Which gas is useful for inhibiting bacterial growth?

Give reasons for the following:
$(i)$ Gold and platinum are used in jewellery.
$(ii)$ Copper cannot displace hydrogen from dilute acids.
$(iii)$ Stainless steel does not rust easily.
$(iv)$ Metals can be given different shapes according to our needs.
$(v)$ Zinc does not give hydrogen gas on reacting with $HNO_3$.

What is the chemical formula of alumina?

Which substance is formed when ammonia gas reacts with hydrogen chloride gas?

Which method is used for the purification of semiconductor metals?

During the electrolytic refining of zinc,it gets