Who discovered hydrogen gas?

Which of the following is a compound of hydrogen and oxygen?

Which gas has a pungent smell that causes choking and coughing?

An electrolytic cell consists of
$(i)$ positively charged cathode
$(ii)$ negatively charged anode
$(iii)$ positively charged anode
$(iv)$ negatively charged cathode

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 $HNO_3$.

Why are sulphide and carbonate ores converted into oxides?