(a) If sodium metal is kept in the open, it reacts vigorously with air and produces harmful effects. Hence to avoid such effects and to prevent accidental fires, they are kept immersed in kerosene.

(b) When alumninium powder is added in a copper sulphate solution, the blue colour of the solution goes away. The solution become colourless. Here, the displacement reaction is taking place. Aluminium being more reactive displaces copper from its own sulphate. As a result, aluminium sulphate is formed. The reaction is:

CuSO₄ + Al → Al₂(SO₄)₃ + Cu