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NCERT Solutions for class 7 Science Physical and chemical changes

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Keywords

Physical changes: When physical properties of a substance changes it is called physical change. Example: Folding of paper.

Chemical changes: The changes in which new substance are formed are called chemical changes. Example: rusting of iron.

Galvanisation: The process of depositing the layer of zinc on iron is called galvanization.

Crystallisation: A process in which crystals of a substance appears is called crystallization. The salt in obtained from sea in the same manner.

Properties of physical changes

  1. No new substance is formed in physical changes.
  2. Physical changes can be reversed and hence they are called reversible changes.
  3. In physical changes just the physical properties such as shape and size of a substance changes.
  4. No heat or light is produced in physical changes.

Properties of chemical changes

  1. In chemical changes new substances are formed.
  2. Chemical change cannot be reversed and hence they are called irreversible change.
  3. Heat and light is produced in chemical changes.
  4. Sometimes sound may be produced.
  5. The change in colour may take place.
  6. Gases may also evolve in chemical change.
  7. Example of chemical change is burning of coal.

Some chemical reactions are given below

  1. Magnesium (Mg) + Oxygen (O2) -> Magnesium oxide (MgO)
  2. Magnesium oxide (MgO) + Water (H2O) → Magnesium hydroxide [Mg(OH)2 ]
  3. Carbon dioxide (CO2 ) + Lime water [Ca(OH)2 ] → Calcium Carbonate (CaCO3 ) + Water (H2O)

Usefulness of chemical changes in real life

  1. Ripening of fruits is chemical changes.
  2. The medicines are formed by chemical reactions.
  3. Our digestion system works on chemical reactions.
  4. Useful materials such as plastic and detergents are formed through chemical reactions.

Rusting of iron

The iron objects that comes in contact with moisture (or water) and oxygen gets a brownish layer deposited on its surface. This brownish layer is called rust. And the process is called rusting.

Prevention from rusting

Two methods can be used to prevent rusting:

  1. Coating the iron object with paint can prevent it from rusting.
  2. Galvanisation or depositing the layer of zinc on iron objects can also prevent rusting.

What is crystallisation?

It is the process of obtaining clear crystals of a substance. Crystallisation is a physical change because no new substance is formed in this process.

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