible.
- Efflorescence — loses water of crystallisation to the air: washing soda, Glauber's salt, green vitriol.
- Deliquescence — absorbs moisture and dissolves in it: sodium hydroxide, potassium hydroxide, calcium chloride, magnesium chloride, ferric chloride.
- Hygroscopy — absorbs moisture without dissolving: concentrated sulphuric acid, quicklime, phosphorus pentoxide, silica gel, anhydrous copper sulphate.
- The two tests: gains or loses water, and dissolves or not. Deliquescence and hygroscopy differ only on the second.
- Hygroscopic substances are used as drying agents; deliquescent and efflorescent ones need airtight storage. Pure common salt is not deliquescent — its magnesium chloride impurity is.
Sort a list of a dozen substances into the three categories, giving both facts for each — that pair of facts is the whole answer the examiner wants.