Video Transcript
The melting point of sucrose is 186
degrees Celsius. What is the expected melting point
of a slightly impure sucrose sample? (A) It should be up to a few
degrees higher than 186 degrees Celsius. (B) It should be exactly equal to
186 degrees Celsius. (C) It should be up to a few
degrees lower than 186 degrees Celsius. (D) It should be over 250 degrees
Celsius. Or (E) it should be lower than 100
degrees Celsius.
Sucrose is a sugar that has the
molecular formula C12H22O11. To answer this question, we need to
understand how impurities would affect the melting point of a sucrose sample. The melting point of a substance
depends on the forces of attraction between neighboring particles.
Let’s consider a pure sample of
sucrose, a sample which only contains sucrose molecules. Sucrose molecules experience strong
forces of attraction with one another called hydrogen bonding. 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 interactions between the molecules. In order for pure sucrose to melt,
the temperature must be raised to 186 degrees Celsius. At this temperature, sufficient
energy has been added to overcome the forces of attraction between the molecules and
the sample melt.
An impure sucrose sample would
contain particles other than sucrose molecules. These particles disrupt the orderly
lattice structure of the solid. This weakens the intermolecular
forces in the crystal lattice. Because the intermolecular forces
are weaker, less energy is required to overcome the forces of attraction. So the melting point should be
lower. In addition, because the impurities
disrupt the lattice structure in a nonuniform way, an impure substance tends to melt
over a range of temperatures rather than just one specific temperature.
In summary, when compared to a pure
solid, an impure solid will have a slightly lower melting point that occurs over a
range of temperatures. We can use this information to
determine that an impure sucrose sample melts at a temperature a few degrees below
186 degrees Celsius. Thus, answer choice (C) is the
correct answer. It should be up to a few degrees
lower than 186 degrees Celsius.
