From a BITSAT Mock Test:

Which of the following liquid pairs show positive deviation from Raoult's law?

a) Water — nitric acid
b) Benzene — methanol
c) Water — hydrochloric acid
d) Acetone — chloroform

The correct answer is b) Benzene — methanol. How to identify if the mixture of a given pair of solvents will show positive or negative deviation from Raoult's law?

  • 1
    $\begingroup$ Azeotropes cannot be expected or foreseen. It is an experimental reality. $\endgroup$
    – Maurice
    Jul 31 at 17:05
  • 1
    $\begingroup$ See this link: sharadpra.wordpress.com/2017/02/07/… $\endgroup$
    – AChem
    Jul 31 at 17:12
  • 1
    $\begingroup$ I guess I have even seen using terms +ve and -ve exclusively from users from India. But, AFAIK, always in context of charges and electrochemistry. This is the first time in Raoult law context. I may guess the meaning intuitively, but what exactly is meant by that? $\endgroup$
    – Poutnik
    Jul 31 at 17:33
  • 2
    $\begingroup$ @Maurice That's nonsense. Corner cases are hard to tell, but combinations of strongly interacting compounds (e.g. acids, negative), and compounds with strongly different polarity (ethanol/water, positive) are always aceotropic. $\endgroup$
    – Karl
    Jul 31 at 19:25
  • 1
    $\begingroup$ The key is estimation of intra and inter solvent attraction of molecules. Strong intersolvent interaction ( strong acid ionization counts here) leads do negative deviation and vice versa. Quantification of deviation to decide if azeotrope is formed cannot be easily done without advanced calculations or measurement. $\endgroup$
    – Poutnik
    Aug 1 at 8:31


Browse other questions tagged or ask your own question.