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.
And if you want the precise astronomical moment when spring starts (which is when the sun crosses the equator and heads north), that will be at 10.45pm on that date. In 2015, the date will be easy to remember because March 20 also happens to be when we will experience Europeâs biggest solar eclipse since 1999. Spring will last until the Summer Solstice, which in 2015 will be on June 21. That will be the longest day and signal the start of summer. From that point, the days begin to get shorter until daylight and night are once again approximately of equal length on the Autumnal Equinox on September 23.