Physical And Chemical Changes

Question 1

What are desirable changes?

Answer 1

Changes that are useful to human beings are said to be desirable changes.
Examples The ripening of fruits, cooking of food etc. are all desirable changes,

Question 2

What are undesirable changes?

Answer 2

A change that is harmful to human beings or causes destruction is called an undesirable change.
Examples Cyclonic storm (Fig. 2.3), spoilage of food, rotting of fruits etc.
are undesirable changes

Question 3

At what is reversible change?

Answer 3

Reversible alla
Reversible changes
A change that can be reversed (returned to its original form) by reversing the conditions is called a reversible change.
Examples The melting of ice (Fig. 2.5), evaporation of water, etc. are all reversible changes

Question 4

What is an irreversible change

Answer 4

Irreversible changes
A change that cannot be reversed is called an irreversible change.
Examples The burning of paper (Fig. 2.6), ageing of living beings, ripening of fruit rusting of iron, etc. are irreversible changes.

Question 5

What is a physical change?

Answer 5

A physical change is a change in which no new substance is formed.
All changes of state are physical changes.

Question 6

What are the characteristics of a physical changes

Answer 6

Characteristics of a physical change
The following are the characteristics of a physical change.
1. Generally, the change is temporary and reversible.
2. No new substance is formed.
3. The chemical composition of the original substance is not altered.
4. A physical change may cause changes in state, shape, size and colour of a substance.

Question 7

What is a chemical change?

Answer 7

A chemical change is a permanent change in which the original substance loses its own composition and properties.
During this change, one or more new substances with composition and properties different from the original substances are formed.

Question 8

What dies wood form when it is burnt in the air?

Answer 8

Carbon dioxide and water vapour along with smoke and ashes

Question 9

State the characteristics of chemical change

Answer 9

Characteristics of a chemical change
The following are the characteristics of a chemical cha
1. The change is permanent and cannot be reversed b physical methods.
2. One or more new substances are formed with com and properties different from the original substanc
3. A chemical change involves an exchange of energy form of heat or light.