Question

Which substance turns red litmus paper blue in aqueous solution?

• A. Alkali
• B. Acid
• C. Salt
• D. None of these

Hint:

Litmus paper is a quick test, but for accuracy, synthetic indicators like phenolphthalein and methyl orange are also used.

Answer 1

The Correct Option is A

Step 1: Conceptual Background:
Litmus paper is a common natural indicator used to test whether a solution is acidic or basic.
Acids turn blue litmus red.
Bases (alkalis) turn red litmus blue.
Step 2: Analysis of Options:
Alkali: A soluble base, e.g., NaOH, KOH. It turns red litmus blue, confirming alkalinity.
Acid: Does the opposite — turns blue litmus red.
Salt: Most salts are neutral and do not change litmus color.
Step 3: Example:
When red litmus paper is dipped in sodium hydroxide solution (alkali), it turns blue, indicating a basic solution.
Step 4: Final Answer:
The substance that turns red litmus paper blue is an Alkali.