Many times when a substance crystallizes into a solid, molecules of water are included in the crystal. These molecules of water combine with the substance in a fixed ratio, similar to the fixed ratios between the atoms in a molecule. Whenever weighing or doing calculations based on compounds that have waters of hydration, the amount of water in the crystals must be taken into consideration.
When writing formulas for these compounds, the waters of hydration are shown by placing a dot (or dash) after the formula for the compound, followed by the formula for water with a coefficient to indicate the number of waters of hydration. For example, cupric sulfate forms crystals that contain five molecules of water for each molecule of cupric sulfate--its formula is written CuSO45H2O.
Compounds that contain waters of hydration are called hydrates. (If all the water has been removed by drying, they are called anhydrous.) When writing the names for these compounds, the number of waters of hydration is indicated by using number prefixes. Thus, the name for CuSO45H2O is cupric sulfate pentahydrate. Another number prefix seen occasionally in the names of hydrates is hemi-, which means one-half (½).
- AlCl36H2O is aluminum chloride hexahydrate
- Mg3(PO4)25H2O is magnesium phosphate pentahydrate
- Na2HPO47H2O is disodium hydrogen phosphate heptahydrate
- FeSO47H2O is ferrous sulfate heptahydrate
- Na2CO31OH2O is sodium carbonate decahydrate
- CaSO4½H2O is calcium sulfate hemihydrate (two molecules of calcium sulfate for each molecule of water)