Critical Opalescence

What it shows:

The demonstration shows density fluctuations in liquids. These fluctuations are particularly spectacular near critical points. A binary fluid mixture of methanol (29% by weight) and cyclohexane (71%) becomes opalescent when heated up to its critical temperature (about 45˚C) ... the fluids become miscible above this temperature.

How it works:

The two fluids are sealed in a special vial, able to withstand elevated pressure. The fluids are immiscible at room temperature. When brought up to 45˚C, they become miscible and cloudy (opalescent). Above the critical temperature they appear as a uniform clear fluid. The whole process reverses when the vial is allowed to cool down.

Setting it up:

The phenomenon can be shown with an overhead projector or video projection. Gently heat the glass vial with a hair dryer.


specific gravity of methanol CH3 OH = 0.7918, molecular weight 32

specific gravity of cyclohexane C6 H12 = 0.779, molecular weight 84

The mixture is highly volatile. If not capped, the components evaporate at different rates, and the critical temperature changes. If securely capped, the pressure becomes high if you go well above the mixing temperature (so don't!).