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.
Click here for more Autumn photos. In the United States and the rest of the northern hemisphere, the first day of the autumn season is the day of the year when the Sun crosses the celestial equator moving southward (on September 22nd or 23rd). This day is known as the Autumnal Equinox.