An element $A$ forms two oxides $AO$ and $AO_2$. The oxide $AO$ is neutral,whereas the oxide $AO_2$ is acidic in nature. Would you call element $A$ a metal or a non-metal?

Which of the following is not a use of sulfur?

Name the metal which:
$(a)$ becomes blackish after a certain time.
$(b)$ develops a green coating on exposure to air.
$(c)$ does not corrode at all.
$(d)$ is used in making steel.
$(e)$ is non-lustrous.

State reasons for the following:
$(i)$ Metals are good conductors of heat.
$(ii)$ Addition of some silver to pure gold for making ornaments.
$(iii)$ Inability of non-metals to displace hydrogen from dilute sulphuric acid.

A non-metal $A$, which is the largest constituent of air, when heated with $H_2$ in a $1 : 3$ ratio in the presence of a catalyst $(Fe)$, gives a gas $B$. On heating with $O_2$, it gives an oxide $C$. If this oxide is passed into water in the presence of air, it gives an acid $D$, which acts as a strong oxidising agent.
$(a)$ Identify $A$, $B$, $C$, and $D$.
$(b)$ To which group of the periodic table does this non-metal belong?

$(a)$ Why does calcium start floating when added to water?
$(b)$ Most metals do not release hydrogen gas when reacting with nitric acid. Why?
$(c)$ Write the chemical equation for the reaction of iron with steam. Name the compound of iron obtained.