Critical Opalescence
If you look at a normally clear gas/liquid at the critical point, something interesting happens – the gas becomes milky, cloudy.
So what? Doesn’t the steam coming off of boiling water look milky?
Not the steam – that milky-ness actually comes from little water droplets – liquid water – condensing over the water – making a little cloud. This is something different.
Look in the picture of carbon dioxide above and at the critical point – can you see a boundary between the liquid and gaseous states? NO! It’s all the same.
Ask yourself...
Describe what happens in the pictures of the carbon dioxide as you lower the temperature from super(above)critical to sub(below)critical.
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What’s going on?
The full answer to this question took over a century (and a few Nobel prizes) to develop. BUT we can get a basic idea pretty quickly.