Answer: Le Chatelier's principle states that a system at equilibrium will respond to any stress or change in conditions by shifting the equilibrium position to counteract the stress and restore equilibrium. In the case of indicators, the color of the indicator molecule depends on its protonation state, which in turn depends on the pH of the solution.
At relatively low pH, the solution is acidic and has a high concentration of H+ ions. In this condition, the equilibrium of the indicator molecule will shift towards the protonated form (HIn), as per Le Chatelier's principle. The H+ ions from the solution will combine with the indicator molecule to form HIn, which has a different color than its deprotonated form (In-).
Therefore, at low pH, the dominant form of the indicator is HIn. On the other hand, at high pH, the solution is basic and has a low concentration of H+ ions. In this condition, the equilibrium of the indicator molecule will shift towards the deprotonated form (In-), as per Le Chatelier's principle.
The low concentration of H+ ions in the solution makes it difficult for the HIn molecules to remain protonated, and they will undergo deprotonation to form In-. At high pH, the dominant form of the indicator is In-.
In summary, Le Chatelier's principle explains why the dominant form of the indicator molecule changes as the pH of the solution changes. At low pH, the equilibrium shifts towards the protonated form (HIn), and at high pH, the equilibrium shifts towards the deprotonated form (In-).