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Blue copper sulfate crystals are one of the prettiest things in a school chemistry lab. Heat them gently and something surprising happens: they crumble into a dull white powder, and droplets of water appear at the cool mouth of the test tube. Add a few drops of water back and the powder turns blue again, getting noticeably warm.
The water was part of the crystal all along. Solids like this are called hydrates.
What a hydrate is
A hydrate is an ionic compound that has water molecules built into its crystal structure in a fixed ratio. That water is called water of crystallization (or water of hydration). It isn’t “wetness” — the crystals feel completely dry — but it’s an integral part of the solid.
The formula shows the water after a raised dot:
CuSO₄·5H₂O — copper(II) sulfate pentahydrate: five water molecules for every CuSO₄ unit.
The version with the water removed is called anhydrous (“without water”): anhydrous CuSO₄ is white.
Naming hydrates
The number of water molecules is given with a Greek prefix plus “hydrate”:
| Number | Name |
|---|---|
| 1 | monohydrate |
| 2 | dihydrate |
| 3 | trihydrate |
| 4 | tetrahydrate |
| 5 | pentahydrate |
| 6 | hexahydrate |
| 7 | heptahydrate |
| 8 | octahydrate |
| 10 | decahydrate |
Some common examples:
- CuSO₄·5H₂O — copper(II) sulfate pentahydrate (blue crystals)
- MgSO₄·7H₂O — magnesium sulfate heptahydrate, Epsom salt
- CaSO₄·2H₂O — calcium sulfate dihydrate, gypsum. Plaster of Paris is the hemihydrate, CaSO₄·½H₂O; mixing it with water turns it back into gypsum, which is why plaster sets.
- Na₂CO₃·10H₂O — sodium carbonate decahydrate, washing soda
- CoCl₂·6H₂O — cobalt(II) chloride hexahydrate, pink; the anhydrous form is blue, which is why cobalt chloride paper is used to test for water
Molar mass of a hydrate
The water counts. The dot means “plus”, and the number in front multiplies the whole water molecule:
CuSO₄·5H₂O
- CuSO₄: 63.546 + 32.06 + 4(15.999) = 159.60 g/mol
- 5H₂O: 5 × 18.015 = 90.08 g/mol
- Total: 249.68 g/mol
Forgetting the water is the most common mistake in hydrate problems. If a question gives you a mass of CuSO₄·5H₂O and you divide by 159.60, your moles will be about 56% too high.
Percent water in a hydrate
% water = mass of water in one formula unit ÷ molar mass of hydrate × 100
For CuSO₄·5H₂O: 90.08 ÷ 249.68 × 100 = 36.1% water.
So if you heat 10.0 g of the blue crystals until all the water is gone, about 3.6 g of water leaves and 6.4 g of white powder remains.
Finding the formula of an unknown hydrate
This is a classic lab experiment. You weigh a hydrate, heat it until its mass stops changing (so all the water has gone), and weigh the anhydrous residue.
A 5.00 g sample of hydrated magnesium sulfate, MgSO₄·xH₂O, is heated. The anhydrous MgSO₄ left behind weighs 2.44 g. Find x.
- Mass of water lost: 5.00 − 2.44 = 2.56 g
- Moles of water: 2.56 ÷ 18.015 = 0.1421 mol
- Moles of anhydrous salt: MgSO₄ = 120.37 g/mol → 2.44 ÷ 120.37 = 0.02027 mol
- Ratio: 0.1421 ÷ 0.02027 = 7.01
So x = 7, and the hydrate is MgSO₄·7H₂O — Epsom salt.
In a real experiment the ratio rarely comes out as a perfect whole number. Values like 6.8 usually mean not quite all the water was driven off; values above the expected whole number can mean the salt started to decompose.
Why hydrates matter
- Accurate weighing. If a recipe calls for a certain number of moles of a salt, you need to know whether your bottle contains the anhydrous form or a hydrate — they have very different molar masses. Always check the label.
- Drying agents. Anhydrous salts like calcium chloride and magnesium sulfate pull water out of the air or out of organic liquids by forming hydrates.
- Building materials. Gypsum plaster and cement both depend on hydration reactions to set.
- Deliquescence and efflorescence. Some anhydrous salts absorb so much moisture that they dissolve in it (deliquescence); some hydrates lose water to dry air and crumble to powder (efflorescence), as washing soda does.
Quick answers
Is the water in a hydrate “free” water? No. It’s held in the crystal lattice in a fixed ratio, often bonded to the metal ion. That’s why the crystals are dry to the touch.
Does the dot mean multiply? No — it means the two parts are combined in the crystal. For calculating mass, you add them.
Why does anhydrous copper sulfate get hot when water is added? Forming the hydrate releases energy: it’s an exothermic process, the reverse of the heating that removed the water.
Calculators that handle hydrates
The molar mass calculator and percent composition calculator both accept hydrate formulas typed with a dot (CuSO4·5H2O, or CuSO4.5H2O), and include the water in every result.
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