Question

The indicator which does not change colour on passage of HCl gas is ______.

Options

• methyl orange

• moist blue litmus

• phenolphthalein

Answer 1

The indicator which does not change colour on passage of HCl gas is phenolphthalein.

Explanation:

Phenolphthalein is a pH indicator that remains colourless in both neutral and acidic solutions, changing to pink only in basic conditions. When acidic hydrogen chloride (HCl) gas passes through it, the solution becomes highly acidic, so the indicator stays colourless. In contrast, methyl orange turns red or pink, and moist blue litmus paper turns red when it encounters the acidic gas.