Question

why do solid expand and contract on heating and cooling.

Answer 1

Solids expand on heating because the kinetic energy increase and the inter molecular space thus increase reducing inter molecular force of attraction. an don cooling the potential energy increases making extensive inter molecular force of attraction and less inter molecular space.
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Answer 2

because in solid the intermolecular force of attraction is more and intermolucular space is less that is why they contract in case if we heat it by changing it state in liquid there intermolecular force of attraction will be less and inyermolecular space will be more so they expand