The s-Block Elements

Question 185
CBSEENCH11007163

How will you distinguish between:
(i) Slaked lime    (ii) Milk of lime      (iii) Lime water?

Solution

(i) Slaked lime:  It is a white amorphous solid formed when quicklime is added to water.
  CaO with Quick space lime below space plus space straight H subscript 2 straight O space space rightwards arrow space space space space stack Ca left parenthesis OH right parenthesis subscript 2 with Slaked space lime below
(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. 

Question 186
CBSEENCH11007164

What is Gypsum? How is it prepared? What are its properties and uses?

Solution
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.
CaCO subscript 3 left parenthesis straight s right parenthesis space plus space straight H subscript 2 SO subscript 4 left parenthesis aq right parenthesis space space rightwards arrow space space CaSO subscript 4 left parenthesis straight s right parenthesis space plus space CO subscript 2 left parenthesis straight g right parenthesis space plus space straight H subscript 2 straight O left parenthesis straight l right parenthesis
Ca left parenthesis OH right parenthesis subscript 2 left parenthesis aq right parenthesis space plus space straight H subscript 2 SO subscript 4 left parenthesis aq right parenthesis space rightwards arrow space space space CaSO subscript 4 left parenthesis straight s right parenthesis space plus space 2 straight H subscript 2 straight O left parenthesis straight l right parenthesis
Properties:
(i) It is a white crystalline solid sparingly soluble in water.
(ii) On heating to 393 K, it changes into CaSO subscript 4. space 1 half straight H subscript 2 straight O left parenthesis Plaster space of space Paris right parenthesis. 
On heating to about 437 K, it changes into anhydrous calcium sulphate. 

                        stack CaSO subscript 4. space 2 straight H subscript 2 straight O with Gypsum below space rightwards arrow with 390 on top stack space CaSO subscript 4.1 half straight H subscript 2 straight O with Plaster space of space Paris below space space rightwards arrow from 473 straight K to Above of space CaSO subscript 4
 
Uses. It is used: 
(i) in the manufacture of cement. 
(ii) for manufacturing Plaster of Paris
(iii) as a drying agent. 
(iv) for the preparation of ammonia, bleaching powder slaked lime and basic calcium nitrate. 
Question 187
CBSEENCH11007165

What is Plaster of Paris? How is it prepared? What are its properties and uses?

Solution

Plaster of Paris is calcium sulphate hemihydrate.
CaSO subscript 4. space 1 half straight H subscript 2 straight O space or space left parenthesis CaSO subscript 4 right parenthesis subscript 2. space straight H subscript 2 straight O
Preparation:
(i) From gypsum (CaSO4. 2H2O): When gypsum is heated to about 393 - 403 K, partial dehydration takes place with the formation of Plaster of Paris. 
2 CaSO subscript 4. space 2 straight H subscript 2 straight O space rightwards arrow from 393 minus 403 space straight K to Heat of space stack 2 CaSO subscript 4. space straight H subscript 2 straight O with Plaster space of space Paris below space plus space 3 straight H subscript 2 straight O
(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.
2 CaSO subscript 4. space 2 straight H subscript 2 straight O space space space rightwards arrow from 393 minus space 403 straight K to Heat of space stack left parenthesis CaSO subscript 4 right parenthesis subscript 2 space. with Plaster space of space Paris below space straight H subscript 2 straight O space plus space 3 straight H subscript 2 straight O
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.
stack 2 CaSO subscript 4. space 1 half straight H subscript 2 straight O with Plaster space of space Paris below space plus space 3 straight H subscript 2 straight O space space rightwards arrow space space space stack 2 CaSO subscript 4. space 2 straight H subscript 2 straight O with Gypsum below
(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. 

Question 188
CBSEENCH11007166

What is the effect of heat on the following compounds? (Write equations for the reactions):
(i) Calcium Carbonate
(ii) Magnesium chloride hexahydrate
(iii) Gypsum
(iv) Magnesium sulphate heptahydrate

Solution
(i) Calcium carbonate on heating decomposes to produce calcium oxide (quick lime) and carbon dioxide.
CaCO subscript 3 space rightwards arrow with 1070 space minus 1270 straight K on top space CaO space plus space CO subscript 2
(ii)  MgCl2.6H2O, on heating undergoes hydrolysis by its own water of crystallisation to give magnesium oxide and hyrogen chloride.
                  MgCl subscript 2. space end subscript 6 straight H subscript 2 straight O space rightwards arrow with Heat on top space MgO space plus space 2 HCl space plus space 5 straight H subscript 2 straight O
(iii) On heating to 390K, it forms a compound having the composition CaSO subscript 4. space 1 half straight H subscript 2 straight O left parenthesis known space as space Plaster space of space Paris right parenthesis which on heating above 473 K, changes into anhydrous calcium sulphate. 
               CaSO subscript 4. space 2 straight H subscript 2 straight O space rightwards arrow with 390 space straight K on top space CaSO subscript 4. space 1 half straight H subscript 2 straight O space space rightwards arrow from 473 space straight K to above of space CaSO subscript 4
(iv) On heating to 423 K, it changes into monohydrate which on further heating to 473K, forms anhydrous salt. 
          MgSO subscript 4. space 7 straight H subscript 2 straight O space space rightwards arrow with 423 space straight K on top space MgSO subscript 4. space straight H subscript 2 straight O space rightwards arrow with 473 space straight K on top space MgSO subscript 4 space plus space straight H subscript 2 straight O
Anhydrous salt on further heating, forms MgO.
              MgSO subscript 4 space rightwards arrow with Heat on top space MgO space plus space SO subscript 3

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