The following experiment is described in a paper by E.S.R. Gopal of the Indian Institute of Science; you can download it for free (Resonance 5 (4), April 2000, 37-45).
We said that the critical point of many common substances is beyond the temperatures and pressures of the human habitat. BUT that doesn’t mean that there aren’t systems with critical points at comfortable temperatures and pressures. One such system is the binary liquid (“two liquid”) combination of methyl alcohol, CH3OH, and carbon disulfide, CS2. Where before, we thought about the liquid, gas, and supercritical phases of the water example, now, we think in terms of two substances either being dissolved in one another or separate. Above the critical temperature, methyl alcohol dissolves in carbon disulfide – one single phase. Below it, the two substances separate into two distinct liquids – two phases. The same kind of opalescence happens at the critical point for the same reason.
Experiment:
- In a glass beaker, combine methyl alcohol, CH3OH, with carbon disulfide, CS2, in the weight ratio of 20:80. Seal the beaker with a stopper.
- Heat the mixture in a bath of warm water to about 60 °C – well above the critical point. The methyl alcohol should dissolve in the carbon disulfide so that the mixture appears as one liquid phase.
- Slowly cool the mixture down. Close to the critical point at 36 °C, you should start to see a cloudy area appear. This is where the boundary between the two substances will eventually form.
- The cloud will get bigger and bigger until, right at the critical point, it takes up most of the space of the mixture.
- Once the temperature crosses below the critical point, two distinct regions in the mixture should begin to appear, with a dark region in the cloud separating them.
- Finally, well below the critical point, there should be two clearly distinguishable liquids, one sitting on top of the other.
- Don’t worry if the critical point isn’t exactly at 36 °C! Its location is very sensitive to impurities in the CH3OH and CS2 – even a very little bit of water in the mix can change the critical temperature by a couple of degrees Celsius.
External links:
A description of an experiment in critical opalescence for a binary fluid (different from one included in these pages):
Description of binary liquid phase transition and critical point: