Why can a liquid near its critical point become cloudy or dark?

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Diagram showing four physical states of matter: solid with tightly packed molecules, liquid with loosely packed molecules, gas with widely spaced molecules, and plasma with free electrons and nuclei, each labeled with descriptions and an arrow indicating increasing energy. — phase States of matter.

The phenomenon where a liquid becomes cloudy or dark as it approaches its critical point is known as critical opalescence.

As a substance nears this point, its compressibility becomes effectively infinite, meaning the pressure is no longer sufficient to restrain local fluctuations in density. These fluctuations grow in size until they are comparable to the wavelength of light. Because these density variations are large enough to strongly scatter light, the substance—which is typically transparent—appears cloudy, opaque, or dark brown in color.1

At the critical point itself, the distinction between the liquid and gas phases vanishes, as their densities become identical and the boundary between them disappears.21