Chapter 2: Exploring Substances: Acidic, Basic, and Neutral

Introduction

Substances around us can be acidic, basic, or neutral. Knowing their properties helps in daily life, experiments, and chemical reactions. Indicators are used to test the nature of a substance.

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1. Types of Substances

Acids

• Taste sour
• Turn blue litmus red
• React with metals to produce hydrogen gas
• Examples: Lemon juice, Vinegar, Hydrochloric acid

Bases (Alkalis)

• Taste bitter
• Feel soapy/slippery
• Turn red litmus blue
• Examples: Baking soda, Soap solution, Sodium hydroxide

Neutral Substances

• Neither acidic nor basic
• Do not change litmus color
• Examples: Water, Sugar solution, Salt solution

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2. Indicators

• Litmus paper: Red or blue; detects acidic or basic nature
• Phenolphthalein: Colorless in acid, pink in base
• Methyl orange: Red in acid, yellow in base

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3. pH Scale

• Measures how acidic or basic a substance is
• Scale from 0 to 14
  • 0–6 → Acidic
  • 7 → Neutral
  • 8–14 → Basic

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4. Neutralization

• Reaction of acid and base to produce salt and water
• Example: HCl + NaOH → NaCl + H₂O

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5. Key Points to Remember

• Acids, bases, and neutral substances differ in taste, feel, and reaction with indicators.
• pH indicates the strength of acids or bases.
• Neutralization is important in daily life (antacids, soil treatment, cleaning products).

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Questions – Exploring Substances: Acidic, Basic, and Neutral

1. Very Short Answer (1 mark)

1. Give one example of an acid.
2. Give one example of a base.
3. What is a neutral substance?
4. Which indicator turns blue in acid?
5. What is the pH of a neutral substance?
6. Name one use of neutralization in daily life.
7. Which base feels slippery?
8. Give one example of a neutral liquid.
9. Which indicator turns red in acid?
10. Give one example of an acid found in food.

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2. Multiple Choice Questions (MCQ)

1. Which of the following is acidic?
   a) Lemon juice
   b) Soap solution
   c) Water
   d) Baking soda
2. Which of these is a base?
   a) Vinegar
   b) Hydrochloric acid
   c) Soap solution
   d) Lemon juice
3. pH value of a strong base is:
   a) 2
   b) 7
   c) 12
   d) 5
4. Neutralization produces:
   a) Acid only
   b) Base only
   c) Salt and water
   d) Gas only
5. Phenolphthalein is colorless in:
   a) Acid
   b) Base
   c) Neutral solution
   d) Both acid and base

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3. Fill in the Blanks

1. Bases turn __________ litmus paper blue.
2. Acids taste __________.
3. Neutral substances do not change the color of __________.
4. The pH of lemon juice is around __________.
5. HCl + NaOH → __________ + __________

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4. True/False Type

1. All acids are harmful. (True/False)
2. Bases feel slippery. (True/False)
3. Water is neutral. (True/False)
4. pH 14 indicates a strong acid. (True/False)
5. Neutralization produces salt and water. (True/False)

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5. Short Answer Questions (2–3 marks)

1. Define an acid and give two examples.
2. Define a base and give two examples.
3. What is an indicator? Name two indicators.
4. Define neutralization with an example.
5. Explain the pH scale and its significance.

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6. Long Answer Questions (4–5 marks)

1. Explain the properties of acids and bases with examples.
2. Describe how indicators can be used to test substances.
3. What is neutralization? Explain its applications in daily life.
4. Explain the importance of pH in our environment.
5. How can acids and bases be safely used in laboratories and homes?

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7. Higher-Order Thinking (HOT) & Application

1. A student tests three liquids with litmus: red litmus turns blue, blue litmus turns red, and no change occurs. Identify each liquid.
2. How does adding lime (CaO) to acidic soil help plants?
3. Why is pH monitoring important in swimming pools and aquariums?

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8. Passage-Based Questions

Read the passage and answer:
Acids are sour and turn blue litmus red. Bases are bitter, feel slippery, and turn red litmus blue. Neutral substances do not change litmus color. Indicators and the pH scale help identify the strength of acids and bases. Neutralization is the reaction between an acid and a base producing salt and water.

1. Give one example of an acid and a base from the passage.
2. What happens when red litmus is added to a base?
3. Define neutralization.
4. What does pH indicate?

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Answer Key – Exploring Substances: Acidic, Basic, and Neutral

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1. Very Short Answer

1. Lemon juice
2. Soap solution
3. Substance neither acidic nor basic, e.g., water
4. Red litmus
5. 7
6. Using antacids, treating acidic soil
7. Soap solution
8. Water
9. Blue litmus
10. Vinegar

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2. MCQ Answers

1. a) Lemon juice
2. c) Soap solution
3. c) 12
4. c) Salt and water
5. a) Acid

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3. Fill in the Blanks

1. red
2. sour
3. litmus
4. 2–3
5. NaCl, H₂O

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4. True/False

1. False
2. True
3. True
4. False
5. True

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5. Short Answer

1. Acid – tastes sour, turns blue litmus red; e.g., Lemon juice, Vinegar
2. Base – tastes bitter, slippery, turns red litmus blue; e.g., Soap, Baking soda
3. Indicator – substance that changes color in acid or base; e.g., Litmus, Phenolphthalein
4. Neutralization – acid + base → salt + water; e.g., HCl + NaOH → NaCl + H₂O
5. pH scale measures acidity or basicity of a substance (0–14)

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6. Long Answer

1. Acids: sour, react with metals, turn blue litmus red; Bases: bitter, slippery, turn red litmus blue
2. Indicators change color in acids/bases: Litmus (red/blue), Phenolphthalein (colorless/pink)
3. Neutralization is used in antacids, agriculture, and cleaning products
4. pH important for soil, water, and health
5. Safety: Wear gloves, handle acids and bases carefully, proper storage

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7. HOT & Application

1. Red→Blue: Base; Blue→Red: Acid; No change: Neutral
2. Lime neutralizes excess acid in soil, improving plant growth
3. pH ensures safe water for swimming and healthy conditions in aquariums

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8. Passage-Based

1. Acid – Lemon juice; Base – Soap solution
2. It turns blue
3. Reaction between acid and base forming salt and water
4. Indicates how acidic or basic a substance is