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.

Na2CO3.10H2O ----------------------> Na2CO3 .H2O Examples: 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 ^ /||\ || 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 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.