Changes Around Us
There are two types of changes, viz. reversible and irreversible.
Reversible Change
When a change can be reversed, it is called reversible change. Examples: Melting of ice, folding of paper, water turning into vapour, inflation of balloon, etc.
We get water after melting of ice. If you will put water in refrigerator, you will once again get ice. Similarly, water vapour can be condensed to get water.
Irreversible Change
When a change cannot be reversed, it is called irreversible change. Examples: Cooking, growth in a living being, burning of paper, etc.
Once you cook rice, you cannot get raw rice back from it. Once your burn a piece of paper, you cannot get back paper from it.
Changes can also be divided into following two types:
Physical Change
Fig: Physical Change
When no new substance is formed after the change, it is called physical change. Physical changes are usually reversible. Examples: Melting of ice, melting of wax, etc.
Ice and liquid water are made of the same material, i.e. water. No new substance is formed when water freezes to become ice or ice melts to become water. Similarly, no new substance is produced after melting of wax. So, these are examples of physical change.
Chemcial Change
Fig: Chemical Change
When a new substance is formed after the change, it is called chemical change. Chemical changes cannot be reversed by simple physical means. Many chemical changes are irreversible. Examples: Burning of candle, rusting of iron, etc.
When a candle, or a piece of paper is burnt then a new substance is formed. When iron gets rusted, the brown layer of rust is not iron but a new substance. So, these are examples of chemical change.
Causes of Change
There are various causes of change. Some of them are given below:
Force: Force can bring changes in certain things. For example: when you blow air in a balloon, the force of air inflates the balloon. When a potter applies force on a lump of soil, he gives shape to the lump. When a goldsmith hammers a piece of gold, he makes intricate designs on it. When a ball of dough it pressed with the rolling pin, it turns into flat bread; which is ready to cook.
Temperature: When ice is kept on room temperature, it melts and turns into water. When candle begins burning, its wax melts because of higher temperature. When a piece of paper is burnt, it turns into a new substance because of heat. Ironsmith uses heat to change a lump of iron into useful items. Iron is heated till it becomes red hot and then is beaten to give it a particular shape. Spade, knife, sickle, etc. are made from iron in this way.