(i) The black powder acts as a catalyst. The catalysts accelerate the speed of reactions without itself undergoing any permanent changes
(ii) Black powder is manganese dioxide (Mn02).
(iii) Black powder cannot be used for all decomposition reactions because catalysts are highly specific in nature. A catalyst which can catalyse one reaction may have no effect on another reaction even, if that reaction is very similar.
(iv) Glycerol slows down the decomposition of H₂O₂ and it is called negative catalyst

(i) Zinc is needed in our diet for digestion of proteins. There are more than 20 zinc-containing enzymes in the human body which are responsible for proper absorption of CO2 by red blood cells in muscles and other tissues and for maintaining proper pH. Some of the zinc-containing enzymes play an important role in the digestion of proteins by animals.
(ii) Zinc phosphide is used as a rodent poison.
(iii) Zinc oxide (white zinc) is used in paints.
(iv) ZnSO₄(aq) is colorless because it has filled d-subshell.

Copper has small size compared to alkali metals in their period. Its electronic configuration
is [Ar] 3d104s1. As filled 3d-subshell is less effective in shielding the outer s-electrons than a s- and p-subshell, so s-electron in copper is more tightly held by the nucleus than in alkali metals. Hence, its first ionization energy is higher than for alkali metals. However, second and third ionization energy values of copper are lower as compared to those of alkali metals due to the removal of electrons from diffused d-orbitals.

In cuprous chloride (CuCI), Cu⁺ ion has fully filled 3d-subshell and therefore, it cannot absorb energy for the d-d transition. Therefore, it is colorless. In cupric chloride (CuCI₂), Cu²⁺ has 3d9 configuration having one unpaired electron and therefore, it can absorb energy for the d-d transition. Hence, it is blue coloured.

Ag⁺ ion is stronger Lewis acid than Na⁺. Ag⁺ has 18-electron shell configuration (known as pseudo inert gas configuration) and causes greater polarization than Na⁺, an ion with 8-electron shell configuration. This is due to the fact that in the case of 18-electrons shell ion there are 10 d-electrons in addition to 88-and p-electrons. The d-electrons do not shield the nuclear charge effectively and therefore, they have increased the effective nuclear charge. Hence they cause greater polarization and therefore, Ag⁺ is a stronger Lewis acid than Na⁺ ion.