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Unit 1
Elements of Chemical Structure and Inorganic Nomenclature

1-14. WATERS OF HYDRATION

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 ().

Examples