a. Methyl orange (whose structure is given in Section 7.12) is an acid-base indicator. In...

a. Methyl orange (whose structure is given in Section 7.12) is an acid-base indicator. In solutions of pH > 4, it is red; in solutions of pH<4, it is yellow. Account for the change in color.

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b. Phenolphthalein, which is another indicator, exhibits a much more dramatic color change. In solutions of pH<8.5, it is colorless; in solutions of pH > 8.5, it is deep red-purple. Account for the change in color.