Look at shifts in the following chemical equation when answering the questions: HIn(aq) ⇄ H + (aq) + In- (aq) (In stands for indicator, not the element indium) yellow violet 1. Add 5 mL of distilled water to a medium test tube. 2. Add a few drops of methyl violet indicator. What is the color of the solution? 3. Add 6 M HCl to the solution, drop by drop, until it changes color completely according to the chemical equation above. What species causes the color change? 4. Add 6 M NaOH until the solution changes back to its original color. Explain how [H+ ] changed, and how that change altered the color of the solution.

When methyl violet is added to water the solution is violet in colour. When HCl is added, it turns dark yellow. Addition of NaOH turns the colour to violet again.

Explanation:

The sequence of colour changes outlined in the answer stems from Le Chaterliers principle.

The equilibrium set up in a solution of yellow violet is;

HMV (aq) > H^+(aq) + MV^- (aq)

HMV is yellow in colour while MV^- is violet in colour. Addition of HMV to water leads to its dissolution and release of MV^- ion, hence the solution turns violet. Similarly, when HCl is added, H^+ concentration increases and HMV is formed again, hence the solution turns yellow. If NaOH is added, the solution becomes alkaline, the equilibrium shifts in favour of more MV^- and the solution turns violet again.

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