Class 7 Science Notes Chapter 6 (Chapter 6) – Examplar Problems Book

Detailed Notes with MCQs of Chapter 6, 'Physical and Chemical Changes' from your Science Exemplar book. This is an important chapter, not just for your school exams but also for various government competitive exams where basic science concepts are tested. Pay close attention as we break it down.

Chapter 6: Physical and Chemical Changes - Detailed Notes for Exam Preparation

1. Introduction: Changes Around Us

• We observe many changes happening around us constantly. These changes can be broadly classified into two types: Physical Changes and Chemical Changes.
• Understanding the difference is crucial.

2. Physical Changes

• Definition: A change in which only the physical properties of a substance change, and no new substance is formed.
• Characteristics:
  • Usually reversible (e.g., ice melts to water, water freezes to ice).
  • No change in the chemical composition of the substance.
  • Properties like shape, size, state (solid, liquid, gas), colour (sometimes, like heating a metal temporarily), texture may change.
  • No significant energy change (heat/light) usually occurs, or if it does, it's easily reversible.
• Examples (Important for Exams):
  • Melting of ice or wax
  • Boiling of water
  • Freezing of water
  • Condensation of steam
  • Sublimation of camphor or naphthalene balls (Solid directly to Gas)
  • Dissolving sugar or salt in water (Can be recovered by evaporation/crystallization)
  • Cutting paper or wood
  • Tearing cloth
  • Stretching a rubber band
  • Breaking a glass tumbler
  • Glowing of an electric bulb filament (Gets hot and glows, but returns to normal when switched off - no new substance)
  • Inflating a balloon

3. Chemical Changes

• Definition: A change in which one or more new substances with entirely different properties (both physical and chemical) are formed.
• Characteristics:
  • Usually irreversible (e.g., burnt paper cannot be turned back into paper).
  • The chemical composition of the original substance(s) changes completely.
  • New substances are formed with different properties.
  • Often accompanied by one or more of the following indicators (These are key identifiers!):
    • Evolution of gas: (e.g., reaction of baking soda with vinegar releases CO₂)
    • Formation of a precipitate: (An insoluble solid formed during a reaction in a solution)
    • Change in colour: (e.g., rusting of iron, ripening of fruit)
    • Change in smell: (e.g., spoiling of food, burning)
    • Production or absorption of heat (Exothermic/Endothermic): (e.g., burning produces heat, some reactions feel cold)
    • Production of light: (e.g., burning magnesium ribbon, fireworks)
    • Production of sound: (e.g., bursting of crackers)
• Examples (Important for Exams):
  • Burning of wood, paper, coal, magnesium ribbon (produces ash, gases, light, heat)
  • Rusting of iron
  • Digestion of food in our body
  • Cooking of food (e.g., cooking an egg, baking a cake)
  • Fermentation (e.g., making curd from milk, formation of alcohol from sugar)
  • Ripening of fruits
  • Spoilage of food
  • Reaction between baking soda (Sodium bicarbonate) and vinegar (Acetic acid) - produces CO₂
  • Reaction between copper sulphate solution and iron nail (Blue solution turns green, reddish-brown deposit on nail)
  • Photosynthesis in plants
  • Respiration in living organisms

