Page 39 - ICSE Chemistry 8

CHANGES INVOLVING ENERGY CHANGES

Most of the changes involve release or absorp on of energy. Energy, usually in the form of heat, is either
released or absorbed during such changes.

Changes in which Energy is Absorbed

Physical changes may involve change of energy. Mel ng of ice, boiling of water and evapora on are some
changes which absorb heat. This is because when a change of state occurs from solid to liquid or liquid
to gas, the intermolecular force of a rac on decreases resul ng in an increase in the intermolecular
space. This is possible only when heat energy is absorbed by a solid or liquid. Some chemical changes like
photosynthesis occur by absorbing energy in the form of sunlight.
Changes in which Energy is Released

Physical change like freezing of water releases energy in the form of heat. This is because when water
freezes into ice, i.e., water changes its state from liquid to solid, the intermolecular force of a rac on
increases and intermolecular space decreases. This is possible only when heat energy is released by the
liquid or gas. Chemical changes like combus on and diges on of food also involve release of energy.

Take a Break!

Write true (T) or false (F).
1. Breaking of a glass plate is an example of a physical change. ___________

2. Evapora on of water is a physical change which is man-made and undesirable. ___________
3. Freezing is a process in which a substance changes from the liquid state to the
solid state on cooling. ___________
4. Naphthalene sublimes at room temperature. ___________

5. A chemical change is permanent and reversible. ___________
6. Baking a chapa is a chemical change. ___________

7. Boiling of water is a physical change in which energy is released in the form
of heat. ___________

8. Combus on is a chemical change that involves release of energy. ___________

Terms to Know

Reversible changes: Changes in which we can get back the original substance by reversing the ac on

Irreversible changes: Changes in which we cannot get back the original substance by reversing the ac on
Periodic changes: Changes that occur a er a regular interval of me
Non-periodic changes: Changes that do not occur a er a regular interval of me, i.e., occur irregularly

Desirable changes: Changes that are benefi cial to us
Undesirable changes: Changes that are not benefi cial to us
Slow changes: Changes that take place over a period of me

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