Video Transcript
Which technique could be used to
obtain pure liquid acetaldehyde, boiling point 20 degrees Celsius, from a mixture of
acetaldehyde and ethanol, boiling point 78 degrees Celsius? (A) Evaporation, (B)
centrifugation, (C) filtration (D) crystallization, (E) distillation.
To answer this question, we need to
determine which separation technique could be used to separate acetaldehyde from a
mixture of acetaldehyde and ethanol.
Acetaldehyde and ethanol are
miscible liquids. This means that when the two
liquids are combined, they form a homogeneous mixture. So, we need to determine which
technique can separate two miscible liquids.
During evaporation, the mixture is
heated in an open container. As acetaldehyde and ethanol have
different boiling points the acetaldehyde will boil first. But as the container is open, the
acetaldehyde will turn into a gas and escape into the room. So we won’t be able to obtain pure
liquid acetaldehyde using evaporation. This separation technique is
typically used to separate a soluble solid from a liquid rather than two miscible
liquids.
During centrifugation, the mixture
is spun at a high speed inside of a centrifuge. The spinning forces the more dense
substance to the bottom of the test tube. Acetaldehyde and ethanol have
similar densities, so they can’t be separated using centrifugation. This technique is typically used to
separate finely divided insoluble solids from a liquid.
During filtration, the mixture is
poured through a filter. Large solid particles will be
trapped by the filter paper, while liquids and dissolved ions will pass through into
the flask. As both acetaldehyde and ethanol
are liquids, they cannot be separated using filtration. Filtration is typically used to
separate large, insoluble solid particles from a liquid.
Crystallization is similar to
evaporation. The mixture is heated until
crystals begin to form on the edge of the dish. Then, the mixture is allowed to
cool, and the resulting crystals are collected. When acetaldehyde and ethanol are
heated, the liquids will boil and no crystals will form so they cannot be separated
using crystallization. Crystallization is typically used
to separate a soluble solid from a liquid.
During distillation, the mixture is
heated in a distilling flask. When the temperature passes the
boiling point of the more volatile liquid, this liquid will turn into a vapor. In this question, the more volatile
liquid is acetaldehyde, which has a boiling point of 20 degrees Celsius. So, when the temperature is greater
than 20 degrees Celsius, the acetaldehyde will be turned into a vapor. As heating continues the
acetaldehyde vapor rises up and out of the flask until it reaches the condenser, at
which point the vapor is cooled and converted into liquid acetaldehyde. The liquid acetaldehyde can then be
collected in a conical flask.
It’s important during this process
for the temperature to remain above the boiling point of acetaldehyde but below the
boiling point of ethanol. If the temperature passes 78
degrees Celsius, then the ethanol will also boil and the mixture will not be
separated. So, distillation is a process that
can separate miscible liquids so long as they have distinctly different boiling
points.
In conclusion, the technique the
could be used to obtain pure liquid acetaldehyde from a mixture of acetaldehyde and
ethanol is distillation, answer choice (E).
