Video Transcript
Which of the following statements
best describes what an impurity is? (A) A substance that can be
separated from another substance. (B) A wanted substance mixed within
a desired substance. (C) A substance finally dispersed
in another substance. (D) An unwanted substance mixed
within a desired substance. Or (E) a substance dissolved in
another substance.
To answer this question, we need to
identify which statement is the most accurate description of the term impurity. To help us better understand the
term impurity, let’s consider two beakers.
The beaker on the left contains
distilled water, so this beaker only contains water molecules. This is an example of a pure
substance, a substance that contains only one type of element or compound. The beaker on the right contains a
sample of river water. This sample contains water
molecules in addition to dissolved ions and dispersed sand and silt particles.
River water is an example of an
impure substance, a mixture of a desired substance with one or more undesired
substances. These undesired substances are
known as impurities. In the example of river water, the
water is the desired substance, and the sand, silt, and dissolved ions are the
impurities.
With this information in mind,
let’s consider the answer choices. Answer choice (A) states that an
impurity is a substance that can be separated from another substance. While it is true that sand, silt,
and dissolved ions can be separated from the water, it is also true that the water
can be separated from the sand, silt, and dissolved ions. So, statement (A) can be used to
describe both the desired substance and the impurities, so it cannot be the answer
to this question.
Statement (B) states that an
impurity is a wanted substance mixed within a desired substance. As we know that impurities are
undesired substances mixed within a desired substance, we can eliminate answer
choice (B).
Answer choices (C) and (E) are
similar. One describes impurities as finely
dispersed in a substance, while the other describes impurities as dissolved in a
substance. We can see in the example of river
water that impurities can be both dissolved, like the ions dissolved in water, and
dispersed, like the sand and silt particles dispersed in the water. So, neither of these statements is
a completely accurate description of an impurity.
This leaves us with answer choice
(D). We know that an impurity is an
undesired or unwanted substance mixed within a desired substance, so this statement
is the correct answer. Therefore the statement that best
describes an impurity is answer choice (D), an unwanted substance mixed within a
desired substance.
