Video Transcript
Which of the following substances
can be considered chemically pure? (A) Air, (B) apple juice, (C)
brass, (D) a bar of 24-karat gold, or (E) seawater.
To answer this question, we need to
identify which of the answer choices is a chemically pure substance. Scientists tend to define a pure
substance as a material that contains only one type of element or compound. Distilled water is an example of a
pure substance. We can say that distilled water is
a pure substance because it does not contain any contaminants or minerals, just H2O
molecules. In contrast, an impure substance is
a mixture of two or more different substances. Salt water is an example of an
impure substance. Salt water is an impure substance
because it is a mixture of two different substances: salt, NaCl, and water, H2O.
With this information in mind,
let’s take a look at each of the answer choices. Air is a mixture of gases including
nitrogen, oxygen, carbon dioxide, and argon, just to name a few. As air is a mixture of different
gases, it is not chemically pure and cannot be the answer to this question.
The label on a carton of 100
percent apple juice might list the only ingredient as apple juice concentrate. Even though the carton may say 100
percent and there’s only one ingredient listed, apple juice is an impure
substance. This is because apple juice
concentrate contains various different substances such as water, different sugars,
starches, vitamins, and minerals. As apple juice is not chemically
pure, it cannot be the answer to this question.
Brass is an alloy commonly used in
musical instruments that is a combination of copper and zinc elements. Brass is yet another example of an
impure substance, so it cannot be the answer to this question.
A karat is a measure of the purity
of gold. One karat indicates that one
twenty-fourth of the sample is pure gold. So a sample of 24-karat gold is 100
percent pure gold. As a bar of 24-karat gold is 100
percent gold, it is a pure substance. So answer choice (D) is likely the
answer to the question. But just to be sure, let’s take a
look at answer choice (E) seawater.
Seawater is a mixture of water,
various salts such as sodium chloride, and other substances like dissolved minerals,
particles, and atmospheric gases. Therefore, seawater is not
chemically pure and cannot be the answer to this question.
In conclusion, we have determined
that the substance that can be considered chemically pure is answer choice (D), a
bar of 24-karat gold.
