The particles begin to move at a higher speed. It is a physical change.
Temperature is defined as the average kinetic energy of the particles of a body. Hence, when a body is heated, the kinetic energy and consequently the speed of its particles increases.
This increase in particle speed is reversed when the system cools down. Recall that a physical change is easily reversible while a chemical change is not easily reversible.
As such, this increase of particle speed with temperature is a physical and not a chemical change.
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