When we heat the substance, it tends to expand because the increase in the molecule's energy creates more vibrations and movements, usually creating more distance between themselves. Similarly, it contracts when cooled.
The expansion of a substance on heating is called, the thermal expansion of that substance.
Particles - before and after applying the heat
Thermal expansion of a solid
The change in length, area or volume due to contraction or expansion is directly proportional to temperature change.
We can see an everyday example of thermal expansion in the kitchen.
Almost everyone has had the experience of trying to open a tight metal lid on a glass container, unsuccessfully.
Did you have that experience? What have you done to open it?
Most of you can open it after running hot water over the lid: it gives way and opens at last.
Experiment to understand thermal expansion in solid
When the hot water is poured over the lid, which is made of steel, having a high coefficient of expansion starts to expand.
On the other hand, glass with a low coefficient of expansion is relatively low expansion compared to the lid. It helps to open the container.