Question Video: Identifying the Effect That Impurities Do Not Have on Boiling Points or Melting Points Chemistry

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 [E] Decreasing the melting point

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Video Transcript

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 point.

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 temperature.

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