Final answer:
In a column still, the heads are collected (4) out of the top of the rectifier.
Step-by-step explanation:
In a column still, the components of a mixture are separated based on their volatilities through a process called distillation. The heads, which usually contain volatile components including impurities and undesired elements with lower boiling points, are collected at specific points based on the design of the still.
The correct answer to the question is that the heads are collected out of the top of the rectifier. The rectifier is a part of the still where further purification takes place after the initial separation in the analyzer. The heads being the most volatile compounds are drawn off the top of the rectifier where they are less dense and have been separated from the heavier components, also known as 'tails', which exit at the bottom of the still.
Do not confuse this with the collecting of gases over water in a pneumatic trough or the downward displacement of water, which are methods used in laboratory settings to collect gases, not the heads in distillation processes.