Talk about mutual iron powder

Potassium ferricyanide, testing ferrous ion, turning blue.

Potassium thiocyanate, test iron ion, turn red.

Methyl orange turns red at pH≤3. 1, orange at pH≥3. 1 ~ 4.4, and yellow at pH≥4.4, which is often used as an indicator of acid dripping alkali. Similarly, phenolphthalein can be used as an indicator of alkali dripping acid.

Magenta solution can be used to detect sulfur dioxide and chlorine, and it will fade when it meets.

Clear limewater, stop carbon dioxide and become turbid.

Glowing copper oxide, testing carbon monoxide, copper oxide from black to red.

Barium chloride solution, test sulfur trioxide gas.

The landlord's question is correct. Chlorine dissolves in water to produce hydrochloric acid and hypochlorous acid, which is acidic, but hypochlorous acid can bleach the indicator.