Physical and Chemical Changes

Start music   Jan 11 2017 || 1:07 PM

Types of changes

  1. Physical change
  2. Chemical change

 

Physical change

  • Change in a substance’s physical properties such as shape, colour, and size
  • In such a change, no new substance is formed
  • Eg: Crystallisation

 

Crystallisation

  • Crystals of pure substances can be formed from their solutions. The is called crystallisation
  • Example: copper sulphate crystals from copper sulphate solution

 

Chemical Change

  • A change in which one or more substances are formed is called as chemical change.
  • It is also called as chemical reaction
  • Examples:
    • Burning of a ribbon of magnesium
      • Magnesium (Mg) + Oxygen (O2) à Magnesium oxide (MgO)
      • Magnesium oxide (MgO) + Water (H2O) à Magnesium hydroxide [Mg(OH)2]
      • Copper sulphate solution (blue) + Iron à Iron sulphate solution (green) + Copper
      • Vinegar (Acetic acid) + Baking soda (Sodium hydrogencarbonate) à Carbon dioxide + other substances
      • Carbon dioxide (CO2) + Lime water [Ca(OH)2] à Calcium Carbonate (CaCO3) + Water (H2O) (Lime water turns milky)
  • Turning lime water milky is the test for ‘carbon dioxide’

 

Properties of a chemical reaction

  • New products are formed
  • Heat, light or any other radiation may be given off
  • Sound may be produced.
  • A change in smell may take place or
  • A new smell may be given off.
  • A colour change may take place.
  • A gas may be formed.

 

Rusting of iron

 

Iron (Fe) + Oxygen (O2, from the air) + water (H2O) à Rust (iron oxide Fe2O3)

 

  • For rusting, the presence of both oxygen and water (or water vapour) is essential
  • High moisture content makes rusting faster
  • Saltwater makes rusting faster

 

Preventing rusting

  • Prevent iron articles from coming in contact with oxygen, or water, or both.
  • Examples:
    • Apply a coat of paint or grease on iron items
    • Deposit a layer of a metal like chromium or zinc on iron
    • Process of depositing a layer of zinc on iron is called galvanisation