Compounds of alkaline earth metals

Compounds of alkaline earth metals

Magnesium sulphate, epsom slat, MgSO₄. 7H₂O

It is prepared by dissolving magnesium oxide or carbonate in dilute sulphuric acid.

MgO + H2SO4          -- > MgSO4+ H2O

Uses

1.     As a purgative

2.     In dyeing and tanning processes and in dressing cotton goods.

3.     Platinised MgSO4 is used as a catalyst.

Calcium oxide, CaO, quicklime

It is prepared by burning limestone in specially designed kilns.

CaCO_{3      -- 1070K} -- >   CaO + CO₂

Properties

1.     Lime is a white porous solid

2.     On adding water it gives a hissing sound and becomes very hot. The fine powder obtained is known as slaked lime and consists of calcium hydroxide Ca(OH)₂. This process is called slaking of lime.

CaO + H₂O -- > Ca(OH)₂

The paste of lime in water is called milk of lime whereas the filtered and clear solution is known as lime water.

3.     With chlorine it gives bleaching powder CaOCl₂ . H₂O

4.     With carbondioxide, it forms calcium carbonate while with sulphurdioxide, calcium sulphite is obtained.

5.     Moist hydrochloric acid gas reacts with it to give calcium chloride but there is no action with the dry gas.

CaO + 2 HCl  -- >     CaCl2 + H2O

Uses

1.     For the manufacture of calcium chloride, cement, mortar and glass.

2.     For drying gases and alcohol.

3.     As milk of lime, used in refining sugar and white washing.

4.     As lime water, used as a reagent in laboratory and in medicine.

Calcium sulphate, CaSO₄

It occurs as Anhydrite, CaSO₄ and Gypsum CaSO₄. 2H₂O. It may be prepared by adding dilute sulphuric acid to the solution of a calcium salt.

CaCO₃ + H₂SO₄     CaSO₄ + H₂O + CO₂

Uses of Gypsum

1.     It is used

2.    For preparing plasters

3.    As a retardant for the setting of cement

Plaster of Paris

When Gypsum is heated to about 393K it loses 1½ molecules of water and forms plaster of paris with the formula CaSO₄ ½H₂O, Calcium Sulphate hemihydrate. The substance is known as plaster of paris because the large deposits of Gypsum used for the manufacture of plaster are at Montmeite (Paris).

When plaster of paris is wetted with, it forms a plastic mass which sets in from 5 to 15 minutes to a white porous hard mass. A slight expansion occurs during the setting so that it will take sharp impression of a mould. The process of setting takes place in two steps, the setting step and the hardening step. The final product of setting is gypsum.

CaSO4.½H2O ---- (3/2 H₂O ) -- >

setting step

hardening

CasO4.2H2O   -----   -à  CaSO4.2 H2O

Step

The setting step may be catalysed by NaCl while it is retarded by borax or alum.

Uses : It is used

1.     In surgery for plastering the fractured bones.

2.     In making moulds for statues, in dentistry etc.

In making false ceilings.