Efflorescence, Deliquescence &
Exsiccation
By: Dr. Tahseen Ismail
EFFLORESCENCE
• The large no of chemical compound exists
in hydrated or anhydrous form. In hydrated
form the water molecule attached to the
compound are called the water of
crystallization. The surrounding atmosphere
also has vapours and definite vapour
pressure, due to water in atmosphere. If the
vapour pressure of the hydrated substance
is greater than the surrounding atmosphere,
than the hydrated substance loses water
molecule to become less hydrous and to
make an equilibrium between the hydrated
substance and the atmosphere.
EFFLORESCENCE
• This phenomenon is called efflorescence.
• In simple words efflorescence is the loss of
water of crystallization from the hydrated
substances into the atmosphere so that an
equilibrium is formed b/w the hydrated
substance and the surrounding
atmosphere.
Examples:
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Examples of Efflorescent substances are;
1. Borax Na2B4O7.H2O (Hydrated substance)
2. Na2SO4.10H2O
3. Na2CO3.10H2O
e.g. ZnSO4.7H2O and MgSO4.7H2O
Na2CO3.10H2O loses 9 water molecules first and
then becomes completely anhydrous as follows;
Spontaneous
Na2CO3.10H2O ----------------------> Na2CO3 .H2O
Efflorescence
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Na2CO3 (Anhydrous0
• Since the vapor pressure of the
hydrated substance is greater then the
atmospheric vapor pressure, so there
is a spontaneous loss of water of
crystallization till the equilibrium is
obtained. After the equilibrium
formation number more loss of water
molecules occur.
Prevention of Efflorescence
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Efflorescence or loss of water of
crystallization can be prevented by the
following ways:
By closing the container immediately
after use.
By filling the container completely so
that no space for air storage is
available above the liquid in the
container.
Deliquescence
• Deliquescence is a reverse of
efflorescence.
• If a hydrated substance has a lower
vapour pressure, than the surrounding
atmosphere than the water molecule
transfer from the atmosphere to the less
hydrated substance to make them more
hydrated and to produce a equilbrium.This
phenomenon is called deliquescence. e.g.:
NaOH
Prevention of Deliquescence
• By closing the container immediately after
using it.
• By filling the container completely so that
no space for air storage is available above
the liquid in the container.
• By placing the dehydrating agent e.g:
(Silica gel)to absorb water.
Exsiccation
• Exsiccation is the process of removing the water of
crystallization from the hydrated crystalline substances
by heating and making them less hydrous or
anhydrous.
• The examples of exsiccated substances include
exsiccated ferrous sulphate, exsiccated magnesium
sulphate, exsiccated sodium sulphate, exsiccated
sodium carbonate, exsiccated alum and anhydrous
sodium arsenate.
• In the laboratory, exsiccation may be carried out by
taking a weighed amount of the substance in a tared
dish which is heated on water bath, sand bath or in an
oven, with continuous stirring until a constant weight is
obtained or until the calculated loss in weight has
taken place.
Exsiccation
• The temperature required to remove 'water of crystallization is
very important. For example, copper sulphate, CuSO4 . 5H20,
when heated at about 30°C loses two molecules of water of
crystallization, at 100°C it loses two more water molecules and
the last water molecule is removed when it is heated at 200°C,
thus forming CuSO4. 3H2O, CuSO4.H20 and CuSO4
respectively.
• Similarly ferrous sulphate FeSO4.7H2O when heated on water
bath loses six molecules of water of crystallization 'but when
further heated to remove last water molecule, decomposition
takes place. Hence ferrous sulphate with one molecule of water
of crystallization FeSO4.H2O is called exsiccated ferrous
sulphate.
• Since the exsiccated salts are very hygroscopic, they must be
stored in well closed containers.
Exsiccation
Applications
• 1. Exsiccation is done to get an anhydrous
product required in the formulation of certain
medicaments.
• 2. It is also carried out to reduce the bulk and
weight of certain drugs so that they can be
easily administered.
• 3. Generally on exsiccation a fine powder of the
substance is obtained.