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Irreversible change
The changes that cannot be reversed to form its original shape and size is termed as irreversible change.
Example:
Burning of a candle, rusting of iron, and burning of paper.
This type of change is permanent and cannot be undone by any means. Sometimes, a new substance is formed in this type of change.
1. Bursting a balloon:
If a balloon is blown to its full size and pricked with the pointed tip of a pencil or needle, it cannot regain its original form.
2. Cutting a paper:
In a sheet of paper, the shape of an aeroplane is drawn and cut along its outline.
3. Baking a roti:
When a dough is baked into a roti, it cannot be changed into a dough again. These changes cannot be reversed.
4. Pot making:
A potter makes a pot using a lump of clay. The shaped pot can be rolled again into a lump which is a reversible process. But when the same pot is baked in an oven, it cannot be shaped into a lump of clay again. So, this process becomes irreversible.
Chemical change
Example:
Burning of matchstick, cooking of vegetables, and formation of curd from milk.
Expansion and contraction
When a substance is heated, it becomes larger in size. This process, when an object expands on heating, is known as expansion. In general, all the metals expands on heating.
When a substance is cooled, it shrinks in size. This process, when an object contracts on cooling, is known as contraction.
A tool with the iron blade fixed to the wooden handle is used to dig the soil. In all these tools, the ring of the blade will be smaller in size than the wooden handle. The blade is fixed to the handle by heating the iron ring to expand. It is then fixed to the handle and then cooled down to contract so that it fits properly.
With the same technique, the metal rim is fixed on a wooden wheel of a cart. The rim is fixed onto the wheel by heating it. After fixing the rim to the wheel, cold water is poured on the rim to contract so that it fits tightly to the wheel.
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