Hydrates


Some compounds form crystals with a definite proportion of water, which are called hydrates. Zinc sulfite is an example. The formula for solid zinc sulfite is ZnSO3⋅2H2O. Note the centered dot between ZnSO3 and 2H2O. The 2 in front of the formula for water multiplies both H2 and O by 2, although there are no parentheses. Greek prefixes are used to show the number of water molecules in hydrates. The name of this compound is zinc sulfite dihydrate. As usual, 1 is understood if no number is written. The compound MgSO4⋅H2O has one molecule of water for each unit of MgSO4 and is called magnesium sulfate monohydrate.