Video Transcript
Which of the following best
describes what a precipitation reaction is? (A) The formation of a soluble salt
from the mixing of two solutions. (B) The formation of a gas from the
mixing of two solutions. (C) The formation of a covalent
compound from the mixing of two solutions. (D) The formation of water from the
mixing of two solutions. (E) The formation of an insoluble
salt, i.e., a solid, from the mixing of two solutions.
In this question, we need to recall
what a precipitation reaction is and then determine which of the given statements
best describes it. Often, when two aqueous solutions
containing dissolved ions are mixed, the resulting solution is also soluble. In these reactions all the chemical
species are soluble in water. However, sometimes when two aqueous
solutions containing dissolved ions are mixed, a reaction occurs which produces an
insoluble salt. This insoluble salt is known as a
precipitate and separates out from the solution.
From this description, we can see
that a precipitation reaction is one which produces a precipitate. In other words, it is a reaction
that produces an insoluble salt that separates out from solution.
Now that we have discussed a
precipitation reaction, let us review the answer choices. Let us take a look at answer choice
(A). In a precipitate reaction, an
insoluble salt is formed from the mixing of two aqueous solutions. Answer choice (A) is related to a
soluble salt. Therefore, we can conclude
statement (A) as incorrect.
Statement (B) and (D) describe the
formation of a gas or water from the mixing of two solutions. While one of the products from
mixing two solutions can indeed be water or a gas, such as carbon dioxide or
hydrogen, this is not the defining feature of a precipitation reaction, which is an
insoluble salt. We can therefore discount these
statements.
Statement (C) talks about the
formation of a covalent compound. Often, the precipitate produced is
an ionic compound. Some of the polyatomic ions in
solution might indeed contain covalent bonds. However, the formation of a
covalent compound is not the description of a precipitation reaction. Therefore, statement (C), the
formation of a covalent compound from the mixing of two solutions, is the incorrect
answer choice.
Finally, we have established that a
precipitate is an insoluble salt formed from the mixing of two aqueous
solutions. Therefore, we can confirm that the
statement that best describes a precipitation reaction is statement (E): the
formation of an insoluble salt, i.e., a solid, from the mixing of two solutions.
