Video Transcript
In this video, we will learn how to
identify a selection of aqueous negative ions based on their reactivity and the
color and solubility of their salts.
In chemistry, qualitative analysis
can be used to determine the identity of an unknown chemical. We can use chemical tests to
identify the different positive and negative ions present. During a chemical test, a chemical
reaction will take place, and we might observe a number of different changes. Heat might be given out, we may see
a color change, or a gas might be produced. We could see some bubbles in the
test tube, which is referred to as effervescence, and we might even smell a peculiar
odor. Finally, we might notice a solid
being formed inside the test tube, which we refer to as a precipitate.
Anions are negative ions and
sometimes referred to as acidic radicals. If we were to imagine a common
acid, such as sulfuric acid, with the hydrogen ions removed, we would be left with
the sulfate anion or acidic radical. When we test for acidic radicals,
we use a specific order and methodology to do so. This helps us to accurately
identify different ions. We first use a primary test, and if
the result is positive, then we follow up with a confirmatory test to verify the
primary one. We use specific reagents in a fixed
order when testing for unknown acidic radicals. And the order relates to the
stability of the different anions.
Firstly, we use dilute hydrochloric
acid, which will help us to identify carbonate, sulfite, bicarbonate, sulfide,
thiosulfate, and nitrite anions. If the unknown anion has not been
identified using dilute hydrochloric acid, we move on to the second testing reagent,
which is concentrated sulfuric acid. We use concentrated sulfuric acid
to identify halide anions and nitrate anions. If having used dilute hydrochloric
acid and concentrated sulfuric acid we still don’t know what the identity of our
unknown anion is, we finish by using aqueous barium chloride solution, which will
help us to identify sulfates and phosphates.
Let’s look at some of the
observations we would expect to see with some of these tests. We can use dilute hydrochloric acid
to test for bicarbonate, sulfide, thiosulfate, and nitrite ions. And in all these cases, the dilute
acid is added to a solid sample of the unknown salt. Bicarbonate ions react with the
acid to produce carbon dioxide and water. We can test for the carbon dioxide
gas produced by using limewater, which is a saturated solution of calcium
hydroxide. The limewater will turn cloudy or
milky in the presence of carbon dioxide.
However, carbonates also react with
dilute hydrochloric acid in the same way. So we need a confirmatory test to
tell us that this particular unknown anion is specifically a bicarbonate and not a
carbonate. For this confirmatory test, we will
use a solution of magnesium sulfate. When we mix a solution of magnesium
sulfate with a compound containing bicarbonate ions, then magnesium bicarbonate will
be present in the test tube. However, if carbonate ions are
present, they will instantly react with the magnesium ions to form the insoluble
magnesium carbonate, which would form as a white precipitate.
The second stage of this
confirmatory test is then to heat our solution. This causes the thermal
decomposition of the magnesium bicarbonate and once again the formation of an
insoluble white precipitate of magnesium carbonate. So, using this confirmatory test,
we can see the difference between a carbonate and a bicarbonate salt. Carbonate ions are present if the
white precipitate appears instantly. Bicarbonate ions are present if the
white precipitate appears only after heating.
We can also use dilute hydrochloric
acid to test for the presence of sulfide anions. The sulfide ions react with the
acid to form a gas known as hydrogen sulfide. Hydrogen sulfide gas has the
horrible smell of rotten eggs. And while that might give us a clue
while performing the test, using smell and sniffing unknown gases is incredibly
dangerous. And so smell, even if it is very
informative, is never a part of formal qualitative analysis in chemistry.
So what we use instead to test for
the hydrogen sulfide gas is a solution of lead acetate. We take a small piece of filter
paper and soak it in the solution and hold it carefully above the test tube. When the gas comes into contact
with the lead acetate, a further reaction takes place, and lead sulfide is formed on
the piece of filter paper. Lead sulfide has a characteristic
black color. The confirmatory test for sulfide
ions is the use of a solution of silver nitrate. And once again here, we form a
black substance. This time, silver sulfide, which is
a black precipitate, forms inside the test tube.
When we use hydrochloric acid to
test for the presence of thiosulfate anions, we need to look out for a yellow
precipitate. The yellow precipitate is solid
sulfur. But we also get another
strange-smelling gas generated here, which is known as sulfur dioxide. It’s important to remember here
that the sulfite anion also generates sulfur dioxide. So it’s the yellow precipitate, or
solid sulfur, that tells us in this primary test that we are dealing with a
thiosulfate anion and not a sulfite anion.
We can test the sulfur dioxide gas
coming off using filter paper that we’ve wetted with potassium dichromate. Potassium dichromate is an orange
solution and when reduced by the acidic sulfur dioxide gas turns from orange to
green. While the production of the
pale-yellow precipitate is a very good indicator when testing for thiosulfate, we
also need to perform our confirmatory test.
