[mcjhn (OP)]

Does anyone know exactly how this works?

I searched the forum and there have been some mentions of this experiment but I still have a question

Is the result entirely down to changing temperature?
expansion > cools > vapour condenses > droplets form?
compression > heats > droplets vaporise?

You can do a similar experiment just with water but I was told a cloud requires smoke particles (from a match) to act as Cloud Condensation Nuclei (same job as silver iodide in cloud making machines).
(Description of experiment here > http://www.sep.org.uk/catalyst/articles/catalyst_21_4_484.pdf [nofollow])

Is it because the change in vapour pressure is greater for alcohol?

any help much appreciated


graph done by geogebra

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I think your analysis is correct.