• Some changes cannot be reversed. These are called irreversible changes.
• Irreversible changes are permanent. For example, an unripe mango is green and sour, but on ripening it usually becomes yellow and sweet.
Ripening of fruits:
• Ripening of fruits is an irreversible change because it is not possible to get unripe fruits from ripened or mature fruits.
Growth of a baby:
• Growth of a baby into an adult is irreversible because the man/woman cannot turn back into a baby.
Pencils and erasers:
• Pencils and erasers get reduced in size with regular use. This is also an irreversible change.
• There are a large number of irreversible changes that take place around us.
• These result in a new material being produced, which may or may not be useful.
Cooking of food:
• Cooking of food takes place on heating uncooked food. For example, making chapattis from flour, cooking rice, baking a cake, making an egg omelette or a fish fry are different forms of cooking.
• We cannot get back the ingredients in their original form after cooking.
• So, cooking of food is an irreversible change.
• Buds change into flowers.
• Blooming of flowers is an irreversible change because flowers cannot change back into buds.
Curdling or souring of milk:
• What happens to the raw milk kept overnight and not refrigerated particularly in summers?
• There are some changes that take place in the milk.
• This is called curdling or souring of milk, which is an irreversible process.
• Curdling of milk is also done by adding lemon juice to milk for making cottage cheese or paneer.
• Heating can cause an irreversible change.
For example, we heat a raw egg to cook it.
• Mixing substances can cause an irreversible change.
• For example, when vinegar and sodium bicarbonate are mixed, the mixture changes and carbon dioxide is evolved.
• If we burn a paper, it turns into ash.
• It is a new substance that differs from the paper in its physical and chemical properties.
• This ash cannot be turned back into paper.
• Burning is therefore an irreversible change.