Class 7 Chemistry Physical and Chemical Changes Chemical change

Chemical change

A change leading to the production of a new substance is known as a chemical change. These changes are irreversible. Some of the chemical changes occurring in our day to day life include rusting of iron, silver jewellery getting tarnished or copper articles getting covered by green layer.

  • In case of rusting of iron, the iron reacts with the oxygen present in air and moisture and develops rust (hydrated iron (III) oxide).

Fig. Iron chain left in moist air got rusted

Corrosion causes damage to car bodies, bridges, iron railings, ships and to all objects made of metals, specially those of iron.

  • In case of rusting of copper, the metallic copper reacts with oxygen, carbon-dioxide and atmospheric moisture and develops a green coloured coating of copper hydroxide and copper carbonate.

Fig. Copper developing green coloured rust on exposure to moist air

  • In case of tarnishing of silver articles, the metallic silver reacts with hydrogen sulphide or sulphur present in air and gets tarnished.

Fig. Tarnished silver Vs polished silver


  • Burning of magnesium is another chemical change. When a ribbon of magnesium is brought near flame it starts burning with a brilliant white light leaving behind a powdery ash on complete burning. Magnesium burns in air to form magnesium oxide.

2Mg + O2 - --> 2MgO

            On dissolving the ash in water it forms a new substance.

MgO +H2O - -> Mg(OH)2


  • Rotten fruit

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