Q.Which of the following statements is incorrect?

The key idea is Le Chatelier’s principle and the effect of temperature on equilibrium constants.

• Statement (i) is correct: in an insulated flask, no heat exchange occurs, so the ice–water equilibrium remains undisturbed — masses stay constant.
• Statement (ii) is incorrect. The equilibrium is $\ce{Fe^{3+} + SCN^- <=> [Fe(SCN)]^{2+}}$ (red). Oxalic acid forms a stable complex with $\ce{Fe^{3+}}$, removing it from equilibrium. The red colour fades, not increases. …

The question tests your understanding of chemical equilibrium — specifically, how different factors (temperature, catalysts, concentration changes) affect the position of equilibrium and the equilibrium constant itself. Let’s examine each statement carefully.

1. Statement (i): In a perfectly insulated flask containing an equilibrium mixture of ice and water, the system is at constant temperature (0°C) and no heat can enter or leave. The equilibrium between solid and liquid water is dynamic: some ice melts and some water freezes at equal rates. Since no heat exchange occurs, the amounts of ice and water remain constant over time. This is correct.

2. Statement (ii): This involves the equilibrium between iron(III) ions and thiocyanate ions:

The complex $[\text{Fe(SCN)}]^{2+}$ is deep red. Oxalic acid ($\text{H}_2\text{C}_2\text{O}_4$) reacts with $\text{Fe}^{3+}$ to form a stable, colourless complex $[\text{Fe(C}_2\text{O}_4)_3]^{3-}$. This removes $\text{Fe}^{3+}$ from the equilibrium, shifting it to the left (Le Chatelier’s principle). Consequently, the concentration of the red complex decreases, so the red colour fades — it does not intensify. This statement is incorrect.