Explain the following statements:

i. Most metal oxides are insoluble in water but some of these dissolve in water. What are these oxides and their solutions in water called?

ii. At ordinary temperature, the surface of metals such as magnesium, aluminium and zinc, etc. is covered with a then layer. What is the composition of this layer? State its importance.

iii. Some alkali metals can be cut with a knife.

Explain the following statements:

i. Most metal oxides are insoluble in water but some of these dissolve in water. What are these oxides and their solutions in water called?

ii. At ordinary temperature, the surface of metals such as magnesium, aluminium and zinc, etc. is covered with a then layer. What is the composition of this layer? State its importance.

iii. Some alkali metals can be cut with a knife.

Why sodium chloride has high melting point?

Metals are refined by using different methods. Which of the following metals are refined by electrolytic refining?

(i) Au

(ii) Cu

(iii) Na

(iv) K

Silver articles become black on prolonged exposure to air. This is due to the formation of

State reasons for the following observations:

(a) The shining surface of some metals becomes dull when exposed to air for a long time.

(b) Zinc fails to evolve hydrogen gas on reacting with dilute nitric acid.

(c) metal sulphides occur mainly in rocks but metal halides occur mostly in sea and lake waters.