Properties of Acids and Bases (HCl & NaOH): Litmus MCQs Quiz | Class 10

This quiz is designed for Class X Science (Code 086), covering Unit I: Practicals, specifically the topic ‘Properties of Acids and Bases (HCl & NaOH): Litmus MCQs Quiz’. It focuses on understanding blue and red litmus paper changes and making inferences. Answer all 10 multiple-choice questions, then submit to see your score and review correct answers. You can also download a detailed PDF answer sheet for future reference.

Understanding Acids and Bases with Litmus

Acids and bases are fundamental chemical substances encountered frequently in chemistry and everyday life. One of the simplest and most common ways to distinguish between them in a laboratory setting is by using indicators, with litmus paper being a classic example. This practical involves observing how hydrochloric acid (HCl), a strong acid, and sodium hydroxide (NaOH), a strong base, interact with red and blue litmus papers, allowing us to infer their chemical nature.

Key Concepts of Litmus Paper Indicators:

  • Litmus Paper: Litmus is a natural indicator derived from lichens. It is available as small strips of paper, typically in two forms: blue litmus paper and red litmus paper.
  • Color Change in Acids: When blue litmus paper comes into contact with an acidic solution, it changes color from blue to red. Acids are characterized by a pH less than 7.
  • Color Change in Bases: When red litmus paper comes into contact with a basic (or alkaline) solution, it changes color from red to blue. Bases are characterized by a pH greater than 7.
  • No Change in Neutral Solutions: If a solution is neutral (pH 7, like pure water), neither red nor blue litmus paper will show a color change. Blue litmus remains blue, and red litmus remains red.
  • Specific Examples:
    • Hydrochloric Acid (HCl): A strong acid. It will turn blue litmus red. Red litmus will remain red.
    • Sodium Hydroxide (NaOH): A strong base. It will turn red litmus blue. Blue litmus will remain blue.

Making Inferences from Litmus Tests:

The changes in litmus paper color allow us to make direct inferences about the nature of an unknown solution:

  1. If blue litmus turns red, the solution is definitively acidic.
  2. If red litmus turns blue, the solution is definitively basic.
  3. If blue litmus remains blue, the solution could be either basic or neutral. Further testing with red litmus is needed.
  4. If red litmus remains red, the solution could be either acidic or neutral. Further testing with blue litmus is needed.
  5. If both blue litmus remains blue AND red litmus remains red, the solution is neutral.

Summary Table: Litmus Paper Color Changes

Solution Type Effect on Blue Litmus Paper Effect on Red Litmus Paper
Acidic Turns Red Remains Red
Basic Remains Blue Turns Blue
Neutral Remains Blue Remains Red

Quick Revision Points:

  • Acids generally taste sour and are corrosive.
  • Bases generally taste bitter, feel slippery, and are caustic.
  • Litmus paper is a convenient and cost-effective pH indicator.
  • Always handle acids and bases with care as they can be corrosive.
  • HCl is a mineral acid; NaOH is an alkali (soluble base).

Extra Practice Questions:

  1. What would happen if you dipped both red and blue litmus papers into distilled water?
  2. If a solution is unknown, and it turns blue litmus red, what can you definitively say about its nature?
  3. A liquid has a pH of 13. How would it react with red litmus paper?
  4. Why is it important to use both red and blue litmus papers to confirm the nature of a solution?
  5. Describe the litmus test results for a solution of baking soda (sodium bicarbonate).