The molecules and atoms of this substance in which heat has been added to, start to speed up when they begin to vibrate quicker. Hence, the space between these atoms become higher. As a result, the object expands in any phase (gas, liquid, solid). This also explains why molecules move more rapid in hot water than in cold and why powdered things dissolve quicker (like yeast or powdered milk) in hot water than in cold. Also, the mass remains the same when heat is added or removed.
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- Dotz