Critical opalescence is caused by a large change in the refractive index. In this experiment we mix hexane and methanol in the molar ratio of .435 moles to .665 moles respectively. Methanol has 40.45 ml / mole. Hexane has 130.55 ml / mole. This means that 17.60 ml of methanol and 85.51 ml of hexane will give you the .435 : .665 mole ratio. The critical opalescence will occur at 42.4 C. The easiest way to do this is to heat the mixture above 42.4 C. so that the mixture is clear and appears as one liquid. (To do this heating, set the hot plate at 225 on the dial and place the flask containing the liquids on it for several minutes until you can swirl the mixture and it remains clear.  Make sure that the stopper is loose in the flask as you do the heating and cooling otherwise there is danger of pressure breaking the flask.)   Place the flask into the clamp and focus the camera on the bottom end of the flask so that you can do a continual observation as this experiment will take up to 30 minutes to go to completion.  Let the mixture cool and observe the critical opalescence at the transition temperature. Depending on existing pressure and experimental conditions this may vary slightly from the 42.4 C reported in the literature. As the mixture continues to cool the two liquids will separate into distinct layers.

The Astronomy Connection:  Apparently, this is not only a demonstration of the opacity mechanism of Cepheid variables, but also the phase transition of the universe at recombination.