One of the characteristics of the
thiosulfate anion is that it is able to reduce iodine to iodide. An iodine solution commonly has a
dark-brown appearance. And when mixed with a solution
containing thiosulfate ions, the iodine is decolorized. Nitrite anions can also be detected
using dilute hydrochloric acid. During the reaction, a chemical
known as nitrous acid is produced. This acid is unstable and
decomposes, releasing nitrogen monoxide gas. In turn, this gas reacts with
oxygen in the air and produces nitrogen dioxide, which has a characteristic
orangey-brown color.
The confirmatory test for the
nitrite ion involves passing the orange-brown nitrogen dioxide through a solution of
potassium permanganate. We acidify the potassium
permanganate with concentrated sulfuric acid. And as the acidic nitrogen dioxide
gas passes through this solution, the characteristic deep-purple color of the
permanganate is lost and replaced by a pale-pink, almost colorless, solution, which
contains manganese two plus ions.
As we follow our testing
methodology, we move from testing with dilute hydrochloric acid to testing with
concentrated sulfuric acid. If having tested with both of those
acids we had still not managed to identify our unknown anion, we would move forward
to the third stage of testing and begin to use barium chloride solution. Barium chloride solution can be
used to test for phosphate and sulfate anions. The first step of this test is to
add some barium chloride solution to the unknown solution thought to contain either
phosphate or sulfate anions. If either of these two anions are
present, then a white precipitate will form, either barium phosphate or barium
sulfate.
In order to differentiate between
these two similar white precipitates, we add some dilute hydrochloric acid. When we add the acid to the white
precipitate of barium phosphate, the white precipitate will disappear. The solid barium phosphate reacts
with the acid to produce phosphoric acid and dissolve the barium ions. This reaction will not take place
with barium sulfate, and the white precipitate will remain.
The confirmatory test for phosphate
anions involves the addition of silver nitrate solution. When silver cations come into
contact with phosphate anions, a precipitate of silver phosphate is formed, which
has a yellow color. As we did before, we can dissolve
this precipitate by using either nitric acid or ammonia solution. If we use nitric acid, we form
silver nitrate and phosphoric acid. And in the case of the ammonia
solution, we form a silver complex.
Before we summarize what we’ve
learned about further tests for anions in this video, let’s take a look at a
question.
The image shows a series of tests
carried out on an unknown sodium salt, X. What formula is the unknown sodium
salt likely to have?
In this question, we are told that
an unknown sodium salt is being tested with a series of chemical tests. In the image, we see that the
sodium salt is dissolved in a solution. Based on the tests used and the
results of the tests shown in the provided image, we will need to determine the
identity and chemical formula of the sodium salt X. Moving from X to the left, silver
nitrate is added to the solution containing X. The result is a black mixture.
Let’s suppose that the salt X
contains sulfide ions. After adding silver nitrate, the
sulfide ions present in the unknown solution will react with the silver ions from
silver nitrate. The result of the reaction is the
precipitate silver sulfide, which is black in color. The result of the test with silver
nitrate suggests that X contains sulfide ions.
Let’s continue to record the
chemical reactions occurring in the remaining chemical tests. When hydrochloric acid is added to
a fresh sample of the solution containing X, a smelly gas is produced. When hydrochloric acid is added to
the solution containing sulfide ions, a chemical reaction occurs between the sulfide
ions and the hydrogen ions from the acid. The reaction results in the
formation of hydrogen sulfide gas, or H2S, which has the unpleasant odor of rotten
eggs.
According to the diagram, when
lead(II) acetate is added to the mixture that produced the gas, a different black
mixture forms. If there are still sulfide ions or
hydrogen sulfide in the mixture in the test tube, then a chemical reaction will
occur with the lead(II) acetate. The products of the reaction are
acetic acid and lead(II) sulfide, or PbS. Lead(II) sulfide is a black
precipitate that gives the mixture in the test tube its black color.
The results of the three tests
shown in the image give evidence that the sodium salt sample contains sulfide
ions. Therefore, the identity of X is the
salt sodium sulfide, which has the chemical formula Na2S. What formula is the unknown sodium
salt likely to have? Na2S.
Let’s summarize what we have
learned about the further tests for anions. Dilute hydrochloric acid,
concentrated sulfuric acid, and barium chloride solutions can be used to identify
unknown anions. Dilute hydrochloric acid is a
primary test of carbonate, sulfite, bicarbonate, sulfide, bisulfate, and nitrite
anions. Concentrated sulfuric acid is the
primary test used for halides and nitrate anions. Barium chloride is used as a
primary test to identify sulfate and phosphate anions. We use confirmatory tests to verify
the results of primary tests.
