The colour of litmus in an alkaline medium is

1. Analyze the question

The question asks for the specific color change observed when litmus, a common acid-base indicator, is placed in an alkaline (basic) solution.

2. Explain the concept

Indicators are substances that change color depending on the pH of the solution they are in. Litmus is a natural dye extracted from lichens. Its behavior is summarized by the following rule:

• In Acidic medium: Litmus turns Red.
• In Neutral medium: Litmus remains Purple.
• In Alkaline (Basic) medium: Litmus turns Blue.

An alkaline medium is characterized by a pH greater than 7, which causes the litmus molecules to undergo a structural change that reflects blue light.

3. Show the solution

• Step 1: Identify the nature of the medium. The question specifies an "alkaline medium."
• Step 2: Recall the indicator property of litmus. Litmus is a standard indicator used to distinguish between acids and bases.
• Step 3: Apply the property. Since the medium is alkaline, the litmus will shift from its neutral purple state to its basic state.
• Step 4: Conclusion. The color of litmus in a base is blue.

4. Identify the answer

Comparing our conclusion to the given options: A. Red (Acidic) B. Yellow (Common color for Methyl Orange in acid) C. Pink (Common color for Phenolphthalein in acid) D. Blue (Alkaline)

The correct option is D.

5. Exam Tip

To remember this easily, use the mnemonic "B-A-B": Base turns litmus Blue. Conversely, remember that Acids turn litmus Red (think of "Acid" and "Red" both having the letter 'd').