Does adding solutes to water alter its triple point? If yes, how do we find the new temperature and pressure of this point?


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    $\begingroup$ Adding things to water changes much more than you expected. It is not that the triple point will just change. No, it will cease to be a point. $\endgroup$ – Ivan Neretin Jun 13 at 14:27
  • $\begingroup$ @IvanNeretin Are you implying that when I dissolve solvents in water, it will not have a triple point at all? $\endgroup$ – NoLand'sMan Jun 13 at 14:30
  • $\begingroup$ It neither "yes" nor "no"; it is complicated. $\endgroup$ – Ivan Neretin Jun 13 at 14:56
  • $\begingroup$ @IvanNeretin Can you provide a few sources/websites/papers which are related to this question? $\endgroup$ – NoLand'sMan Jun 13 at 14:59

Triple points are typically defined as points on the phase diagram of a pure substance where three phases coexist. Their existence requires that the phase rule be observed, which is $$f=C+2-P$$ where $$\begin{align}f &: \text{number of degrees of freedom} \\ C &: \text{number of components} \\ P &: \text{number of phases}\end{align}$$ The number of degrees of freedom is the number of intensive variables that can be altered independently while remaining in a given phase.

In general, when three phases coexist, $$f=C-1$$ If the system consists of a pure substance, $f=0$, as expected (the coexistence region corresponds to a point). If you have a mixture of $C$ components (labeled $i$), you will observe a different triple point for each choice of $C-1$ independent mole fractions $\chi_i$. But for each combination of mole fractions, coexistence of three phases will still occur at regions corresponding to single points.

  • $\begingroup$ Interesting...But, I found this(chemed.chem.purdue.edu/genchem/topicreview/bp/ch15/…) website which says that there will simply be a depression in the boiling and melting points. So, the triple point will just decrease. Im confused. $\endgroup$ – NoLand'sMan Jun 18 at 15:13
  • $\begingroup$ Both (my answer and that website's explanation of freezing point depression) are correct and consistent. The explanation in the link answers your second question (I only answered the first - sorry!!). $\endgroup$ – Buck Thorn Jun 18 at 17:28
  • $\begingroup$ I would read that page very carefully, there is a lot of information crammed into the phase diagram, but if you parse it apart you should come to understand what's going on. $\endgroup$ – Buck Thorn Jun 18 at 17:30
  • $\begingroup$ How can they both be the same? According to the gibbs equation, there must be a line representing triple points. But, in the website there is only one point.. $\endgroup$ – NoLand'sMan Jun 19 at 9:20
  • $\begingroup$ Does the equation only apply to large concentrations of solute? $\endgroup$ – NoLand'sMan Jun 19 at 9:54

Yes, the triple point of the system will change. Instead of a single point specified by temperature and pressure at which multiple phases of water coexist, the addition of solutes (not solvents) will cause the conditions for 3 phases to exist to become a range of temperatures and pressures.

  • $\begingroup$ But how do we find this range of temperature and pressures? $\endgroup$ – NoLand'sMan Jun 18 at 9:25

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