4. Rusting of Iron - A Specific Chemical Change

• What is Rust? Rust is a reddish-brown flaky substance formed on the surface of iron objects when exposed to air and moisture. Chemically, rust is hydrated iron(III) oxide (Fe₂O₃·xH₂O).
• Conditions Necessary for Rusting:
  • Presence of Oxygen (from air)
  • Presence of Water or Water Vapour (moisture)
  • Note: Rusting is faster in salty water (like near coastal areas) or acidic conditions (acid rain).
• Chemical Equation (Simplified):
  Iron (Fe) + Oxygen (O₂) + Water (H₂O) → Rust (Hydrated Iron Oxide - Fe₂O₃·xH₂O)
• Harmful Effects: Rusting weakens iron structures like bridges, ships, car bodies, railings, etc., leading to economic loss.
• Prevention of Rusting (Very Important for Exams):
  • Painting: Applying a coat of paint prevents direct contact of iron with air and moisture.
  • Oiling or Greasing: Applying oil or grease creates a barrier layer. Used for tools and machine parts.
  • Galvanization: Coating iron with a layer of a more reactive metal, usually Zinc (Zn). Even if the zinc layer is scratched, zinc corrodes preferentially, protecting the iron (sacrificial protection). Commonly used for pipes, buckets, dustbins.
  • Chrome Plating: Depositing a layer of chromium. Gives a shiny finish and prevents rust (used in car parts, taps).
  • Alloying: Mixing iron with other metals (like chromium, nickel) and non-metals (like carbon) to form an alloy. Stainless Steel is an alloy of iron with carbon, chromium, and nickel, which does not rust easily. Used for utensils, surgical instruments.
  • Tinning: Coating iron with tin (used for food cans, as tin is non-toxic).

5. Crystallization

• Definition: The process of obtaining large, pure crystals of a substance from its saturated solution.
• Is it a Physical or Chemical Change? Crystallization is a Physical Change. Although the shape changes (from dissolved state to crystal), the chemical substance remains the same (e.g., copper sulphate remains copper sulphate).
• Significance: It is a purification technique. Impurities are left behind in the solution as pure crystals form.
• Example: Obtaining pure crystals of copper sulphate (CuSO₄) or common salt (NaCl) from their impure solutions.

6. Difference between Physical and Chemical Changes (Summary Table)

Multiple Choice Questions (MCQs)

Here are 10 MCQs based on the chapter for your practice:

1. Which of the following is a physical change?
   a) Rusting of iron
   b) Burning of magnesium ribbon
   c) Melting of wax
   d) Formation of curd from milk

2. When baking soda is mixed with vinegar, bubbles are formed. This is because a new substance is formed which is:
   a) Oxygen
   b) Carbon dioxide
   c) Water vapour
   d) Hydrogen

3. Galvanization is a process used to prevent the rusting of iron by coating it with a layer of:
   a) Copper
   b) Aluminium
   c) Zinc
   d) Silver

4. Which of the following indicates that a chemical change has occurred?
   a) Change in state (solid to liquid)
   b) Change in size
   c) Formation of a new substance
   d) Stretching a material

5. Stainless steel is resistant to rust because it is:
   a) An element
   b) A compound of iron
   c) An alloy of iron
   d) Coated with plastic

6. Crystallization is a technique used for:
   a) Separating insoluble impurities
   b) Obtaining pure solids from solutions
   c) Changing the chemical nature of a substance
   d) Preventing rusting

7. Which two conditions are essential for the rusting of iron?
   a) Presence of oxygen and nitrogen
   b) Presence of oxygen and water vapour
   c) Presence of carbon dioxide and heat
   d) Presence of water vapour and sunlight

8. Burning of paper is a chemical change because:
   a) It changes the size of the paper
   b) It produces ash and gases
   c) It can be reversed
   d) It absorbs heat

9. When a magnesium ribbon is burnt in air, it produces:
   a) Magnesium hydroxide
   b) Magnesium carbonate
   c) Magnesium oxide
   d) Magnesium sulphate

10. Dissolving common salt in water is considered a physical change because:
    a) A new substance (salty water) is formed
    b) The change is irreversible
    c) The salt can be recovered by evaporation
    d) Heat is produced during dissolution

Answer Key for MCQs:

1. c) Melting of wax
2. b) Carbon dioxide
3. c) Zinc
4. c) Formation of a new substance
5. c) An alloy of iron
6. b) Obtaining pure solids from solutions
7. b) Presence of oxygen and water vapour
8. b) It produces ash and gases (new substances)
9. c) Magnesium oxide
10. c) The salt can be recovered by evaporation (indicating no new chemical substance was permanently formed)

Study these notes thoroughly. Focus on the definitions, characteristics, key examples, and the specific details about rusting and crystallization. Understanding the core difference between physical and chemical changes is key. Good luck with your preparation!