Red litmus turns blue.
Explanation
Ammonium hydroxide ($NH_4OH$) is a basic substance commonly found in window cleaning solutions. Chemical indicators change colour in the presence of acids or bases to identify the nature of the solution. The specific colour change depends on the pH level of the substance being tested.
Analysis of Indicators:
- Litmus Paper: Litmus is a natural indicator. Basic solutions turn red litmus blue, while acidic solutions turn blue litmus red. Since ammonium hydroxide is a base, it turns red litmus blue.
- Turmeric: Turmeric is a natural indicator that remains yellow in acidic and neutral solutions but turns reddish-brown in basic solutions. Therefore, in ammonium hydroxide, turmeric paste would change colour, not remain yellow.
- Phenolphthalein: Phenolphthalein is a synthetic indicator. It remains colourless in acidic and neutral solutions but turns pink in basic solutions. Thus, in ammonium hydroxide, it would turn pink.
Conclusion:
Red litmus turns blue. correctly describes the behaviour of a basic solution like ammonium hydroxide with red litmus paper.
Key Takeaway:
Bases (alkalis) turn red litmus blue, phenolphthalein pink, and turmeric reddish-brown. Acids turn blue litmus red and keep phenolphthalein colourless.