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nH2O

nH2O is the conventional notation used in chemistry to denote a hydrate: a compound that includes a specific number of water molecules incorporated into its crystal structure as water of crystallization. The water molecules are integral to the solid’s lattice, not separate reactants, and the value of n reflects the stoichiometry of hydration. For many hydrates, n is fixed, but some substances can exist in several hydration states that depend on temperature and ambient humidity.

Common hydrated salts include copper(II) sulfate pentahydrate, CuSO4·5H2O; magnesium sulfate heptahydrate, MgSO4·7H2O; calcium chloride dihydrate, CaCl2·2H2O;

Hydrates form when a compound crystallizes from water or absorbs water from the environment. Dehydration occurs

Water content is typically determined by loss on drying or thermogravimetric analysis, and proper control of

and
sodium
sulfate
decahydrate,
Na2SO4·10H2O.
The
dot
in
the
formula
separates
the
main
formula
from
its
water
of
crystallization.
In
cases
where
the
exact
water
content
varies
with
conditions,
a
general
form
such
as
X·nH2O
is
used
to
indicate
hydration
without
specifying
n.
when
the
compound
is
heated
or
placed
under
low
humidity,
yielding
anhydrous
or
lower-hydration-state
forms.
Rehydration
can
then
occur
upon
exposure
to
moisture.
The
hydration
state
influences
physical
properties
such
as
stability,
solubility,
and
sometimes
color.
hydration
is
important
for
the
handling,
storage,
and
application
of
hydrated
reagents
and
minerals.
Hydrates
are
relevant
in
both
laboratory
and
industrial
contexts,
where
the
presence
or
absence
of
water
of
crystallization
can
affect
performance
and
reliability.