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Heating a pure substance at it's triple point

  1. Apr 27, 2014 #1
    Suppose we heat a pure substance in solid phase at a constant pressure corresponding to it's triple point data up to it's triple point temperature where all the three phases of the substance are known to exist in equilibrium.

    Will further addition of heat at the same pressure result in departure from triple point temperature?

    If not will there be transition among the three phases in equilibrium such that the relative quantities of solid,liquid and vapor phases varies with further addition of heat?

    From the PVT surface of a pure substance such as water it can be observed at an exact temperature of 273.16 K(0.01 °C) and a partial vapor pressure of 611.73 pascals (ca. 6.1173 millibars, 0.0060373 atm) water will exist as ice, liquid water and water vapor in equilibrium.At the corresponding pressure and temperature we have the triple point line along the P-V plot which suggests addition of heat results in a change in specific volume of the mixture which is only possible if a phase transition is in operation.What would happen if we go on heating the mixture along it's triple point line?
  2. jcsd
  3. Apr 27, 2014 #2

    Simon Bridge

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    It may - depending on the system.
    You would need to specify the details of the containment for example.

    If it moved off it's triple point, then you'd no longer have equilibrium behavior.
    The new behavior will correspond to the new state.

    If you keep heating the substance so that the state moves along the triple point line - the equilibrium condition remains. Compare the triple point lines to isotherms though - consider what sort of processes will be involved in moving along the line. Likely the balance of the equilibrium shifts too... a higher pressure state will involve more vapor in the container.
  4. Apr 27, 2014 #3
    Moving along the triple point line would be essentially an isobaric as well an isothermal process such that the system expands upon heating.

    Is that possible at all for a system to undergo heating on a simultaneous isobaric and isochoric process?

    The triple point line on the PVT surface clearly implies there is rise in specific volume of a three phase system at the corresponding pressure and temperature of triple point data.

    If that is so would the solid and liquid phases transform into vapor phase until the system is all vapor?
  5. Apr 27, 2014 #4

    Simon Bridge

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    In principle - but I bet it's hard to implement.
    The system would have to include different phases of the substance - in a gas, an isotherm is not an isobar.
    Basically, when there are different phases around things can get complicated.

    Changing the state would be expected to change the equilibrium concentrations - there will exist states which are entirely gasseous.
  6. Apr 28, 2014 #5
    If it's so impractical to heat a system along it's triple point line I wonder how was the change in specific volume for a three phase system recorded and plotted on the PVT surface which is basically the triple point line!

    Any thoughts on this?
  7. Apr 28, 2014 #6

    Simon Bridge

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    Real life is messy:

    1. being inconvenient is not the same as being impossible - maybe someone just wanted to make the effort? Pretty much all the standard pure processes you learn in class are difficult to do.

    2. until the line has been determined - it is impossible to deliberately heat something so it's state moves along the line. The line needs to be established by other means so you know it's there. What you do is find lots of combinations of the state variables and wait for equilibrium ... put a dot everywhere you have all three phases present and join the dots.

    You won't get a smooth line - the line of the graph you've seen is an average.
  8. Apr 29, 2014 #7
    That sounds satisfactory

    Thank Mr. Bridge:smile:
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