Class 6 Science
There are two types of changes, viz. reversible and irreversible.
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.
When a change cannot be reversed, it is called irreversible change. Examples: Cooking, growth in a living being, burning of paper, etc.
Changes can also be divided into following two types:
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.
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.
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.
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