Class 6 Science Notes Chapter 6 (Chapter 6) – Examplar Problem Book

Detailed Notes with MCQs of Chapter 6, "Changes Around Us," from your Science Exemplar book. This chapter deals with fundamental concepts about how things transform, which are often tested in competitive exams. Pay close attention to the details and examples.

Chapter 6: Changes Around Us - Detailed Notes for Government Exams

1. Introduction to Changes:

• Change is a constant process occurring everywhere around us. It signifies an alteration or modification from one state or form to another.
• Examples are abundant in daily life: leaves changing colour, flowers blooming, weather changing, cooking food, growing taller, melting ice, burning wood, etc.

2. Types of Changes:
Changes can be broadly categorized based on whether the original substance can be recovered.

• A. Reversible Changes:

  • Definition: These are changes that can be undone or reversed, meaning the substance can return to its original form or state.
  • Characteristics:
    • Usually involves a change in physical properties like shape, size, or state (solid, liquid, gas).
    • No new substance is formed.
    • The chemical composition of the substance remains the same.
  • Examples:
    • Melting: Ice (solid) melts to water (liquid). Water can be frozen back to ice.
    • Freezing: Water (liquid) freezes to ice (solid). Ice can be melted back to water.
    • Boiling/Evaporation: Water (liquid) turns into steam (gas). Steam condenses back to water.
    • Condensation: Steam (gas) turns into water (liquid). Water can be boiled back to steam.
    • Folding/Unfolding: Folding paper changes its shape, but unfolding brings it back.
    • Stretching: Stretching a rubber band or spring changes its shape/size temporarily; it returns to original when force is removed (within limits).
    • Inflation/Deflation: Inflating a balloon changes its size; deflating returns it.
    • Dissolving: Dissolving salt or sugar in water. The salt/sugar can be recovered by evaporating the water.
    • Heating/Cooling Metals: An iron rod expands on heating and contracts back to its original size on cooling.

• B. Irreversible Changes:

  • Definition: These are changes that cannot be undone or reversed. The original substance cannot be easily recovered from the final product.
  • Characteristics:
    • Often involve the formation of one or more new substances with different properties.
    • The chemical composition of the substance changes.
    • Often accompanied by heat, light, sound, or gas production.
  • Examples:
    • Burning: Burning paper, wood, candle wax, or LPG produces ash, carbon dioxide, heat, and light. These cannot be converted back to the original substance.
    • Cooking: Raw food (like egg, rice, vegetables) changes its properties (taste, texture, smell) permanently upon cooking.
    • Rusting: Iron reacts with oxygen and moisture to form rust (iron oxide), a new reddish-brown substance. Rust cannot be easily turned back into iron.
    • Growth: Growth of plants, animals, or humans is irreversible. A tree cannot become a seed again.
    • Ripening: A raw fruit (like a mango) ripens, changing its colour, taste, and texture. A ripe mango cannot become raw again.
    • Curdling: Milk turns into curd (or paneer/cheese) due to bacterial action or adding an acidic substance. Curd cannot be changed back to milk.
    • Making Dough: Mixing flour with water and kneading creates dough. Flour cannot be easily separated back from the dough.
    • Grinding: Grinding wheat grains into flour changes the form permanently.

3. Physical and Chemical Changes (Important Distinction):
This classification often overlaps with reversible/irreversible and is crucial for higher-level understanding.

• Physical Change: A change affecting only the physical properties (like state, shape, size, colour) without forming a new substance. Most physical changes are reversible. Examples: All changes of state (melting, freezing, boiling, etc.), tearing paper, chopping wood, dissolving sugar.
• Chemical Change: A change where one or more new substances with different chemical compositions and properties are formed. Most chemical changes are irreversible. Examples: Burning, cooking, rusting, digestion, photosynthesis, curdling milk.

4. Changes Caused by Heating and Cooling - Expansion & Contraction:

• Expansion: Most substances (solids, liquids, gases) increase in size or volume when heated. This is called expansion.

  • Application 1 (Solids): Fixing an iron rim onto a wooden cartwheel. The rim is made slightly smaller than the wheel. It is heated strongly, causing it to expand. It is then slipped onto the wooden wheel. As it cools, it contracts and grips the wheel tightly.
  • Application 2 (Solids): Gaps are left between sections of railway tracks. This allows space for the rails to expand in hot weather, preventing them from bending or buckling. Similar gaps are seen in bridges.
  • Application 3 (Liquids): Thermometers work on the principle that liquids (like mercury or alcohol) expand uniformly on heating.

