The notation  hydrated compound ânH2O , where n is the number of water molecules per formula unit of the salt, is commonly used to show that a salt is hydrated. The n is usually a low integer, though it is possible for fractional values to occur. For example, in a monohydrate n is one, and in a hexahydrate n is 6.
When does spring start? How you define the first day of spring depends on whether you follow the astronomical or meteorological seasons. This year, spring will take place between March 20 and June 21, if you use the astronomical method. But it starts slightly earlier, between March 1 and May 31, if you follow the meteorological calendar.
Astronomical and solar reckoning. In some cultures in the Northern Hemisphere (e.g. Germany), the astronomical Vernal equinox (varying between 19 and 21 March) is taken to mark the first day of spring, and the Summer solstice (around 21 June) is taken as the first day of summer.