Question

7. Give reasons for the following:

a) Sodium is kept under kerosene.

b) Iron cannot displace zinc from its solution.

c) A copper vessel acquires a dull green coating if exposed to moist air for long.

d) Silver jewellery turns dark over a period of time.

e) Why gold and silver are used for jewellery.​

Answer 1

Answer:

a)it is highly reactive and reacts vigorously with oxygen

b)iron is less reactive metal than zinc.

c)it reacts with oxygen to form a green layer

d)it reacts with oxygen to form a black layer

e)they are less reactive metal and are not corroided with sweat too.

Answer 2

Answer:

à) reaction of sodium, potassium, lithium with oxygen is so violent and it can catches fire. We can not store these metals in water also because it can give exothermic reaction, which is too violent

b.) iron cannot displace zinc from its salt solution because zinc is more reactive than iron. More reactive metal displaces less reactive metal from its salt solution. Zn has displaced copper, but iron cannot do so.

c) When a copper statue (or copper vessel) is exposed to moist air for long, it acquires a dull green coating. The green material is a mixture of copper hydroxide [Cu(OH)2] and copper carbonate (CuC03) formed due to the reaction of copper with moist air.

d) Silver mostly reacts with atmospheric sulphur particles though being less reactive and forms silver sulphite (Ags) which is a black coloured substance and turns the jewelleries into dark black. For this the jewellery must be cleaned with proper reducing agents.

e) They are highly lustrous metals which are resistant to corrosion. They are highly malleable and ductile so can be transformed into any shape or design.