• Contraction: Most substances decrease in size or volume when cooled. This is called contraction. This is the reverse process of expansion. Example: The iron rim contracting on the wheel upon cooling.

5. Other Ways to Bring About Change:

• Mixing Substances: Changes can occur when substances are mixed.
  • Example 1 (Reversible): Dissolving salt in water.
  • Example 2 (Irreversible): Mixing cement with water sets into a hard mass (chemical change). Adding lemon juice to milk causes curdling (chemical change).
• Applying Force/Pressure: Force can change the shape or size of an object.
  • Example 1 (Reversible): Squeezing a sponge, stretching a rubber band.
  • Example 2 (Irreversible): Breaking a piece of chalk or glass. Kneading dough.

Key Points for Exams:

• Clearly distinguish between reversible and irreversible changes with examples.
• Understand that reversible changes are usually physical, and irreversible changes are often chemical.
• Know the concepts of expansion on heating and contraction on cooling, along with practical applications (iron rim, railway tracks, thermometers).
• Recognize that changes can be caused by heating, cooling, mixing, or applying force.

Multiple Choice Questions (MCQs)

Here are 10 MCQs based on Chapter 6 to test your understanding:

1. Which of the following changes cannot be easily reversed?
   (a) Melting of chocolate
   (b) Inflating a bicycle tube
   (c) Burning of Liquefied Petroleum Gas (LPG)
   (d) Folding a piece of paper

2. When you dissolve salt in water, the change is considered:
   (a) Irreversible and Chemical
   (b) Reversible and Physical
   (c) Irreversible and Physical
   (d) Reversible and Chemical

3. The process of iron rusting requires the presence of:
   (a) Oxygen only
   (b) Water/Moisture only
   (c) Both oxygen and water/moisture
   (d) Carbon dioxide and heat

4. Expansion happens when substances are generally:
   (a) Cooled
   (b) Mixed with acid
   (c) Put under pressure
   (d) Heated

5. Which action represents a physical change?
   (a) Baking a cake
   (b) Boiling water into steam
   (c) Digesting food
   (d) Ripening of a banana

6. Why is a gap left at the joint of cement concrete roads or bridges?
   (a) To allow water drainage
   (b) To allow for expansion during hot weather
   (c) To reduce the cost of construction
   (d) To allow for contraction during cold weather

7. Curdling of milk results in the formation of paneer (cottage cheese). This change is:
   (a) Reversible
   (b) Physical
   (c) Irreversible
   (d) Temporary

8. A blacksmith heats an iron ring before putting it on a wooden wheel primarily to:
   (a) Make the iron softer
   (b) Clean the iron surface
   (c) Cause the iron ring to expand
   (d) Cause the iron ring to contract

9. Which of the following is an example of a chemical change?
   (a) Stretching a rubber band
   (b) Grinding wheat into flour
   (c) Formation of clouds
   (d) Cooking vegetables

10. Which statement accurately describes a reversible change?
    (a) A new substance is always formed.
    (b) The change cannot be undone.
    (c) The original substance can be obtained back.
    (d) It is always caused by heating.

Answer Key for MCQs:

1. (c) Burning of LPG is a chemical change producing new substances (CO2, water vapour) and is irreversible.
2. (b) Dissolving salt is a physical change (no new substance) and is reversible (salt can be recovered by evaporation).
3. (c) Rusting is the oxidation of iron, requiring both oxygen (from air) and moisture (water).
4. (d) Heating generally causes substances to expand.
5. (b) Boiling water changes its state (liquid to gas) but not its chemical nature (it's still H2O). Baking, digesting, and ripening are chemical changes.
6. (b) Materials like concrete expand when heated by the sun; the gap prevents buckling.
7. (c) Curdling is a chemical change where milk proteins are altered, forming a new substance (curd/paneer). It cannot be reversed back to milk.
8. (c) Heating causes the iron ring to expand, making it slightly larger so it can fit onto the wheel. It then contracts upon cooling for a tight fit.
9. (d) Cooking involves chemical reactions that change the composition and properties of the vegetables, forming new substances. Grinding and stretching are physical changes. Cloud formation involves changes of state (physical).
10. (c) The defining characteristic of a reversible change is that the original substance or state can be recovered.

Study these notes and practice the MCQs thoroughly. Good luck with your preparation!