Class 6 Science Notes Chapter 6 (Changes Around Us) – Science Book

Detailed Notes with MCQs of Chapter 6, 'Changes Around Us'. This is a fundamental chapter, and understanding the types of changes is crucial, not just for your regular studies but also for various competitive exams. Pay close attention to the definitions and examples.

Chapter 6: Changes Around Us - Detailed Notes for Exam Preparation

1. What is Change?

• Change refers to the act or process where something becomes different.
• Changes are constantly happening around us – in fields (crop changes), in our bodies (growth, nail/hair growth), and with objects (leaves falling, flowers blooming/wilting).

2. Types of Changes:

The primary classification in this chapter is based on whether a change can be undone or not.

• A. Reversible Changes:

  • Definition: Changes that can be reversed or undone to get back the original substance or object in its original form.
  • Characteristics: Often involve changes in shape, size, or state (solid, liquid, gas). No new substance is formed.
  • Examples:
    • Inflating and deflating a balloon (change in shape and size).
    • Folding and unfolding a piece of paper (change in shape).
    • Stretching a rubber band (change in shape and size, returns to original when released).
    • Melting of ice (solid water to liquid water) and freezing of water (liquid water to solid ice) - Change of state.
    • Boiling water (liquid to gas/steam) and condensation (gas/steam to liquid).
    • Dissolving salt or sugar in water (can be reversed by evaporation/crystallization).
    • Heating a metal object (it expands) and cooling it (it contracts back).

• B. Irreversible Changes:

  • Definition: Changes that cannot be reversed or undone by simple physical means to get back the original substance in its original form.
  • Characteristics: Often involve the formation of one or more new substances with different properties. The original substance is permanently altered.
  • Examples:
    • Burning paper or wood (turns into ash and gases – new substances).
    • Cooking food (raw ingredients change into cooked food with different properties). E.g., cooking an egg, baking a chapati.
    • Ripening of fruits (raw mango to ripe mango – taste, color, texture change).
    • Growth of a plant or animal (cannot be reversed back to a seed or baby).
    • Curdling of milk (milk turns into paneer/curd – a new substance).
    • Rusting of iron (iron reacts with oxygen and moisture to form rust – a new substance).
    • Mixing cement with water (sets into a hard mass).

3. Other Ways to Cause Change:

Changes can be brought about by various means:

• Heating:
  • Causes expansion in most materials (increase in size). Example: Heating metal tools before fixing wooden handles, heating the metal rim before fitting it onto a wooden cartwheel.
  • Can cause melting (solid to liquid).
  • Can cause evaporation/boiling (liquid to gas).
  • Can cause burning (a chemical, irreversible change).
  • Can cause cooking (irreversible change).
• Cooling:
  • Causes contraction in most materials (decrease in size). Example: Cooling the hot metal rim makes it contract and fit tightly onto the wheel.
  • Can cause freezing (liquid to solid).
  • Can cause condensation (gas to liquid).
• Mixing Substances:
  • Sometimes mixing leads to reversible changes (e.g., salt and water, sand and water).
  • Sometimes mixing leads to irreversible changes (e.g., cement and water, plaster of Paris and water, adding acid to base).
• Applying Force:
  • Can change the shape or size of an object (e.g., squeezing dough, hammering metal). This can be reversible (stretching a rubber band) or irreversible (breaking glass).

Key Concepts for Exams:

• Distinction: Be very clear about the difference between reversible and irreversible changes. The core idea is whether you can get the original substance back in its original form.
• Examples: Memorize key examples for each type of change. Exams often ask you to classify a given change.
• Expansion & Contraction: Understand this concept, especially the practical application related to cartwheels and fixing tools. Remember heating causes expansion, cooling causes contraction, and this is generally a reversible physical change.
• Formation of New Substances: This is the hallmark of many irreversible changes (often called chemical changes in higher classes).

Multiple Choice Questions (MCQs)

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

1. Which of the following is an example of a reversible change?
   a) Burning of a candle
   b) Melting of wax
   c) Ripening of a mango
   d) Curdling of milk

2. Rolling a chapati from dough and then changing it back into a ball of dough is an example of:
   a) An irreversible change
   b) A chemical change
   c) A reversible change
   d) Both a) and b)

3. When water is heated, it turns into steam. This process is called:
   a) Condensation
   b) Freezing
   c) Evaporation/Boiling
   d) Melting

4. A blacksmith heats the metal rim before fitting it onto a wooden wheel because heating:
   a) Makes the metal rim softer
   b) Causes the metal rim to expand
   c) Causes the metal rim to contract
   d) Cleans the metal rim

5. Which of the following changes cannot be easily reversed?
   a) Folding a paper aeroplane
   b) Inflating a balloon
   c) Dissolving sugar in water
   d) Baking a cake

6. Formation of curd from milk is:
   a) A reversible change
   b) A physical change
   c) An irreversible change
   d) A change caused only by cooling

7. Stretching a rubber band is a reversible change because:
   a) A new substance is formed
   b) It breaks easily
   c) It returns to its original shape and size when the force is removed
   d) It requires heating

8. What happens when a substance is cooled?
   a) It generally expands
   b) It generally contracts
   c) It always melts
   d) It always burns

9. Which of these processes involves an irreversible change?
   a) Melting of ice cream
   b) Drying of wet clothes
   c) Germination of seeds
   d) Opening a door

10. Dissolving salt in water is considered a reversible change because:
    a) Salt disappears completely
    b) The water tastes salty
    c) Salt can be recovered by evaporating the water
    d) It requires stirring

Answers to MCQs:

1. b) Melting of wax
2. c) A reversible change
3. c) Evaporation/Boiling
4. b) Causes the metal rim to expand
5. d) Baking a cake
6. c) An irreversible change
7. c) It returns to its original shape and size when the force is removed
8. b) It generally contracts
9. c) Germination of seeds
10. c) Salt can be recovered by evaporating the water

Study these notes carefully, focusing on the definitions and examples. Understanding the 'why' behind the classification (can it be undone?) is key. Good luck with your preparation!