As we know that in solid, molecules are held together by strong intermolecular attraction. they have definite shape and volume and also they are rigid. so when we fill them in a container they don't properly occupy the space i.e. some spaces are left unoccupied. if after converting them into powder ,we fill them in a container then also a very small spaces are left unoccupied. thus solid can't take the shape of the container properly.
In case of liquids, the molecules are not held together by strong intermolecular attraction . their molecules can move freely. they don't have fixed shape and volume. so whenever we fill them in a container they occupy the shape of the container properly without left any spaces unoccupied.
In case of gases, the molecules are held together by weak intermolecular attraction. also they don't have particular shape and volume. the molecules are far apart from each other and can move freely. so they can occupy the shape of the container in which they placed but not like a liquid do. but a liquefied gas can occupy the shape of the container properly.
Mercury is liquid in room temperature . so when it placed in a thermometer the volume of liquid doesn't affected. it remains same.
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