Demonstration of Critical Opalesence:

Technical Details

The demonstration is described in the book The Theory of Critical Phenomena by JJ Binney, NJ Dowrick, AJ Fisher (who occupies the office next to me!) and MEJ Newman. Published by Oxford University Press in 1992 ISBN0-19-851393-3.

It involves mixing hexane (C6H14) with methanol (CH3OH) in the ratio such that ratio of the number of molecules is 665:435. Using the data for the density and molecular weight it can be shown that this corresponds to a volume ratio of 1 : 4.93.

MW

Density

Boiling Point

Refractive Index

CH3OH

32

791.3 kg m-3

64.7 C

1.3284

C6H14

86

659 kg m-3

68.7 C

1.3749

Data from Kaye and Laby.

In fact the book specifies the use of n-hexane rather than just hexane. The difference is that n-hexane is just one isomer of hexane whereas hexane contains several isomers (i.e. several "versions" of a molecule with the same chemical formula, but arranged in a different shape). This probably helps the demonstration by broadening the range of temperatures at which critical phenomena may be observed. Purists however should use n-hexane (but see P.S. below). At wholesale prices the cost of the chemicals was: 2.5 litres methanol £3.50; 2.5 litres hexane £7.50. In contrast 0.5 litres if n-hexane costs £20.

I mixed the liquids in the ratio 20 cc methanol to 98.6 cc hexane. The precision required did not seem to be too great.The photographs on this site represent my second attempt at the experiment. To observe the phenomena clearly it was important to keep the container still as it cooled.

P.S. Out of interest, I repeated the experiment later using n-hexane. The results were qualitatively indistinguishable.