A drop of a liquid sample was put on the pH paper. It war observed that the colour of the pH paper turns blue. The liquid sample is

Correct option is C

The blue color on pH paper indicates that the liquid sample is basic in nature. Sodium bicarbonate solution (commonly known as baking soda solution) is weakly basic and turns pH paper blue due to its pH value of around 8–9.

• Distilled water is neutral with a pH of 7 and does not cause any significant color change on pH paper.
• Lemon juice is acidic with a pH of around 2–3 and would turn the pH paper red or orange.
• Sodium bicarbonate solution’s weakly basic nature aligns with the observation, making it the correct answer.

Information Booster:

1. pH Scale: Measures the acidity or basicity of a solution on a scale from 0–14.
2. Acidic solutions have pH < 7, neutral solutions have pH = 7, and basic solutions have pH > 7.
3. Sodium bicarbonate solution is commonly used in baking and cleaning due to its weak alkalinity.
4. Distilled water is neutral but can show slight acidity due to dissolved CO₂ forming carbonic acid.
5. pH indicators, such as pH paper, change color in the presence of acids or bases, facilitating easy identification.
6. Lemon juice, a strong natural acid, contains citric acid and is often used for cleaning and culinary purposes.

Additional Knowledge:

1. (a) Distilled water:

   • It is considered neutral with a pH of 7.
   • However, exposure to air can make it slightly acidic as carbon dioxide dissolves, forming carbonic acid.
   • It does not change the color of pH paper significantly, usually showing a faint green corresponding to neutral pH.

2. (b) Lemon juice:

   • Contains citric acid, giving it a pH of 2–3, making it strongly acidic.
   • It is widely used in food for flavoring and in cleaning solutions due to its acidic properties.
   • When applied to pH paper, it turns red or orange, indicating acidity.