When two reactants $A$ and $B$ are mixed to form products $C$ and $D$,what is the value of the reaction quotient $Q$ at the initial state of the reaction?

Identify which of the following reactions represent homogeneous and heterogeneous equilibria:
$(a)$ $H_{2(g)} + I_{2(g)} \rightleftharpoons 2HI_{(g)}$
$(b)$ $Ca(OH)_{2(s)} \rightleftharpoons Ca^{2+}_{(aq)} + 2OH^{-}_{(aq)}$
$(c)$ $CaCO_{3(s)} \rightleftharpoons CaO_{(s)} + CO_{2(g)}$
$(d)$ $Ag_{2}O_{(s)} + 2H^{+}_{(aq)} \rightleftharpoons H_{2}O_{(l)} + 2Ag^{+}_{(aq)}$
$(e)$ $NH_{3(g)} + H_{2}O_{(l)} \rightleftharpoons N{H_{4}}^{+}_{(aq)} + OH^{-}_{(aq)}$
$(f)$ $CH_{3}COOC_{2}H_{5(l)} + H_{2}O_{(l)} \rightleftharpoons CH_{3}COOH_{(l)} + C_{2}H_{5}OH_{(l)}$

In any chemical reaction,equilibrium is supposed to be established when

Explain solid-vapour equilibrium by example.

If a system is at equilibrium,the rate of the forward reaction compared to the reverse reaction is:

For the reaction ${H_2} + {I_2} \rightleftharpoons 2HI$ at $721 \ K$,the value of the equilibrium constant is $50$. If $0.5 \ mol$ of each of ${H_2}$ and ${I_2}$ are added to the system,the value of the equilibrium constant will be .............