Theory:

Thermal Expansion:
The expansion of a substance on heating is called the thermal expansion of that substance.
When we heat a substance, it expands because the increase in the molecule's energy creates more vibrations and movements, usually creating more distance between themselves. Similarly, it contracts when cooled.
 
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Particles before and after applying the heat
 
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Thermal expansion of solids
The change in length, area or volume due to contraction or expansion is directly proportional to temperature change.
Example:
We can see an everyday example of thermal expansion in the kitchen.
Almost everyone would have had the experience of trying unsuccessfully to open a tight metal lid on a glass container.
 
Did you have that experience? What did you do to open it?

Most of you would have it after running hot water over the lid. It gives way and opens at last.
 
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Activity to open a tight metal lid on a glass container

Reason:
Answer:
When hot water is poured over the lid made up of steel, having a high coefficient of expansion, it starts to expand.
 
On the other hand, glass with a low coefficient of expansion is relatively low expansion compared to the lid. Due to this, the lid expands more than the container. This helps to open the container.