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Pressure in a liquid without gravity

  1. Nov 27, 2011 #1
    Say there was a container filled with liquid floating in space. What would the pressure of the liquid be? Also would it be affected by the temperature of the liquid? I would assume so since faster moving particles would hit the walls of the container with more force.

    The reason I'm asking is because with gases you have the ideal gas law where the temperature, pressure, and concentration of the gas are all related to each other. However, in liquids the only calculation for pressure in a static liquid is found by the formula for hydrostatic pressure. Therefore, a non moving liquid in space would have a pressure of 0 which is extremely counterintuitive since the molecules in the liquid would still be impacting the surface of the container.

    What really got me thinking about this was the concept of osmotic pressure. Apparently, the osmotic pressure of blood can be as high as 8atm. This osmotic pressure is affected by the temperature and concentration of the solute in a similar manner as the ideal gas law. Therefore, it's incredibly intuitive that blood would have such a high osmotic pressure since the solute concentration would usually be much higher than that of gas at SATP. Now what is unintuitive is that in order for water to reach a pressure of 8atm there must be a column of it 80m high while it's concentration (density) is much higher than that of solutes in a typical solution. Where does my misunderstanding lie?
    Last edited: Nov 27, 2011
  2. jcsd
  3. Nov 27, 2011 #2

    Andy Resnick

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    Assuming the tank kept the fluid from freezing/boiling etc, the pressure can be anything from the vapor pressure of the liquid all the way up to the bursting limit of the tank. The only thing you've removed is the hydrostatic pressure head.
  4. Nov 27, 2011 #3
    So is there no way to find how the pressure will change with temperature? Also why would the vapour pressure come into play if there is no space in the tank for it to evaporate?

    I've thought of a way to illustrate my point further with osmosis. Consider a closed cylindrical container half filled with 2L of water at room temperature. The height of the water is 1m. Above the water there is some air at 1 atm. Now there is a semipermeable membrane right through the middle separating the water into two sections with 1L on each side. This cylinder filled with water is at the surface of the earth, therefore, you can calculate pressure on the membrane as pgh+1 atm = 9.8kPa +101kPa = 110.8kPa near the bottom of the membrane (less as you go higher up the water column). Now if you were to add 0.1 mol of NaCl to one side of the water then the osmotic pressure applied by the solute to the semipermeable membrane, given by the Morse equation, would be 498kPa without even factoring in the hydrostatic pressure of the water. How is it possible for so little solute to trump the pressure of so much water and air?
    Last edited: Nov 27, 2011
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