All substances are composed of atoms that are in constant motion. When a substance is heated, the constituent atoms gain kinetic energy and are able to vibrate faster. This causes an increase in the inter-atomic spacing of these atoms. What this means is that on heating, substances expand to some extent. This is exactly what we observe when we melt a solid or vaporize a liquid.
When a substance cools, the exact opposite phenomenon is observed. The atoms lose kinetic energy when cooled and hence vibrate slowly. This causes a decrease in inter-atomic spacing and hence materials contract when cooled. This is what is observed when we cool a liquid and solidify it. Thus, the vibration of atoms is a function of whether the heat is added to or removed from the substance.
Whether heating or cooling, the total mass of the substance remains the same.
Hope this helps.
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