Which of the following is an effect
that impurities do not typically have on the boiling or melting point of a
substance? (A) Deviating from the reference
value for the melting or boiling point. (B) Making the melting point occur
over a larger temperature range. (C) Increasing the boiling
point. (D) Making the boiling point occur
sharply at a definite temperature. Or (E) decreasing the melting
In this question, we need to
understand how the composition of a substance affects its boiling and melting
points. If a substance contains impurities,
then it is impure. An impure substance is a
combination or mixture of two or more different substances not chemically bonded
together, whereas a pure substance contains only one type of element or
compound. An example would be distilled water
or 100 percent liquid ethanol.
We should appreciate that most pure
solids have one well-defined melting point because they have a well-ordered crystal
structure. There are similar attraction forces
throughout the lattice structures. So all parts of the lattice break
for essentially the same amount of thermal energy.
We should also understand that
these solids ordinarily have a slightly lower melting point if they contain
impurities. The impurities disrupt the regular
lattice structure of a solid. There are weaker forces of
attraction between the ions of an impure solid lattice. So we need less thermal energy to
break apart a solid if it has impurities.
Therefore, the statements in (A)
and (E) are correct. Thus, neither (A) or (E) is the
answer to this question.
Let us now consider choice (B),
making the melting point occur over a larger temperature range. We should appreciate that there are
similar attraction forces throughout the lattice of a pure solid. But this is not the case for the
lattice of an impure solid. Not all parts of an impure solid
break for the same amount of energy. So impure solid substances melt
over a range of temperatures rather than one well-defined temperature. We can apply this information to
figure out that choice (B) is not the answer to this question either.
We should acknowledge that
impurities usually cause a liquid to have a higher boiling point. Additionally, impure liquids boil
over a range of temperatures in the same way that impure solid substances melt over
a range of temperatures. We can apply all of this
information to determine that option (C) is not the answer to this question.
We can then see that choice (D) is
the answer. Impurities make boiling and melting
points occur over a range of temperatures. They do not make substances change
phase sharply at a definite temperature.
Therefore, the effect that
impurities do not typically have on the boiling or melting point of a substance is
answer choice (D), making the boiling point occur sharply at a definite