density is the amount of stuff in an area your sampling. boiling point is the temp at which a liquid hits the laten heat of vapor and not temp change will take place on till all liquid is turned into gas. melting point is the laten heat of fusion where the liquid will not change temp on till its all a solid
for water 32 deg f is where the ice and water stop changing temp on till its all ice or all water, this is called the laten heat of fusion or the melting point.
for water 212 deg f is the boiling point where the water turns into steam and all temp change stops, this is called the laten heat of vaporization.
density is if you had a box and you wanted to fill it with lets say a vapor, one atom of the gas would not show up as a high density, but 6.02 * 10 to the 23 power (one mole) would show something.