Hydrates are ionic compounds that have absorbed water. They are named as the ionic compound followed by a numerical prefix and -hydrate. The numerical prefixes used are listed below (see IUPAC numerical multiplier):
# mono-
# di-
# tri-
# tetra-
# penta-
# hexa-
# hepta-
# octa-
# nona-
# deca-
For example, CuSO·5HO is "copper(II) sulfate pentahydrate".