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• Change – Bringing difference in the form of a material
• Reversible change – changes which can be reversed or undone . e.g. ice can be converted to water and again can be converted to ice by keeping in the refrigerator
• Irreversible change – changes which cannot be reversed or undone. E.g. Formation of curd from milk, growth of plant, etc
  

  Reversible change	Irreversible change
  Melting of ice	Burning of wood
  Boiling of water	Cooked food
  Mixing of salt / sugar in water	Burning of crackers
  Melting of butter	Chopped fruits
  Melting and freezing of icecream	Cutting of tree

• How to bring about a change
  • Force – It is a push or a pull for any object. It can be responsible for change in shapes in many ways. E.g. can start and stop motion of objects. Can change direction and shape of an object.
  • Temperature – Temperature can bring change in the following ways
    • Change in temperature can convert ice into water. Again if temperature increases water which is in the liquid state gets converted into gaseous state i.e. vapours
    • Similarly when vapors are cooled it gets converted into water i.e. in liquid state. When temperature is further reduced water gets converted into ice which is solid in state
  • Mixing or chemical change – when two or more substances are mixed it causes changes in its properties. Some time these changes are reversible, sometimes they are irreversible.
    • Mixing of sugar in water
    • Drop of lemon juice in milk makes curd