Video Transcript
Before a compound is analyzed using
a flame test, it is typically dissolved in hydrochloric acid. What is the main reason for
preparing the sample in this way? (A) The resulting salts are more
flammable. (B) Ions in the resulting salts
produce more intense colors due to their greater charge. (C) Treatment with hydrochloric
acid removes impurities. (D) Treatment with hydrochloric
acid displaces anions that could alter the flame color. (E) The resulting salts are more
easily vaporized.
To answer this question, we need to
understand what happens during a flame test. A flame test is a qualitative test
used in chemistry to identify a metal from its emission spectrum. A qualitative test doesn’t tell us
how much metal is present in a sample. It just gives us an idea of which
metals may be present. During a flame test, a piece of
platinum or nichrome wire that is perfectly clean is dipped into some hydrochloric
acid. Whilst the tip of the wire is
coated with hydrochloric acid, it is used to pick up a few crystals of a solid
sample. Alternatively, it may be dipped
into a solution of the sample to be tested.
The sample is then introduced into
a hot or nonluminous Bunsen burner flame. The color observed in the hot part
of the flame will depend upon the identity of the metal in the sample. In a typical metal salt, positive
metal ions occupy fixed positions in the ionic lattice. In a flame test, the metal ions in
the sample must be atomized in the hot Bunsen burner flame for the flame test to
work properly. By first treating a solid metal
salt with hydrochloric acid, the solid salt is converted into more volatile metal
chlorides. This means the metal ions in the
sample vaporize easily, which facilitates the atomization process in the hot Bunsen
burner flame. So we’ll apply this knowledge to
see which statement best suggests why the hydrochloric acid is used in the flame
test.
Statement (A) suggests that using
hydrochloric acid produces resulting salts that are more flammable. The word “flammable” suggests that
the substance can catch fire or it burns well. Although the hot Bunsen burner
flame results from the combustion of flammable gases, the metal ions in the sample
are not burned at all. The metal ions are in fact atomized
in the hot Bunsen flame so that the flame test works properly. Answer (A) is not the correct
answer.
Statement (B) suggests that ions in
the resulting salts treated with hydrochloric acid produce more intense colors
during the flame test due to their greater charge. When a metal salt is treated with
hydrochloric acid, it is converted into a metal chloride salt. However, in the metal chloride, the
metal ion has the same charge as it had originally. This statement suggests that
hydrochloric acid is increasing the charge on the metal ion. That is, it’s behaving as an
oxidizing agent. This is simply not the case. Since the charge on the metal ion
in the chloride salt does not change, this statement is not the correct answer.
Statement (C) suggests that
treatment with hydrochloric acid can remove impurities. If a sample contains impurities, it
may contain other metal salts. If other metal ions are present,
they may produce different colors in the flame test. It’s impossible to identify an
individual metal if a mixture of colors are seen in the flame test. It’s very important that there are
no impurities in the sample in a flame test. If a sample is suspected to be
impure, it must first be recrystallized to purify it. Although hydrochloric acid cleans
the metal wire between flame tests, it will not remove impurities from the
sample. This is not the correct answer.
Statement (D) suggests that
treatment with hydrochloric acid displaces anions that could alter the flame
color. Anions are negatively charged
ions. They’re most commonly produced by
nonmetals. In fact, most anions do not produce
colored flames. The excitable electrons in anions
emit light energy that is not in the visible section of the electromagnetic
spectrum. Anions can be present in a flame
test, and they won’t interfere with the colors we’re trying to observe produced by
metal ions. This statement is therefore not a
correct answer either.
Statement (E) suggests that
treatment with hydrochloric acid produces salts that are more easily vaporized in
the flame test. We’ve already seen that during a
flame test, a solid ionic salt must be vaporized. This is essential if the metal ions
are to be atomized in the hot Bunsen burner flame. This process happens easily if
chloride salts are used because they are more volatile. This simply means they turn from a
solid to a gas easily when heated strongly. Samples are treated with
hydrochloric acid to help them vaporize more easily.
Before a compound is analyzed using
a flame test, it is typically dissolved in hydrochloric acid. What is the main reason for
preparing the sample in this way? The resulting salts are more easily
vaporized is the correct answer.
