Category : 6th Class
Changes Around Us
We observe changes around us all the time. Changes may occur in shape, size, mass, density, colour, position, temperature, structure or in composition of a substance. So we can define a change as:
Transformation in one or more than one properties of a substance is called change'.
Types of Changes
All the changes have been classified into two categories on the basis of, either the changes can be reversed to bring back the original substance, or not.
Reversible Changes
The changes which can be made to occur in the opposite direction are called reversible changes. In reversible change we can bring back the original substance. Melting of ice, freezing of water, dissolution of salt in water, increase in temperature of a metal rod etc. are the examples of reversible changes. Irreversible Changes.
Irreversible changes
The changes which cannot be made to occur in the opposite direction are called irreversible changes. Burning of wood, ripening of fruit, turning milk sour etc. are the examples of irreversible changes. On the basis of either new substance is formed or not, all the changes has been classified into two groups.
Physical Changes
The change in which molecules of a substance do not undergo any change or no new substances are formed, are called physical changes. Melting of ice, freezing of water, evaporation of water, dissolution of salt in water are examples of physical changes.
Chemical changes
The change, in which molecules of substance undergo change or new substances are formed, are called chemical changes. Burning of paper, rusting of iron, growth in animals, spoil of food are the examples of chemical changes.
Which one of the following is a reversible change?
(a) Dissolution of common salt into water
(b) Burning plastics
(c) Decaying of organic substances
(d) All of these
(e) None of these
Answer: (a)
Melting of coal tar on heating is an example of:
(a) Reversible change
(b) Irreversible change
(c) Chemical change
(d) All of these
(e) None of these
Answer: (a)
Transformation in one or more than one properties of a substance is known as change.
The changes which can be reversed to bring back original substance is known as reversible change.
Irreversible changes can be reversed to bring back original substance.
New substance is formed when chemical changes occur.
No new substance is formed when a physical change occur.
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