Chemguide: Support for CIE A level Chemistry
Learning outcome 2.3.4
The meanings of anhydrous, hydrated, and water of crystallisation
An anhydrous substance is one that doesn't contain water. The term is most commonly used in inorganic chemistry - for example, you will come across anhydrous copper(II) sulfate and anhydrous sodium carbonate.
Anhydrous copper(II) sulfate is simply CuSO4, and anhydrous sodium carbonate is Na2CO3.
In organic chemistry, the term is sometimes used where it is essential that a solvent is absolutely free of water - for example, anhydrous ether (ethoxyethane) means that the ether contains no water (or at least an amount of water low enough to be acceptable).
Water of crystallisation
Water of crystallisation is water which is locked into a crystal in a fixed way. So for example, crystals of copper(II) sulfate have the formula CuSO4·5H2O.
You may also find this written with a comma rather than a full stop as CuSO4,5H2O. Ideally, the full stop should be centred half-way up the formula as CuSO4·5H2O, but that is a bit of a bother if you are typing because you have to use a special character. Most people probably wouldn't notice.
Note: If anyone spots a case where a CIE mark scheme has penalised people for writing the dot in the wrong place, please let me know urgently using the email address on this page.
CuSO4·5H2O is the typical blue copper(II) sulfate crystals. Anhydrous copper(II) sulfate is a white powder.
I mentioned anhydrous sodium carbonate above, Na2CO3. Crystals of sodium carbonate have 10 molecules of water of crystallisation, Na2CO3·10H2O.
A substance is hydrated if water, or the elements of water (hydrogen and oxygen in the ratio of 2:1), is added to it.
So if you add water to anhydrous copper(II) sulfate to give blue CuSO4·5H2O, you have hydrated the copper(II) sulfate.
Copper ions in solution are also hydrated. Rather than going around as Cu2+(aq), the copper(II) ions are wrapped up in 6 molecules of water, Cu(H2O)62+(aq). The water molecules are bonded to the copper(II) ion, not just vaguely surrounding it.
In organic chemistry, the word hydrated is used in the sense of adding the elements of water.
For example, in industry the gas ethene, C2H4, is hydrated by reacting it with steam to give ethanol, C2H5OH, better written as CH3CH2OH.
In cases like this, the water isn't present in the new compound in the form of water. Here, a hydrogen from the water has added to one of the carbons, and OH to the other one.
© Jim Clark 2019