What is the difference between a dense fluid and a gas?
A dense fluid refers to a fluid that has nearly constant density as a function of pressure. The density of a gas is a strong function of pressure. An ideal gas has a density of:
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However, liquids such as water have densities that change only very slightly with pressure. The small change that is observed arises from the isothermal compressibility. This effect causes the density of water to increase by about 4% at the bottom of the deepest part of the ocean where the pressure is 800 atm. That is a huge pressure and yet the density of water is nearly the same as it is at sea level. By constrast, the density of the gas in the atmosphere is a strong function of elevation. We also derive this formula known as the barometric pressure formula. The barometric pressure formula is quite useful for many problems involving real work applications.
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