Describe briefly the preparation and uses of magnesium chloride (MgCl2.6H2O).
Preparation: (i) In the laboratory, magnesium chloride is prepared by the action of hydrochloric acid on magnesium oxide or carbonate.
The reaction mixture is cooled and concentrated when crystals of MgCl2.6H2O separate out. (ii) From carnallite: The mineral carnallite (KCl.MgCl2.6H2O) is powdered and then boiled with water. On cooling KCl is crystallised out while magnesium chloride is left in the mother liquor. The mother liquor is separated, concentrated by evaporation and cooled when crystals of MgCl2.6H2O separate out. (iii) From sea water: Sea water is concentrated and then treated with lime when magnesium hydroxide gets precipitated.
The precipitate of Mg(OH)2 is dissolved in HCl when a solution of magnesium chloride is obtained. The solution of magnesium chloride on concentration and cooling yields crystals of MgCl2.6H2O.
It is used: (i) in the preparation of magnesia cement. (ii) in the extraction of magnesium metal.
How is slaked lime prepared? What are its properties and uses?
Slaked lime is prepared by the following methods: (i) From quick lime: By treating quicklime with water, slaked lime is formed.
When water is added to quicklime, a huge amount of heat is produced along with the hissing sound. (ii) From calcium chloride: By treating calcium chloride with caustic soda, slaked lime is formed.
Properties: (i) Slaked lime is a white amorphous powder. (ii) A suspension of slaked lime in water is called milk of lime. (iii) The aqueous layer which is decanted from the precipitated calcium hydroxide is called lime water. (iv) On passing carbon dioxide through lime water, the lime water turns milky due to the formation of insoluble calcium carbonate.
If carbon dioxide is passed in excess, a clear solution is again obtained. This is because the insoluble calcium carbonate changes into soluble calcium bicarbonate.
If the clear solution is heated, it again turns milky due to the decomposition of calcium bicarbonate into calcium carbonate.
(v) Slaked lime reacts with chlorine to form bleaching powder.
Uses. It is used: (i) for the preparation of ammonia (ii) for white washing (iii) in the purification of sugar (iv) in the softening of hard water.
What is lime stone? How is it prepared? Give its properties and uses.
Limestone or marble is calcium carbonate (CaCO3). Preparation: In the laboratory, it is prepared by the action of sodium carbonate on calcium chloride when a white precipitate of calcium carbonate is obtained.
Properties: (i) It is a white powder, sparingly soluble in water. (ii) On heating to about 1200K, it decomposes to form calcium oxide.
Uses: It is used (i) in the form of marble for making floors. (ii) in toothpaste. (iii) in the form of fine powder in face powders and (iv) in the form of limestone for making lime, cement and glass.