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Science: Class 7 — Chapter 5: Physical and Chemical Changes

5.1 Physical Changes

Introduction to Changes

  • Everyday changes involve one or more substances.
  • Changes are broadly classified into two kinds: physical and chemical.

Activity 5.1: Paper Cutting

  • Cutting paper into pieces is a change in size.
  • No new substance is formed; the property of the paper remains unchanged.

Activity 5.2: Chalk Dust

  • Crushing chalk into dust and reforming it into chalk.
  • This change is reversible; chalk can be recovered from dust.

Activity 5.3: Ice and Water

  • Melting ice into water and then freezing water back into ice.
  • This demonstrates a change in state (solid to liquid and back), which is reversible.

Activity 5.4: Boiling Water

  • Boiling water produces steam.
  • Steam condenses into water droplets on a cooler surface.
  • This is a change in state (liquid to gas and back).

Activity 5.5: Heating a Blade

  • Heating a hack-saw blade changes its color.
  • The blade regains its original color after cooling.
  • This is a temporary change in color.

Definition of Physical Change

  • Physical properties: Characteristics such as shape, size, colour, and state of a substance.
  • Physical change: A change in which a substance undergoes a change in its physical properties.
  • Key characteristics:
    • Generally reversible.
    • No new substance is formed.
5.2 Chemical Change

Introduction to Chemical Change

  • Rusting of iron is a common example of a chemical change.
  • Rusting slowly destroys iron articles, causing significant monetary loss.

Activity 5.6: Burning Magnesium Ribbon

  • Magnesium ribbon burns with a brilliant white light, leaving a powdery ash.
  • Equation: Magnesium (Mg) + Oxygen (O₂) → Magnesium oxide (MgO)
  • The ash (magnesium oxide) is a new substance.
  • Dissolving ash in water forms a solution that turns red litmus blue, indicating it is basic.
  • Equation: Magnesium oxide (MgO) + Water (H₂O) → Magnesium hydroxide [Mg(OH)₂]
  • Magnesium hydroxide is also a new substance and a base.

Activity 5.7: Copper Sulphate and Iron Reaction

  • Adding an iron nail to a blue copper sulphate solution.
  • The solution's color changes from blue to green due to the formation of iron sulphate (a new substance).
  • A brown deposit of copper (another new substance) forms on the iron nail.
  • Equation: Copper sulphate solution (blue) + Iron → Iron sulphate solution (green) + Copper (brown deposit)

Activity 5.8: Vinegar and Baking Soda Reaction

  • Mixing vinegar (acetic acid) and baking soda (sodium hydrogencarbonate) produces a hissing sound and gas bubbles.
  • Equation: Vinegar (Acetic acid) + Baking soda (Sodium hydrogencarbonate) → Carbon dioxide + other substances
  • Passing the evolved gas (carbon dioxide) through lime water.
  • The lime water turns milky due to the formation of calcium carbonate (a new substance).
  • Equation: Carbon dioxide (CO₂) + Lime water [Ca(OH)₂] → Calcium Carbonate (CaCO₃) + Water (H₂O)
  • Turning lime water milky is a standard test for carbon dioxide.

Definition of Chemical Change

  • Chemical change (or chemical reaction): A change in which one or more new substances are formed.
  • Importance: All new substances are formed through chemical changes (e.g., digestion, fruit ripening, medicine, plastics, detergents).
  • Accompaniments of a chemical change:
    • Heat, light, or other radiation (e.g., ultraviolet) may be given off or absorbed.
    • Sound may be produced.
    • A change in smell or a new smell may be given off.
    • A color change may take place.
    • A gas may be formed.
  • Examples: Burning of magnesium ribbon, coal, wood, leaves (always accompanied by heat production).
  • Other examples: Explosion of fireworks, spoiling of food, browning of cut fruits/vegetables, neutralization.
  • Ozone layer: Ozone absorbs harmful ultraviolet radiation and breaks down into oxygen, which is a chemical change.
5.3 Rusting of Iron

Process of Rusting

  • Rusting: A chemical change that affects iron articles, forming a brownish substance called rust.
  • Equation: Iron (Fe) + Oxygen (O₂, from air) + Water (H₂O) → Rust (Iron oxide Fe₂O₃)
  • Essential conditions for rusting: Presence of both oxygen and water (or water vapour).
  • Rusting is faster in humid conditions (high moisture content in air).

Prevention of Rusting

  • Prevent iron articles from coming in contact with oxygen or water (or both).
  • Methods:
    • Applying a coat of paint or grease regularly.
    • Depositing a layer of another metal (like chromium or zinc) on iron.
  • Galvanisation: The process of depositing a layer of zinc on iron to prevent rusting (e.g., galvanised iron pipes).
  • Rusting of ships is faster due to salt water.
  • Stainless steel: An alloy of iron with carbon, chromium, nickel, and manganese, which does not rust.
5.4 Crystallisation

Definition and Process

  • Salt obtained by evaporation of sea water is often impure and has unclear crystals.
  • Crystallisation: The process of obtaining large crystals of pure substances from their solutions.
  • Crystallisation is an example of a physical change.

Activity 5.9: Preparing Copper Sulphate Crystals

  • Dissolve copper sulphate powder in hot water containing a few drops of dilute sulphuric acid until no more powder dissolves.
  • Filter the solution and allow it to cool undisturbed.
  • Large, pure crystals of copper sulphate will form.
What You Have Learnt