When we talk about ionic compounds (the ones that make nice, regular crystal structures), we teach that they are formed from the electrostatic attraction between oppositely charged ions. This is true. It is also true that the ions align themselves very nicely to create crystalline structures that maximize the interactions between oppositely charged ions and minimize the repulsions between same-charged ions. What doesn’t always get mentioned is what happens when there is empty space left over in between the ions.
When the ions that make up a crystal are not the right size to pack closely together, relatively large spaces can be left between them. If that space is large enough, water molecules will fill the void. Because of water is a natural dipole, its poles are attracted to the charged ions that make up a crystal, and it will pretty much stay put once it has been absorbed into the crystal. The only practical way to remove it is by heating the crystal to turn the water to a gas, which then escapes. (This can be hazardous, for if the water evaporates too quickly, steam pressure can build up inside and cause the crystal to crack, pop/explode, and send sharp pieces flying.)
When there is no water with a crystal, the compound is called “anhydrous” (without water). When the crystal has water trapped inside, it is referred to as a hydrate, or a hydrated compound.
Because the structure of the crystal is regular, there are a regular number of spaces available for the water molecules to fit into. This number will vary for different compounds, but will always be a constant in a particular compound. Because of this, water molecules will be present in a specific ratio to the molecules of ionic compound. (In a crystal, individual molecules of the ionic compound do not really exist, but the number of ions can be used to compute how many there would be if they were all separate, so we use that.) An example would be the hydrate of copper (II) sulfate. There are always five water molecules for every CuSO4.
The presence of water in a crystal changes some of its physical characteristics. It is probably obvious that by cramming water into the spaces, a hydrate will be denser than the anhydrous form of the same compound. More importantly though is that many hydrates will have a color that is not observed in the anhydrous form. Referring back to CuSO4, which is colorless/white on its own, the addition of water turns it to a beautiful royal blue.
The chemical formula for a hydrate is easily written. First you write the chemical formula of the compound, then a middle dot (which represents that the water is coordinated, but not bonded) and then the number of water molecules. I’ll have to use an asterisk instead of a middle dot here, but you’ll get the picture.
As we said, copper (II) sulfate gets five water molecules each, so it is written:
CuSO4 * 5 H2O
Naming such a compound is also simple. First you name the compound in the usual manner, then you say hydrate where the prefix indicates how many water molecules there are. Here’s your list of prefixes:
1 – mono
2 – di
3 – tri
4 – tetra
5 – penta
6 – hexa
7 – hepta
8 – octa
9 – nona
10 – deca
11 – undeca
12 – dodeca
So our CuSO4 * 5 H2O is called “copper (II) sulfate pentahydrate”.