The s-Block Elements

(i) Slaked lime:  It is a white amorphous solid formed when quicklime is added to water.
  
(ii) Milk of lime: It is the suspension of slaked lime in water.
(iii) Lime water: When milk of lime is kept for some time undisturbed in a beaker and the solution formed on the surface is decanted. It is called lime water. 

Gypsum is hydrated calcium sulphate, CaSO₄.2H₂O
Preparation: It can be prepared by the action of dilute sulphuric acid on calcium carbonate or calcium hydroxide.

Properties:
(i) It is a white crystalline solid sparingly soluble in water.
(ii) On heating to 393 K, it changes into  

Plaster of Paris is calcium sulphate hemihydrate.

Preparation:
(i) From gypsum (CaSO_4. 2H₂O): When gypsum is heated to about 393 - 403 K, partial dehydration takes place with the formation of Plaster of Paris. 

(ii) On a large scale, gypsum is gradually heated in a large steel vessel, holding several tonnes of material. This steel vessel is provided with a mechanical stirrer. During heating, the gypsum is stirred mechanically and the temperature is maintained between 393 - 403 K.

The temperature should be controlled carefully between 393 - 403 K, otherwise above this temperature (say 473 K), the whole of the water of hydration is lost and the gypsum gets dead burnt.

Properties:
(i) It is a white powder.
(ii) When mixed with water, Plaster of Paris quickly solidifies to gypsum with the evolution of heat and also expands slightly.

(iii) The action of heat: When plaster of Paris is heated at 473 K, it forms anhydrous calcium sulphate. 
Uses. (i) It is used in surgery for plastering fractured parts of the body.
         (ii) It is used for making casts for statues and for preparing blackboard chalks. 

(i) Calcium carbonate on heating decomposes to produce calcium oxide (quick lime) and carbon dioxide.

(ii)  MgCl₂.6H₂O, on heating undergoes hydrolysis by its own water of crystallisation to give magnesium oxide and hyrogen chloride.
                  
(iii) On heating to 390K, it forms a compound having the composition  which on heating above 473 K, changes into anhydrous calcium sulphate. 
               
(iv) On heating to 423 K, it changes into monohydrate which on further heating to 473K, forms anhydrous salt. 
          
Anhydrous salt on further heating, forms MgO.