What happens when:
$(i)$ $A$ piece of calcium is dropped in water.
$(ii)$ Sodium is kept exposed in air.
$(iii)$ Iron nail is dipped in copper sulphate solution.
$(iv)$ Potassium is dropped in cold water.
$(v)$ Carbon dioxide is compressed in water at high pressure.
Give chemical equations involved in all the above reactions and balance them.

(N/A) $(i)$ Calcium starts floating because the bubbles of hydrogen gas formed stick to its surface.
$Ca(s) + 2H_2O(l) \longrightarrow Ca(OH)_2(aq) + H_2(g)$
$(ii)$ In dry air,sodium reacts with oxygen to form sodium oxide. In moist air,it reacts with water vapor to form sodium hydroxide.
$4Na(s) + O_2(g) \longrightarrow 2Na_2O(s)$ (In dry air)
$2Na(s) + 2H_2O(l) \longrightarrow 2NaOH(aq) + H_2(g) + \text{Heat}$ (In moist air)
$(iii)$ Iron displaces copper from copper sulphate solution due to higher reactivity.
$Fe(s) + CuSO_4(aq) \longrightarrow FeSO_4(aq) + Cu(s)$
$(iv)$ Potassium reacts violently with cold water,releasing hydrogen gas and heat.
$2K(s) + 2H_2O(l) \longrightarrow 2KOH(aq) + H_2(g) + \text{Heat}$
$(v)$ Carbon dioxide dissolves in water under high pressure to form carbonic acid.
$CO_2(g) + H_2O(l) \longrightarrow H_2CO_3(aq)$