Chemical changes


Physical and Chemical Change I - Concepts
Class - 7th ISO Subjects
 
 
Concept Explanation
 

Chemical Changes

Chemical Change:

A chemical change occurs when two or more chemical substances react to produce new substance with a different set of properties. It is permanent which cannot be reversed. Examples:

  • Reddish – Brown Decomposition on Iron Objects: Iron reacts with atmospheric oxygen in the presence of moisture to form a reddish-brown layer of iron oxide. This layer is called rust and process is called rusting. Oxygen and water are the two essential conditions for rusting of iron. Oiling and painting of iron objects protect their surfaces from coming in contact with moisture and air and prevent rusting.
  • Browning of Vegetables and Fruit Surfaces: When we exposed the cut surface of a brinjal to the air for some time, the surface has turned brown, due to a chemical reaction between certain compounds present in brinjal and atmospheric oxygen to produce a brown colour pigment “melanin” that cause browning of the surface. Cut vegetables and fruits soaked in water can reduce the level of browning because water restrict the amount of oxygen to come in contact with surface of cut vegetables and fruits.
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    Sample Questions
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    Question : 1

    Chemical change are

    Right Option : A
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    Question : 2

    If two substances are bought in contact . it will result in

    Right Option : C
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    Question : 3

    Which of the following are correct :

    (a) On cutting brinjal the atmospheric oxygen react to produce a brown colour pigment “melanin” that cause browning of the surface. 

    (b) The chemical change is irreversible and permanent.

    (c) The enzymes present in these micro organisms convert milk sugar (lactose) to lactic acid.

    Right Option : D
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