In a series of solutions with pH of 2.5, 3.5, 7.0 and 8.0, which is likely to turn red moist litmus paper blue?

1. Analyze the question

The question asks you to identify which of the given pH values corresponds to a substance that will turn red moist litmus paper blue. You are provided with four pH values: 2.5, 3.5, 7.0, and 8.0.

2. Explain the concept

To solve this, you need to understand the pH scale and the behavior of litmus paper:

• The pH Scale: This scale measures the acidity or alkalinity of a solution. • pH < 7: The solution is acidic. • pH = 7: The solution is neutral (like pure water). • pH > 7: The solution is alkaline (basic).
• Litmus Paper Test: • Blue litmus paper turns red in acidic solutions. • Red litmus paper turns blue in alkaline (basic) solutions.

Since the question asks for a substance that turns red litmus paper blue, we are looking for a substance that is alkaline (basic). Therefore, we need to find the pH value that is greater than 7.

3. Show the solution

• pH 2.5: Since 2.5 < 7, this solution is acidic. It will turn blue litmus paper red.
• pH 3.5: Since 3.5 < 7, this solution is acidic. It will turn blue litmus paper red.
• pH 7.0: This is neutral. It will not change the color of either red or blue litmus paper.
• pH 8.0: Since 8.0 > 7, this solution is alkaline (basic). Alkaline solutions turn red litmus paper blue.

4. Identify the answer

Comparing our findings with the options provided, the only value that represents an alkaline solution is 8.0. This matches option B.

5. Tip

Remember the mnemonic "B-A-R": Blue to Acid turns Red. If you remember that blue litmus turns red in acid, you can easily deduce that the opposite (red to blue) must happen in a base!