Physical and Chemical Changes Class 7 Science Notes - Chapter 6

Physical vs. Chemical Changes

Introduction
There are several changes we come across daily e.g dissolving sugar in water, or flattening a metal rod by beating it. These involve changes in the form of the substance. Changes can be classified as:
(i) Physical
(ii) Chemical

Physical properties
Physical properties include size, shape, colour and state (solid/liquid/gas) of a substance.

Physical change

  • Any change to the physical properties of a substance is called a physical change.
  • Physical changes are usually reversible as no new substance is formed. It is the same substance but with changed physical properties.

Chemical change

  • A change in which one or more new substances are formed is called as a chemical change.
  • 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

Reaction between baking soda and vinegar

When a pinch of baking soda is added to vinegar we hear a hissing sound and observe the formation of bubbles.

– Vinegar (Acetic Acid) + Baking Soda (Sodium bicarbonate)  → CO2 (Carbon dioxide) + Other products
The carbon dioxide produced during the reaction of Vinegar and baking soda, when passed through lime water gives calcium carbonate, as follows:
– CO2 + Ca(OH)2 (lime water) → Calcium Carbonate (CaCO3) + H2O
– 
The calcium carbonate turns lime water milky.
Observations that indicate a chemical change

  • Heat or light is absorbed or given out during a chemical reaction.
  • Production of sound
  • Production of gases or precipitates
  • Production of smell
  • A colour change may occur

Rusting

  • When substances made of Iron are exposed to oxygen and moisture in the atmosphere, it forms a red layer which is called rust.
  • The formation of rust can be represented by the following reaction:
  • 4Fe + 3O2 → 2Fe2O3. The chemical formula for rust is Fe2O3.nH2O. More the moisture in the air, quicker the formation of rust.

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. Hence ships need to replace their iron body every year.

Crystallisation
– The process of separation of salts from their solution is called as crystallisation. It is a purification technique that purifies seawater or separates crystals from impure samples. It is a physical change.

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