Video Transcript
Which of the following can help in
testing the purity of a sample? (A) The sample’s boiling point
only. (B) The sample’s melting point
only. (C) The sample’s boiling point and
melting point. (D) The sample’s color. (E) The sample’s physical
state.
Purity is a measure of the extent
to which a substance is free from impurities or undesired substances. The question asks which of the
following can be used to help in testing the purity of a sample. Let’s consider two beakers, one
containing pure water and one containing salt water, which could be said to contain
salt impurities. The two substances have the same
physical appearance and physical state. So the sample’s color and the
sample’s physical state cannot help us in testing the purity of the sample.
Let’s consider the boiling point of
these two samples. Boiling point is the temperature at
which the vapor pressure of a liquid equals the surrounding pressure. For pure water at one atmosphere,
the boiling point is 100 degrees Celsius. But when salt, an impurity, is
added to water, the boiling point is no longer 100 degrees Celsius. This is because the impurities
interfere with the ability of the water molecules to escape the liquid phase, which
tends to decrease the vapor pressure. When the vapor pressure decreases
due to adding a solute, the solution will need to be heated to a higher temperature
in order for the liquid to boil. In general, the more solute that is
dissolved or the more impure the substance, the higher the boiling point.
So we now know that boiling point
can be used to help in testing the purity of a sample. But what about melting point? To help us understand how
impurities might affect the melting point of a sample, let’s examine pure sucrose
and impure sucrose. The molecules of sucrose in a pure
sample, represented by the squares in this figure, are arranged in an orderly
crystal lattice that maximizes the intermolecular forces between the molecules. In order for the sample to melt,
energy must be supplied to overcome the intermolecular forces between the
molecules. A pure sample of sucrose will melt
at a temperature of 186 degrees Celsius.
In an impure sucrose sample, the
impurities disrupt the orderly crystal lattice, weakening the intermolecular forces
in the solid. Because the intermolecular forces
are weaker and the impurities are irregularly dispersed throughout the solid, an
impure sample will have a slightly lower melting point that occurs over a range of
temperatures instead of sharply at just one value. So, the melting point of a sample
can also be used to help test the purity.
Therefore, to help in testing the
purity of a sample, we can use both the sample’s boiling point and melting point,
answer choice (C).
