Carrying out this test, students can realize how heating a liquid leads to the phase transition from liquid state to gas; then the vapours provokes the opposite phase transition: condensation. These two processes are used to separate different substances in the distillation technique: a mixture of substances with different boiling points ins poured into the still to obtain the separation. In this case the substance with the higher boiling point is the first one passing to gas state; then it will be recovered by condensation in the large test tube.
Understanding the different boiling point versus the substance
Separating two different substances by distillation