About $.......\, \%$ impurity remains in alumina obtained by Bayer's method. (in $\%$)

Which of the following gases is of brown colour?

Which element forms the basis of components like vitamins,proteins,and carbohydrates?

Give reasons for the following:
$(a)$ School bells are made of metals.
$(b)$ Electrical wires are made of copper.

An electrolytic cell consists of:
$(i)$ Positively charged cathode
$(ii)$ Negatively charged anode
$(iii)$ Positively charged anode
$(iv)$ Negatively charged cathode

What happens when
$(a)$ $ZnCO_{3}$ is heated in the absence of oxygen?
$(b)$ $A$ mixture of $Cu_{2}O$ and $Cu_{2}S$ is heated?