Section outline

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      Important terms:

      1. Changes refer to the transformation in the state or properties of a substance.

      2. Physical properties of a substance are shape, size, colour & state.

      3. Physical change is a change in the physical properties of a substance.

      4. Chemical Change is a change in which one or more new substances are formed.

      5. Reversible change is a change in which the original substance can be obtained back.

      6. Irreversible change is a change in which the original substance cannot be obtained in its exact original form.

      7. Rust is a brownish film which develops on iron when it reacts with oxygen & water.

      8. Crystallization is a process to obtain large crystals of pure substances from their solutions. 

       

       

      Key Points:

      1.       Changes can be physical or chemical, temporary or permanent, reversible or irreversible.

      2.       Rust is iron oxide which is brownish in colour.

       

       

    • Mind Map

    • Quick Revision Notes:
      1. Properties of physical change:
      • Change only in the physical properties of a substance.
      • No change in the chemical composition of the substance.
      • Mostly reversible. 
      • No new substance is formed.
      • Examples: Change in state, dissolving of salt or sugar, shaping clay, all types of mixtures, etc.
      Irreversible physical change:
      • The original size or shape of the substance cannot be obtained back; hence such a physical change is irreversible.
      • Examples: Shredding of paper, grinding or breaking of rock, braking of glass, etc.
       2. Properties of chemical change:
      • It forms a new substance or substances.
      • The new substance has different properties from the original substances.
      • These changes are irreversible.
      • Heat, light or any other energy may be released or absorbed.
      • Sound maybe produced.
      • There may be a change of smell or colour.
      • A gas maybe formed.
      • Examples: ripening of fruits, digestion, curdling of milk, burning of wood, explosion of fireworks, etc.
      3. Rusting:
      • Humidity & salts in water make iron rust faster. 
      • To prevent rusting coat iron with paint or coat with zinc (galvanization) or using stainless steel.
      4. Features of crystallization:
      • It is a physical change.
      •  It is used to purify substances & separate them from impurities.
      • It occurs when solubility limit of a substance is exceeded by cooling a saturated solution or evaporating the solvent.
      • Examples: copper sulphate crystals, salt crystals, sugar crystals, snow, etc.
      5. Chemical equations:
      • Magnesium (Mg) + oxygen (O2) → Magnesium oxide (MgO)
      • Magnesium oxide (MgO) + water (H2O) → Magnesium hydroxide [Mg (OH)2]
      • Copper Sulphate [CuSO4] (blue solution) + Iron (Fe) → Iron Sulphate [FeSO4] (green solution) + Copper (Cu) (brown deposit)
      • Vinegar (acetic acid) + baking soda → CO2 (carbon dioxide) + other substances
      • Calcium Hydroxide [Ca (OH)2] + Carbon dioxide (CO2) → Calcium carbonate [CaCO3] + Water (H2O)
      • 4Fe + 3O2 + x.H2O → 2Fe2O3 (rust)