Class 7 Science Notes Chapter 5 (Chapter 5) – Examplar Problems Book

Detailed Notes with MCQs of Chapter 5: Acids, Bases and Salts from your Science Exemplar book. This is an important chapter, not just for your class exams but also as a foundation for chemistry concepts you'll encounter later, including in competitive government exams. Pay close attention to the details.

Chapter 5: Acids, Bases and Salts - Detailed Notes for Exam Preparation

1. Introduction:

• Many substances we encounter daily can be classified based on their chemical nature as acids, bases, or salts.
• Taste is a preliminary indicator (sour for acids, bitter for bases), but never taste unknown substances, especially in a lab, as they can be harmful.

2. Acids:

• Definition: Chemical substances that are typically sour in taste and turn blue litmus paper red.
• Origin of the word: Derived from the Latin word 'acere' meaning 'sour'.
• Types:
  • Natural/Organic Acids: Found in plants and animals. They are generally weak acids.
    • Acetic Acid: Vinegar
    • Formic Acid: Ant sting
    • Citric Acid: Citrus fruits (Oranges, Lemons)
    • Lactic Acid: Curd
    • Oxalic Acid: Spinach
    • Ascorbic Acid (Vitamin C): Amla, Citrus fruits
    • Tartaric Acid: Tamarind, Grapes, Unripe Mangoes
  • Mineral Acids: Prepared from minerals. They are usually strong acids and highly corrosive. Must be handled with extreme care.
    • Hydrochloric Acid (HCl)
    • Sulphuric Acid (H₂SO₄) - Known as the 'King of Chemicals'.
    • Nitric Acid (HNO₃)
• Properties:
  • Sour taste.
  • Corrosive nature (especially strong mineral acids).
  • Turn blue litmus red.
  • Do not change the colour of red litmus.
  • Turn China rose indicator magenta (dark pink).
  • Do not change the colour of turmeric indicator (remains yellow).
  • Remain colourless with phenolphthalein indicator.
  • Conduct electricity in solution (due to ions).

3. Bases:

• Definition: Chemical substances that are typically bitter in taste, feel soapy to touch, and turn red litmus paper blue.
• Examples:
  • Sodium Hydroxide (NaOH) - Found in soaps, drain cleaners (Strong Base)
  • Potassium Hydroxide (KOH) - Found in soaps (Strong Base)
  • Calcium Hydroxide (Ca(OH)₂) - Lime water (Moderate Base)
  • Magnesium Hydroxide (Mg(OH)₂) - Milk of Magnesia (Antacid) (Weak Base)
  • Ammonium Hydroxide (NH₄OH) - Window cleaner (Weak Base)
  • Sodium Bicarbonate/Hydrogen Carbonate (NaHCO₃) - Baking Soda (Mild Base)
• Properties:
  • Bitter taste.
  • Soapy/slippery feel.
  • Turn red litmus blue.
  • Do not change the colour of blue litmus.
  • Turn China rose indicator green.
  • Turn turmeric indicator red/reddish-brown.
  • Turn phenolphthalein indicator pink.
  • Conduct electricity in solution.

4. Neutral Substances:

• Definition: Substances that are neither acidic nor basic. They do not change the colour of any indicator.
• Examples: Pure water (Distilled water), Sugar solution, Common salt (Sodium Chloride) solution.

5. Indicators:

• Definition: Special substances used to test whether a substance is acidic or basic. They change their colour when added to a solution containing an acidic or a basic substance.
• Types:
  • Natural Indicators:
    • Litmus: Most common indicator. Extracted from lichens. Available as a solution or strips of paper (red and blue).
      • Acid: Turns Blue Litmus Red.
      • Base: Turns Red Litmus Blue.
      • Neutral: No change (Litmus solution is purple).
    • Turmeric (Haldi): Yellow in colour.
      • Acid/Neutral: Remains Yellow.
      • Base: Turns Red/Reddish-brown. (Think of soap stains on turmeric curry marks).
    • China Rose Petals (Gudhal): Extract acts as an indicator.
      • Acid: Turns Magenta (Dark Pink).
      • Base: Turns Green.
  • Synthetic Indicators:
    • Phenolphthalein: A man-made indicator.
      • Acid/Neutral: Remains Colourless.
      • Base: Turns Pink.

