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class 7 science chapter 6

Physical and Chemical Changes Class 7 Science Notes - Chapter 6

Physical and Chemical Changes Class 7 Science Notes - Chapter 6 

Preview of the chapter 

Changes are occurring all around us, such as souring milk, making sugar solutions, setting curd from milk, melting ice, making ice cream, melting wax, stretching rubber bands, evaporation of water, cutting paper, breaking glass panes, bending glass tube by heating, boiling water, and sublimation of camphor. These changes are of two kinds: Physical and Chemical.

Physical Changes

  • Physical changes involve physical properties such as shape, size, colour and state, and no new substance is formed.
  • Physical changes are temporary changes which can be easily reversed to form the original substance. 
  • They are reversible and can cause a temporary change in colour, but little energy is absorbed or evolved.



Chemical Changes

  • Chemical changes are chemical reactions that occur when two substances react chemically to form a new substance with different chemical properties. 
  • Usually a chemical change involves a chemical reaction, which forms new products.
  • example : Rusting of Iron, or burning wood.




Metallic Oxides

Formation of metal oxides are examples of chemical changes. They are formed by the reaction of oxygen in air.
– Burning of Magnesium ribbon:
– 2Mg + O2 → 2MgO
– The product formed is the oxide of magnesium, which is in the form of ash. It does not look anything like the magnesium ribbon used for burning.

Reaction of metallic oxides with water

  • Reaction of metal oxides with water form metal hydroxides.
  • Example dissolving Magnesium oxide in water, by stirring the ash very well with water.
  • MgO + H2O → Mg(OH)2
  • The product formed is basic in nature and turns red litmus paper → blue

Chemical Alterations in Everyday Life

Chemical changes are essential for the discovery of new materials, such as metals, medicine, plastics, and detergents. Burning of magnesium ribbon, coal, wood, and leaves is also a chemical change, resulting in the production of heat and light. Burning of any substance is also a chemical change, accompanied by the production of heat and light.

Rusting

  • Rust is a red layer that forms when iron-based materials are exposed to oxygen and atmospheric moisture.
  • The reaction 4Fe + 3O2 2Fe2O3 can be used to model the formation of rust.
  • Rust's chemical composition is Fe2O3.nH2O. Rust forms more quickly the more moisture there is in the air.
  • Iron (Fe) + Oxygen (O2) (From air) + Water (H2O) → Rust (Iron oxide, Fe2O3)


Preventions of Rusting

  • Rusting can be prevented by not allowing the iron to come in contact with moisture and air.
  • The simplest method is to coat the iron with oil, grease or paint.
  • A more efficient method is to coat the iron with another metal such as zinc or chromium.
  • Rusting of iron can be prevented by allowing it to make stainless steel.

Galvanization

  • The process of depositing zinc on the surface of Iron to prevent rusting is called as galvanisation.
  • Example: Iron water pipes are galvanised.
  • Ships are made out of iron which is galvanised.
  • Due to the presence of salts in seawater, the process of rusting is hastened.

Crystallisation

  • We have learnt in Class VI that salt can be obtained from seawater by the process of evaporation.
  • The salt obtained in this manner is not pure and its crystals are small.
  • Large crystals of pure substances can, however, be obtained from their solutions by the process of crystallisation.

  • The process of crystallisation is used to obtain crystals of a pure solid substance from the impure solid substance.
  • Impure copper sulphate powder can be purified by the process of crystallisation to obtain large crystals of pure copper sulphate.










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