Video Transcript
The graph shows the heating curve
of a one-to-one mixture of two substances. During the distillation experiment
designed to separate these two substances, which substance was collected first?
In a distillation experiment,
substances in a mixture can be separated based on their boiling points. In simple distillation, the most
volatile substance, which is the substance with the lowest boiling point, is
separated by heating the mixture and then condensing and collecting the vapor in a
separate container. The mixture containing A and B is
heated in a round-bottom flask. During this time, the temperature
of the mixture increases. The substance with the lower
boiling point boils first and turns into a gas. The first flat section on the
heating curve represents this phase change. Substance A continues to boil until
all of it has turned into a gas. However, substance B remains a
liquid in the round-bottom flask because its boiling point has not yet been
reached.
The boiling point value of
substance A can be determined by following the dotted line back to the 𝑦-axis on
the graph. At this point in the experiment, we
could separate substance A from substance B by cooling the vapors of A in the
condenser. The vapors will condense into a
liquid and can be collected in the receiving flask. If we wanted liquid B to boil, we’d
have to continue to heat. The second flat portion of the
heating curve represents B boiling and turning from a liquid into a gas. The boiling point value of B can be
determined by following the dotted line back to the 𝑦-axis on the graph. However, since there are only two
substances in the mixture, after liquid A has been separated from liquid B and they
are in separate flasks, we could stop heating and end our experiment.
In conclusion, in simple
distillation, the substance with the lowest boiling point is collected first. Using the heating curve provided,
we can see that the substance with the lowest boiling point is substance A. Therefore, the substance that is
collected first during the distillation experiment is substance A.
