Changes that cannot be reversed are called irreversible changes.
Some examples are as follows:
(i) Milk into curd
(ii) Burning of wood
(iii) Ripening of fruits
(iv) Digestion of food

(a) Heat
(b) Electricity
(c) Light
(d) Force

When sugar is continuously heated in the porcelain dish, it becomes foggy due to water vapour. A black powdery substance is left behind which is charcoal. This process is called charring of sugar.
This is a chemical change.

These two acts are of different kinds. Making a lump of clay into a pot is a reversible change, while baking the pot in an oven is an irreversible change.

Melting of wax is a reversible change because solid wax melts on heating and melted wax becomes solid on cooling. Melting of wax is a change which can be reversed, whereas burning of wax is an irreversible change we cannot combine all the products of burning of wax to get back the original candle. Therefore, burning of wax is a Change which cannot be reversed.

(a) Changes that can be reversed In such changes, we can go back to original state. These changes are also known as reversible changes.
Some examples are as follows:
(i) Melting of ice
(ii) Drying of clothes
(iii) Knitting of sweater
(iv) Heating of milk
(v) Stretching the rubber band
(b) Changes that cannot be reversed In such changes, we cannot go back to original state. These changes are also known as irreversible changes.
Some examples are as follows:
(i) Making curd from milk
(ii) Making flour from grain
(iii) Boiling of eggs
(iv) Growth of plants
(v) Burning of paper or wood

Ice melts when it is heated. Yes, it is reversible

The changes that can be noticed in our body easily are
(i) our nails grow and
(ii) our body growth.

The two changes which cannot be reverse are
(i) Change of milk into curd.
(ii) Rusting of iron

Melting of wax in (c), which on cooling, changes back to solid wax but burning candle and an extinguished candle show chemical irreversible change.

Physical changes A candle is made up of wax. We take a small candle and measure its length with scale. Now fix this candle on a tile kept on table and light it with burning matchstick. Allow the candle to burn for some time. Now we absorb melting wax comes to side of candle or table. This wax is reversible and we can used this wax.
Chemical changes The wax near the flame burns and gives new substance like carbon dioxide, carbon soot, water vapours, heat and light.

The example of each case is as follows:
(a) Heating of an iron rod
(b) Baking of chapati
(c) Formation of ice from water
(d) Formation of salt solution
(e) Mixing of cement and water