Physics Thermometric prop.

You have not given under what condition your question is made. I have to assume that it is under a general situation.

If that is the case, it is simply the result of the Law of Conservation of Mass. Temperature change only indicates a change (increase or decrease) of energy of the object, hence it will not affect the mass of the object.

More precisely, a change of temperature only leads to a change of volume (either expansion or contraction), which in turn causes a corresponding change of density. The mass of the object, which equals to the density x volume, remains unchaged.