6. Neutralisation:

• Definition: The reaction between an acid and a base is known as neutralisation.
• Products: Salt and Water are always produced in this reaction.
• Heat Evolution: Heat is always evolved (produced) during a neutralisation reaction. This makes it an exothermic reaction. The reaction mixture becomes warm.
• General Equation: Acid + Base → Salt + Water + Heat
• Example:
  • Hydrochloric Acid (HCl) + Sodium Hydroxide (NaOH) → Sodium Chloride (NaCl) (Common Salt) + Water (H₂O) + Heat
• Salt: The salt formed can be acidic, basic, or neutral depending on the strength of the acid and base reacted. (This is a more advanced concept, but good to know that NaCl is a neutral salt).

7. Neutralisation in Everyday Life (Applications - Very Important for Exams):

• Indigestion: Our stomach produces HCl acid to digest food. Too much acid causes indigestion (acidity). We take an antacid, like Milk of Magnesia (Magnesium Hydroxide - a base), which neutralises the excess acid.
• Ant Bite: When an ant bites, it injects Formic Acid into the skin, causing irritation. This can be neutralised by rubbing a mild base like moist baking soda (Sodium Hydrogen Carbonate) or calamine solution (contains Zinc Carbonate).
• Soil Treatment:
  • Plants need a relatively neutral soil pH for optimal growth.
  • Acidic Soil: If soil is too acidic, farmers treat it with bases like quicklime (Calcium Oxide) or slaked lime (Calcium Hydroxide).
  • Basic Soil: If soil is too basic, organic matter (compost) is added. Decomposition of organic matter releases acids, neutralising the excess base.
• Factory Wastes: Wastes from many factories contain acids. If discharged directly into water bodies, they harm aquatic life. Therefore, these wastes are treated with basic substances to neutralise the acids before discharge.

Multiple Choice Questions (MCQs)

Here are 10 MCQs based on Chapter 5 for your practice:

1. Which of the following is a natural indicator?
   (A) Phenolphthalein
   (B) Methyl Orange
   (C) Litmus
   (D) Universal Indicator

2. A solution turns red litmus blue. Its pH is likely to be:
   (A) 1
   (B) 4
   (C) 5
   (D) 10
   (Note: While pH isn't explicitly detailed in Class 7, understanding that bases have pH > 7 helps answer this. Bases turn red litmus blue.)

3. Lime water is:
   (A) Calcium Hydroxide (Ca(OH)₂)
   (B) Sodium Hydroxide (NaOH)
   (C) Magnesium Hydroxide (Mg(OH)₂)
   (D) Ammonium Hydroxide (NH₄OH)

4. When a basic solution is added to turmeric paper, the colour changes to:
   (A) Yellow
   (B) Green
   (C) Reddish-brown
   (D) Blue

5. The reaction between an acid and a base is called:
   (A) Decomposition
   (B) Neutralisation
   (C) Displacement
   (D) Combination

6. Which substance is used to treat the effect of an ant sting?
   (A) Vinegar (Acetic Acid)
   (B) Lemon Juice (Citric Acid)
   (C) Baking Soda (Sodium Hydrogen Carbonate)
   (D) Common Salt (Sodium Chloride)

7. What are the products of a neutralisation reaction?
   (A) Acid and Water
   (B) Base and Water
   (C) Salt and Water
   (D) Acid and Base

8. A farmer finds the soil in his field is too acidic. Which substance would be most suitable to add to the soil?
   (A) Organic Compost
   (B) Slaked Lime (Calcium Hydroxide)
   (C) Common Salt
   (D) Sulphuric Acid

9. What colour does China rose indicator give with an acidic solution?
   (A) Green
   (B) Yellow
   (C) Magenta (Dark Pink)
   (D) Colourless

10. Milk of Magnesia, used as an antacid, contains:
    (A) Magnesium Chloride
    (B) Magnesium Hydroxide
    (C) Magnesium Sulphate
    (D) Magnesium Carbonate

Answers to MCQs:

1. (C) Litmus
2. (D) 10 (Any value > 7 indicates a base)
3. (A) Calcium Hydroxide (Ca(OH)₂)
4. (C) Reddish-brown
5. (B) Neutralisation
6. (C) Baking Soda (Sodium Hydrogen Carbonate)
7. (C) Salt and Water
8. (B) Slaked Lime (Calcium Hydroxide)
9. (C) Magenta (Dark Pink)
10. (B) Magnesium Hydroxide

Study these notes thoroughly. Remember the examples, properties, indicator colour changes, and especially the applications of neutralisation. Good luck with your preparation!