Class 10 Science Notes Chapter 3 (Chapter 3) – Lab Manual (English) Book

Detailed Notes with MCQs of Chapter 3 from your Science Lab Manual. This chapter deals with determining the pH of various substances, a fundamental concept often tested in government exams. Pay close attention to the details.

Chapter 3: Finding the pH of Samples using pH Paper/Universal Indicator

1. Aim:
To determine the pH of the following samples using pH paper and/or universal indicator solution:
* Dilute Hydrochloric acid (HCl)
* Dilute Sodium hydroxide (NaOH) solution
* Dilute Ethanoic acid (CH₃COOH) solution
* Lemon juice
* Water (preferably distilled)
* Dilute Sodium bicarbonate (NaHCO₃) solution

2. Theory:

• pH: pH stands for 'potential of Hydrogen' or 'power of Hydrogen'. It is a measure of the acidity or basicity (alkalinity) of an aqueous solution.
• pH Scale: It is a logarithmic scale ranging from 0 to 14.
  • pH < 7: Solution is acidic. Lower the pH, stronger the acid (higher concentration of H⁺ ions).
  • pH = 7: Solution is neutral (equal concentration of H⁺ and OH⁻ ions, as in pure water at 25°C).
  • pH > 7: Solution is basic or alkaline. Higher the pH, stronger the base (higher concentration of OH⁻ ions or lower concentration of H⁺ ions).
• Indicators: These are substances that change colour depending on the pH of the solution they are in.
  • pH Paper: This is paper impregnated with a mixture of indicators (a universal indicator). When a drop of the test solution is placed on it, the paper changes colour. This colour is then matched with a standard colour chart provided with the pH paper strips to find the approximate pH value.
  • Universal Indicator Solution: This is a solution containing a mixture of indicators. A few drops are added to the test solution, causing the solution itself to change colour. This colour is compared to a standard chart to determine the pH.

3. Materials Required:

• Test tubes and test tube stand
• Droppers
• Glass rod
• White tile or spot plate
• pH paper strips (with standard colour chart)
• Universal indicator solution (with standard colour chart)
• Samples: Dil. HCl, Dil. NaOH, Dil. Ethanoic acid, Lemon juice, Distilled water, Dil. Sodium bicarbonate solution.

4. Procedure:

• Using pH Paper:
  1. Place a strip of pH paper on a clean white tile.
  2. Using a clean dropper, put one drop of the first sample solution onto the pH paper strip. (Ensure a different clean dropper is used for each sample).
  3. Observe the colour change on the pH paper.
  4. Immediately compare the developed colour with the standard pH colour chart.
  5. Record the approximate pH value.
  6. Repeat steps 1-5 for all other samples.
• Using Universal Indicator Solution:
  1. Take about 2 mL of the first sample solution in a clean test tube.
  2. Add 2-3 drops of universal indicator solution using a clean dropper.
  3. Observe the colour change in the solution.
  4. Compare the colour with the standard pH colour chart provided with the indicator solution.
  5. Record the approximate pH value.
  6. Repeat steps 1-5 for all other samples using clean test tubes and droppers each time.

5. Observations (Typical Results):

(Note: Exact colours and pH values might slightly vary depending on the concentration of solutions and the specific brand of indicator used.)

6. Results/Inference:
Based on the observed pH values:

• Dilute HCl, Dilute Ethanoic Acid, and Lemon Juice are acidic. HCl is a strong acid, while Ethanoic acid and Lemon juice are weaker acids.
• Dilute NaOH and Dilute Sodium Bicarbonate solution are basic. NaOH is a strong base, while Sodium Bicarbonate is a weak base.
• Distilled water is neutral.

7. Precautions:

• Use clean and dry test tubes and droppers for each sample to avoid contamination.
• Do not dip the pH paper directly into the solution in the test tube. Use a dropper or glass rod to transfer a drop onto the paper.
• Handle acids and bases carefully as they can be corrosive.
• Compare the colour change immediately as the colour might fade over time.
• Use distilled water for testing 'water' and for preparing dilute solutions, as tap water contains dissolved salts that can affect its pH.
• Match the colour carefully with the standard pH colour chart for accurate results.

Multiple Choice Questions (MCQs):

1. A student tests a sample solution with pH paper and observes the colour changes to red. The sample is most likely:
   a) Dilute NaOH
   b) Distilled Water
   c) Dilute HCl
   d) Dilute Sodium Bicarbonate

2. The pH scale ranges from:
   a) 0 to 7
   b) 7 to 14
   c) 0 to 14
   d) 1 to 14

3. A solution with a pH value of 7 is considered:
   a) Strongly Acidic
   b) Weakly Basic
   c) Neutral
   d) Strongly Basic

4. Universal indicator is:
   a) A single chemical compound
   b) A mixture of several indicators
   c) An indicator only for acids
   d) An indicator only for bases

5. Which of the following substances is expected to have the highest pH value?
   a) Lemon Juice
   b) Dilute Ethanoic Acid
   c) Distilled Water
   d) Dilute NaOH solution

6. A student adds a few drops of universal indicator to a dilute solution of sodium bicarbonate. What colour change would be observed?
   a) Red
   b) Green
   c) Blue/Bluish-Green
   d) Violet

7. Which of the following represents a weak acid?
   a) Dilute HCl (pH ~ 1)
   b) Dilute NaOH (pH ~ 13)
   c) Dilute Ethanoic Acid (pH ~ 4)
   d) Distilled Water (pH ~ 7)

8. When testing pH, why is it important to use a clean dropper for each sample?
   a) To use less sample
   b) To avoid chemical reactions between samples (contamination)
   c) To make the colour change faster
   d) To ensure the dropper doesn't break

9. A solution turns pH paper yellowish-orange. Its approximate pH would be:
   a) 1
   b) 4
   c) 7
   d) 10

10. What does a lower pH value (e.g., pH 1 or 2) indicate about a solution?
    a) It has a high concentration of OH⁻ ions.
    b) It is neutral.
    c) It has a high concentration of H⁺ ions (strongly acidic).
    d) It is weakly basic.

Answer Key for MCQs:

1. c) Dilute HCl
2. c) 0 to 14
3. c) Neutral
4. b) A mixture of several indicators
5. d) Dilute NaOH solution
6. c) Blue/Bluish-Green
7. c) Dilute Ethanoic Acid (pH ~ 4)
8. b) To avoid chemical reactions between samples (contamination)
9. b) 4
10. c) It has a high concentration of H⁺ ions (strongly acidic).

Make sure you understand the theory behind determining pH and the expected results for common substances. This is a practical skill with theoretical importance. Let me know if any part needs further clarification.