Hey guys!
I know the melt of a rock is usually less dense than the actual solid rock but I can't wrap my head around why that is(and yes, I know ice is an exception to this). I was thinking that the more molten it is, the hotter it will be so the molecules will expand and if we have expansion of molecules perhaps we have a bigger volume. If there is a bigger volume then the formula for density (m/v) will then be a smaller number thus giving me a smaller density. Is this correct or is it a wrong way to think about it?
I know the melt of a rock is usually less dense than the actual solid rock but I can't wrap my head around why that is(and yes, I know ice is an exception to this). I was thinking that the more molten it is, the hotter it will be so the molecules will expand and if we have expansion of molecules perhaps we have a bigger volume. If there is a bigger volume then the formula for density (m/v) will then be a smaller number thus giving me a smaller density. Is this correct or is it a wrong way to think about it?
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You're almost right. The molecules don't expand, but their added kinetic energy in the molten state means that there will be more distance between the molecules.
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Density is defined by the formula D=M/V.
Almost all substances (exceptions are water in the area of 4°C and antimony) expand when heated. The mass does not change with temperature as the same number of molecules are still present, but the atoms inside the substance become more energised therefore needing more space to move. They push the outside boundaries making the substance expand. Molecular movement or vibration is a fundamental result that occurs when energy is absorbed by an atom. The energy that is absorbed cannot just disappear; it causes the atom to increase its movement or vibration, and we perceive this as increased volume.
Therefore if you increase the volume and the mass remains constant, the density will decrease.
Almost all substances (exceptions are water in the area of 4°C and antimony) expand when heated. The mass does not change with temperature as the same number of molecules are still present, but the atoms inside the substance become more energised therefore needing more space to move. They push the outside boundaries making the substance expand. Molecular movement or vibration is a fundamental result that occurs when energy is absorbed by an atom. The energy that is absorbed cannot just disappear; it causes the atom to increase its movement or vibration, and we perceive this as increased volume.
Therefore if you increase the volume and the mass remains constant, the density will decrease.