Physical and Chemical Changes

• Introduction to Changes
  • Reversible vs Irreversible changes
  • Changes happen around us constantly
• Physical Changes
  • Definition: Changes affecting only physical properties (shape, size, color, state)
  • Key Characteristic: No new substance is formed
  • Reversibility: Most physical changes are reversible
  • Examples
    • Melting of ice
    • Dissolving sugar in water
    • Breaking a glass piece
    • Boiling of water (steam to water)
    • Cutting of paper
• Chemical Changes
  • Definition: Changes where one or more new substances are formed
  • Other Names: Chemical reactions
  • Key Characteristics
    • Change in chemical composition
    • Heat, light, or sound may be produced or absorbed
    • Change in smell or production of gas
    • Color change may occur
  • Reversibility: Usually irreversible
  • Examples
    • Digestion of food
    • Photosynthesis
    • Burning of a candle or paper
    • Ripening of fruits
• Important Examples & Processes
  • Rusting of Iron
    • Conditions required: Presence of Oxygen ($O_2$) and Moisture ($\text{H}_2\text{O}$)
    • Chemical formula/nature: Formation of Iron oxide ($\text{Fe}_2\text{O}_3$)
    • Prevention Methods
      • Galvanization (depositing a layer of zinc)
      • Painting or greasing
  • Magnesium Ribbon Experiment
    • Burning magnesium gives a dazzling white light
    • Forms Magnesium oxide ash (New substance)
    • Dissolving ash in water forms Magnesium hydroxide (Basic in nature)
  • Reaction of Copper Sulphate and Iron
    • Also known as displacement reaction
    • Blue solution of Copper Sulphate turns green
    • Brown deposit of Copper on the iron nail (used shaving blade)
  • Crystallization
    • Definition: Process of obtaining large crystals of a pure substance from its solution
    • Example: Crystals of Copper Sulphate from its hot solution
    • Type of Change: Physical change (no new substance is formed, only crystal shape)