These two terms has been frequently used in Physical Chemistry by Atkins. He describes Raoult's Law using ideal solution while Henry's law using ideal-dilute solution.

I thought, at first they were same but Atkins in order to contrast the two laws presented graphs in which one curve was for ideal solution & another was for ideal-dilute solution. Though he wrote two different sections for these solutions, I really couldn't contrast what the difference actually is between ideal solution & ideal-dilute solution. I googled it but to vain! Can anyone please help what he is trying to say by these two terms?


1 Answer 1


Raoult's Law is for ideal solutions (all of it: solvent and solute):

Raoult's Law only works for ideal solutions. An ideal solution is defined as one which obeys Raoult's Law. "An ideal solution shows thermodynamic mixing characteristics identical to those of ideal gas mixtures [except] ideal solutions have intermolecular interactions equal to those of the pure components." (all quoted from above link)

As stated here concerning ideal-dilute solutions:

"Many liquid mixtures deviate from the ideal behaviour predicted by Raoult's law. Ideal-dilute solutions are those for which the solvent obeys Raoult's law and the solute Henry's law. The difference in behaviour is because the solvent molecules tend to be surrounded by other solvent molecules so that they behave in an almost ideal way. However, the solute molecules tend also to be surrounded by solvent molecules so that their environment and therefore their thermodynamic behaviour is very unlike that of the pure substance." (all quoted from above link)

In summary: Ideal solutions are those which obey Raoult's Law; ideal-dilute solutions are ones where the solvent obey's Raoult's Law and the solute obeys Henry's Law.

  • $\begingroup$ Doesn't in ideal solution, solute molecules get surrounded by solvent molecules? Mean to say what is actually happening when the solute is in ideal solution compared to the solute in ideal-dilute solution? Both the solutions are dilute. So, why does the difference occur? $\endgroup$
    – user5764
    Commented Jul 13, 2015 at 0:26
  • 2
    $\begingroup$ In an ideal solution, it is assumed that there are no interactions between solvent, solute, and solvent/solute molecules/atoms. The difference is in the solute: if it's ideal, then the system is ideal; if the solute is not ideal, then it's ideal-dilute. A perhaps silly example is: Ideal = helium solvent, neon solute (very low concentration). Ideal-dilute: helium solvent, oxygen solute (very low concentration). $\endgroup$ Commented Jul 13, 2015 at 1:22
  • $\begingroup$ I have understood , sir. In ideal reaction the interaction between solvent-solvent & solvent-solute is same but not in the case of ideal-dilute solution, right? Can't I use Raoult's Law still for solvent in ideal-dilute solution? $\endgroup$
    – user5764
    Commented Jul 13, 2015 at 4:52
  • 1
    $\begingroup$ Correct. And yes, you can use Raoult's Law for the solvent in the ideal-solute case. $\endgroup$ Commented Jul 13, 2015 at 12:00

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