Physical and Chemical Changes-Class-8

Changes are an important part of nature. They takes place all the time. We can classify the changes in the following categories:

1. Natural and man-made changes
2. Slow and fast changes
3. Reversible and irreversible changes
4. Periodic and non-periodic changes
5. Desirable and undesirable changes
6. Physical and Chemical Changes
7. Endothermic and exothermic changes

Natural Changes

Changes that takes place in the nature are known as natural changes.

Example: Change in season, flowering of plants, ripening of fruits, formation of cloud, rainfall, formation of waves, etc.

Man- made Changes

The changes brought about by the activities of man are called man-made changes.

Example: Burning of fuels, Cooking of foods, construction of buildings, farming, preparation of chemical substances in laboratory, etc.

Slow Changes

The changes that takes a long time to occur are called slow changes.

Example: Growth of tree, growth of a baby to a man, rusting of iron, change in season, weathering of rock, etc.

Fast Change

Changes that takes place within a very short period of time are called fast changes.

Example: Burning of paper, bursting of balloons or crackers, occurrence of lightning, breaking of glass, etc.

Reversible Change

Changes that can be reversed by reversing the conditions are called reversible reaction.

Example: Freezing of water, stretching of rubber band, etc.

Irreversible Change

Changes that can not be brought back to their original state after the change are called irreversible changes.

Example: Curdling of milk, burning of paper, growth of plant, etc.

Periodic Changes

Changes which are repeated at regular intervals of time are called periodic changes.

Example: Change in season, changing of day and night, swinging of pendulum, beating of heart, rotation of Earth on its axis, revolution of earth around Sun, etc.

Non-Periodic Change

Changes which are not repeated at regular intervals of time are called non-periodic changes.

Example: Eruption of volcano, earthquake, cyclonic storm, rusting of iron, sneezing, coughing, etc.

Desirable Changes

Changes which are beneficial are called desirable changes.

Example: Formation of cloud, digestion of food, ripening of fruits, etc.

Undesirable Changes

Changes that bring about destruction are called undesirable changes.

Example: Floods, spoiling of foods, earthquake, rusting, etc.

Physical Changes

Physical changes are changes which are temporary, can be easily reversed, no new substance is formed.

Characteristics of Physical Change

1. No new substances are formed during a physical change.
2. It is temporary change.
3. It is reversible by simple physical means.
4. No change in composition of the substance takes place.
5. The properties of the substance remains same.
6. There may or may not be absorption or evolution of energy during a physical change.

Example of physical changes are:

1. The glowing of an electric bulb.
2. Melting of ice.
3. Dissolution of a substance in water.
4. Stretching a rubber band.
5. Melting of butter or ice cream.
6. Magnetizing an iron bar.
7. Expansion of metals on heating and contraction on cooling.
8. Formation of clouds, fog, mist, frost etc.
9. Formation of crystals of sugar and salt.
10. Breaking of class.
11. Beating of metals into thin sheets and drawing them into wires.

Chemical Changes

Chemical changes are changes which are permanent, cannot be reversed, new substance is formed.

Characteristics of Chemical Change

1. New substances are formed during a chemical change.
2. It is permanent change.
3. It is irreversible s.
4. The composition and properties of the substances are altered during a chemical change.
5. Heat and light energy are evolved or absorbed during a chemical change.

Example of chemical changes:

1. Burning of paper, fuels
2. Rusting of iron.
3. Cooking of food.
4. Souring of milk.
5. Burning of magnesium.
6. Fermentation of sugar.
7. Digestion of food.
8. Respiration.
9. germination of seeds.
10. Ripening of fruits.
11. Photosynthesis.
12. Clotting of blood.

• Burning of candle is a chemical process but melting of candle is a physical process.
• Heating solid ammonium chloride to vapour ammonium chloride is physical process. On further heating it breaks to produce ammonia and hydrogen chloride gas. It is a chemical process.

Endothermic Change

A change which proceeds with the absorption of heat energy is called endothermic change.

Example:

1. N₂ + O₂ → 2NO
2. NH₄Cl + H₂O → NH₄OH + HCl
3. CO₂ + H₂O → C₆H₁₂O₆ + O₂
4. Melting of ice
5. Converting hydrated to anhydrous salt.
6. Formation of cation from atom

Exothermic Change

A change which proceeds with the evolution of heat energy is called exothermic change.

Example:

1.Burning fuels like wood, natural gas, or gasoline.

2.The burning of a candle.

3.The reaction between an acid and a base, such as hydrochloric acid (HCl) and sodium hydroxide (NaOH).

4.The oxidation of iron in the presence of oxygen and moisture.

5.The reaction of hydrogen and oxygen gasses.

6.The process of a liquid turning into a solid, like water freezing into ice.

7.The process of a gas turning into a liquid, like water vapor condensing into liquid water.

8.Some solutes, when dissolved into a solvent, will release heat. An example is the dissolving of calcium chloride into water.