When chemists convert ethene (C2H2) to ethanol (CH3CH2OH), they have a mixture of the two gaseous substances. Why do chemists then lower the temperature of the mixture?

to collect liquid ethanol and leave ethene as a gas because ethanol has hydrogen bonds    

Explanation:

The chemist would be lesser than the temperature of the mixture as to collect the liquid ethanol and then leave ethene as a gas since the ethanol is a bond that should be hydrogen. Also -OH that available in the ethanol would be responsible for the hydrogen bonds also it is the main and significant molecular forice

So as per the given situation the above represent the answer

the answer is symmetrical : )