Quick questions on Reaction quotient Q and predicting direction: WACE Year 12 Chemistry

The same comparison drives precipitation prediction in the solubility topic. There the quotient is called the ionic product, written with the same ion-concentration form as $K_{sp}$. If the ionic product exceeds $K_{sp}$ the solution is supersaturated and a precipitate forms (the system shifts to consume ions); if it is below $K_{sp}$ no precipitate forms. Recognising that "$Q$ versus $K$" is a single unifying idea across gaseous equilibria, acid ionisation and solubility is one of the higher-order links WACE rewards.

The reaction quotient gives a quantitative version of Le Chatelier's principle. When you add reactant, $Q$ drops below $K_c$, so the system shifts forward; when you add product, $Q$ rises above $K_c$, so it shifts in reverse. The two ideas describe the same behaviour, but $Q$ lets you put numbers to the prediction.