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Mixture of saturated liquid and saturated vapor

  1. Jun 29, 2006 #1
    In the figure that shows saturated vapor and liquid condition under varying temperature and specific volume (constant temperature), there are 2 lines, 1 is saturated liquid line, the other is saturated vapor line. so the region in between these 2 lines is called the mixture of saturated liquid and vapor.

    There is also statement that says the temperature of a substance which below the boiling temperature (saturated vapor) is called a subcooled liquid. So, that means its a pure liquid (without existance of vapor)?
    "[URL [Broken]

    My question here is, if it is in the region of mixtures (which mean both liquid and vapor is in equilibrium), and the region of mixtures is below the saturated vapor line (boiling temperature), how could it be subcooled liquid (pure liquid) in the second statement?

    So, what is it if the substance phase is in the mixture region under certain temperature and pressure and volume? Do you all understand what I mean, me myself already so confused!!!
    Last edited by a moderator: May 2, 2017
  2. jcsd
  3. Jun 30, 2006 #2


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    It is helpful to look at a P-T diagram.

    The subcooled liquid is 'below' the saturation temperature at a given pressue. In the saturation region, there will be a part that is liquid and a part that is vapor, depending on the total enthalpy (or specific enthalpy) of the mixture. As heat is added to the mixture, the energy goes into a phase transformation at constant temperature (and pressure). Once the mixture is transformed to saturated vapor (100%), further heat (energy) addition makes the vapor superheated and the temperature increases.

    In a gravity field, the denser liquid sits below the vapor. In a flowing system, .e.g a vertical heat exchanger or steam generator, steam bubbles could mix with the liquid. There are various flow regimes according the relatively amounts of vapor and liquid and flow speed.
  4. Jun 30, 2006 #3


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    First of all, I just noticed you have URLs included in the post which you've accidentally hidden by your formatting (I've fixed them in the quote of your post). But having just noticed them, I haven't yet looked at them. I imagine this is just the standard TV diagram for the water-vapor system.

    Secondly, the subcooled liquid is the region to the left of the 2-phase region. Hence, it refers to the liquid at temperatures below the relevant point on the saturated liquid curve. Remember that boiling begins as you enter the 2-phase region from the left (ie: as you pass the sat'd liquid curve).
    Last edited by a moderator: May 2, 2017
  5. Jul 2, 2006 #4
    Still confused...

    I am still not so understand. In a situation where the temperature of liquid is 60oc, and the boiling point of that liquid is 120oC. Can I concluded that the temperature is a pure liquid (not vapor exist)? So as long as the temperature of the liquid is below the boiling point of the liquid, it is 100% liquid?

    I check the data of boiling point for H2SO4 and NaOH in the Perry Chemical Engineers' Handbook, page 2-302, there are figures to show the percentage of conc against enthalpy and temperature. For H2SO4, the 60oC is below the boiling point. For NaOH, the 60oC is at above of the saturated solvent line. What does it means? Are both of them in liquid form for that temperature or....???

    Thank you!!
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