Question

Chemical changes are __________.
A. temporary, reversible and a new substance is produced.
B. always accompanied by exchange of light.
C. permanent, irreversible and a new substance is produced.
D. never accompanied by exchange of light and heat energy.

Answer 1

Hint: There are various changes that occur in nature. Depending upon the nature of the change, it has been classified into two types which are physical and chemical changes. Let us discuss these changes one by one.

Complete answer:
Physical changes are the changes occurring in a substance that the change brings about the change in its physical properties such as changing the state of the substance for example: an ice cube melts into water is a physical change that could be reversed by adjusting the temperature conditions. Now let us understand chemical changes, these are the changes that occur in the substance such that its chemical properties change, resulting in the formation of a new product.
Though no new product is formed during a physical change. For example rusting of iron, here if once the iron corrodes to rust due to the action of moisture then it cannot be changed back to iron. Therefore one tries to prevent it. Hence we get to know that chemical changes are irreversible and permanent in nature. Formation of a new product during chemical reaction either occurs due to decomposition or synthesis. Chemical changes can be both organic and inorganic. Some of the organic chemical changes occurring in nature are ripening of fruit, blooming of a bud to a flower and many more. Inorganic chemical changes include explosions, burning of matchsticks etc.
Therefore from the above examples we can conclude that chemical changes may or may not include exchange of light and heat energy.

Hence the correct option is C.

Note: Irreversible processes take finite time for completion and are carried out rapidly, work obtained in this process is not the maximum. On the other hand reversible processes take infinite time to complete.