Chemical and Physical Changes

Introduction to Changes

  • Changes are observed in everyday life, involving one or more substances.

  • Examples include:

    • Dissolving sugar in water (physical change)

    • Setting curd from milk (physical change)

    • Milk souring (chemical change)

    • Stretched rubber band (physical change).

  • The chapter focuses on physical and chemical changes.

Physical Changes

Definition

  • Physical changes affect physical properties (shape, size, color, state) but do not create new substances.

  • They are generally reversible.

Examples and Activities

  • Activity 6.1: Cutting paper into pieces. You cannot rejoin the pieces, yet the property of the paper remains the same.

  • Activity 6.2: Collecting chalk dust and forming it into a new chalk shape shows no new substance is formed as it’s still chalk.

  • Activity 6.3: Melting ice demonstrates reversible physical change as it returns to solid state when cooled again.

Key Points

  • Physical changes can be observed in ice melting, paper tearing, and chalk crushing.

Chemical Changes

Definition

  • A chemical change occurs when new substances are formed, which typically cannot be reversed easily.

  • A clear example is the rusting of iron:

    • Formula: Iron (Fe) + Oxygen (O2) + Water (H2O) → Rust (iron oxide Fe2O3).

Examples and Activities

  • Activity 6.5: Burning magnesium ribbon shows transformation into magnesium oxide with new properties.

    • Reaction: Magnesium (Mg) + Oxygen (O2) → Magnesium oxide (MgO).

  • Activity 6.6: Burning magnesium and turning ash into an alkaline solution with litmus tests showcases new substances.

  • Activity 6.7: Copper sulphate reacting with iron results in a green solution (iron sulfate) and brown copper deposits.

  • Activity 6.8: Mixing vinegar and baking soda produces carbon dioxide gas, illustrating a chemical change.

    • Reaction: Vinegar (acetic acid) + Baking soda (sodium bicarbonate) → Carbon dioxide + other products.

    • Carbon dioxide in lime water results in milky solution (Calcium carbonate - CaCO3).

Signs of Chemical Changes

  • Produces heat/light/radiation.

  • Changes in smell.

  • Color changes.

  • Gas formation.

Rusting of Iron

Process and Prevention

  • Rusting is a common chemical change that significantly affects iron products, leading to extensive damage and monetary loss.

  • Conditions for rusting: Requires moisture and air. To prevent rusting:

    • Apply paint or grease.

    • Galvanization (coating with zinc).

Additional Notes

  • The ozone layer protects against ultraviolet radiation; breaking down ozone is a chemical change.

  • Photosynthesis and digestion are examples of chemical changes in nature.

Crystallisation

Definition

  • A process of obtaining pure substances from solutions, demonstrating physical change.

  • Activity 6.9: Dissolving copper sulfate in heated water and allowing it to cool results in crystal formation.

Summary of Key Learnings

  • Changes in substances fall into two categories: physical and chemical.

  • Physical changes do not produce new substances and are often reversible.

  • Chemical changes result in the formation of new substances with different properties.

  • Crystallisation can purify substances from their solutions.

Exercises

  1. Classify changes (e.g., Photosynthesis as chemical, dissolving sugar as physical, burning coal as chemical).

  2. True/False statements on changes and properties.

  3. Fill in blanks about concepts related to changes.

  4. Discuss processes that showcase chemical and physical changes, such as baking soda and lemon juice.

Did You Know?

  • The iron pillar near the Qutub Minar in Delhi has not rusted for over 1600 years, showcasing ancient Indian metal technology.