Video Transcript
In this video, we’re going to learn
how to identify a few different positively charged aqueous ions based on the colors
of their solutions and how they react with sodium hydroxide and ammonium hydroxide
solutions. We’ll also look at a special test
just for the ammonium cation. We’ll only be looking at some of
the most common ions you might find in solution in the lab. There are, of course, many more
cations than this, but this selection shows an interesting range of responses. If you use your detective skills,
you may be able to figure out which one of these there is in a particular
solution.
Now, let’s meet the ions. First, we have the aluminum ion
with a charge of three plus. Next, we have the calcium ion with
a charge of two plus. Then, there’s the chromium three
ion with a charge of three plus and two different types of iron ion: iron two and
iron three, with charges of two plus and three plus, respectively. Next, the copper two ion with a
charge of two plus and the last of our metal cations zinc two plus. We’ll also look at the ammonium
ion, NH4+. Of course, to have any of these in
solution, we must be dealing with a dissolved ionic compound. This means if we have some of these
positively charged cations, we’re going to need some anions, which are negatively
charged, for instance, chloride or nitrate anions.
For this video, I’m going to assume
we’re dealing with the chloride salts of all these cations. The chlorides are all soluble in
water or at least slightly soluble. We’ll start off by looking at each
ion individually and then we’ll compare them. But before that, let’s have a look
at the tests. The tests we’ll be using are the
color of the solution of the ions, although we’ll be focusing more on the results of
the chemical tests. Our first chemical test will be
treatment with sodium hydroxide solution, first in dilute form and then to
excess. The metal cations we’ll be looking
at will produce precipitates when treated with dilute sodium hydroxide solution. Some of these precipitates will
then disappear if treated with an excess. And our second chemical test
involves treatment with first dilute ammonium hydroxide and then an excess.
Some of the ions respond
differently to ammonium hydroxide solution than sodium hydroxide solution, allowing
us to distinguish one from another. There will also be a special test
for the ammonium ion, which we’ll come to later. In the lab, you may come across
sodium hydroxide in various forms, for instance, as a white powder or pellets or as
a clear, colorless solution. Ammonium hydroxide looks like
this. It’s also a clear, colorless
solution. If you add ammonia, NH3, to water,
you’ll produce ammonium hydroxide. Sometimes you’ll see ammonia
solution, ammonium hydroxide, or ammonium hydroxide solution; these all refer to the
same thing.
Meanwhile, the term dilute is a
relative term. However, with sodium hydroxide
solution or ammonium hydroxide solution, you might be looking at a concentration of
about 0.1 molar, while a concentrated solution may have a concentration of one or
more molar. If we used a concentrated solution
rather than a dilute solution, we might jump directly to the results of the excess
test. Nonetheless, be wary that these
concentrations are just guidelines. The results of these tests will
depend on the amounts added and the concentration of cation in solution.
Now, it’s time to move on to our
first candidate. A solution of aluminum chloride
will be clear and colorless. If we have a solution that contains
Al3+ ions and we add dilute sodium hydroxide solution, we’ll produce a white
precipitate of AlOH3, aluminum hydroxide. But something slightly different
happens when we add an excess of a sodium hydroxide. If there’s enough sodium hydroxide
in the solution, the aluminum hydroxide precipitate will react further and
disappear. In this video, we won’t be looking
at the nature of the products involved when precipitates redissolve. What we’re focusing on here is the
pattern of precipitates and their color.
Now let’s start with a fresh test
tube of our solution and do the next test. If we add a dilute solution of
ammonium hydroxide to a solution containing Al3+ ions, we’ll form the same white
precipitate of aluminum hydroxide we formed with sodium hydroxide. Just like a sodium hydroxide
solution, ammonium hydroxide acts as a source of the hydroxide ion. However, if you add an excess of
ammonium hydroxide or concentrated ammonium hydroxide, the precipitate will not
redissolve.
Again, we’re not going to go into
the reasons for this; we just have to remember it. In both chemical tests, hydroxide
ions have been added to the aluminum ions, forming a precipitate of aluminum
hydroxide. The hydroxide anion only has a
charge of one minus, so we need three of them to counterbalance the aluminum’s three
plus charge. So here is our balanced ionic
equation, and we need to add in state symbols, taking care to include a solid symbol
for our precipitate.
Now we’ve done one cation, we can
go through the others more quickly. Here is everything we need to fill
in for the next ion, which is calcium two plus. A fresh solution of calcium
chloride is clear and colorless. Treating a solution of calcium two
plus ions with dilute sodium hydroxide will produce a white precipitate of calcium
hydroxide. Adding an excess will not affect
the precipitate; it will not redissolve. On the other hand, adding ammonium
hydroxide solution to a solution of calcium two plus ions will not produce a
precipitate in the first place. Adding an excess will have no extra
effect. Unlike aluminum ions, calcium ions
only have a charge of two plus, so we only need two hydroxide ions to balance the
charges.
Next up is chromium three. A solution of chromium three
chloride will be a dark green, although it’s only slightly soluble. Adding dilute sodium hydroxide
solution to a solution of chromium three plus ions will produce a gray-green
precipitate of chromium three hydroxide. Adding an excess of sodium
hydroxide solution will cause the gray-green precipitate to disappear. If we use dilute ammonium hydroxide
instead, we’ll produce the same gray-green precipitate of chromium three
hydroxide. But adding an excess will have no
effect and the precipitate will remain. Since chromium three ions have a
charge of three plus, we end up with a formula of CrOH3 for the hydroxide.
Next, we have iron two, Fe2+. Solutions of iron two chloride are
green. Adding dilute sodium hydroxide
solution to a solution of Fe2+ will produce a pale green precipitate of iron two
hydroxide but adding an excess will have no further effect. Adding dilute ammonium hydroxide
solution produces the same pale green precipitate, with an excess also having no
further effect. As with calcium, we’ll only need
two hydroxide ions per every two plus ion to balance the charges. The next candidate Fe3+ exhibits
very different colors to Fe2+, so pay careful attention to the differences.
Instead of being green, a solution
of iron three chloride is yellow or brown. The result of a test with dilute
sodium hydroxide solution is a red-brown precipitate of iron three hydroxide, with
an excess producing no further effect. Adding dilute ammonium hydroxide
produces the same red-brown precipitate of iron three hydroxide. And an excess has no further
effect. So we see the same pattern of
precipitations with iron two and iron three but very different colors. As with aluminum and chromium
three, we need three hydroxide ions to balance the charge of Fe3+, giving us
FeOH3.
Next, we’ll look at copper two. Solutions of copper two chloride
are blue. Adding dilute sodium hydroxide
solution to a solution of copper two plus will produce a pale blue precipitate of
copper two hydroxide. And an excess will have no further
effect. Adding dilute ammonium hydroxide
instead will produce the same pale blue precipitate of copper two hydroxide. However, in an excess of ammonium
hydroxide, the precipitate reacts, dissolves, and the solution turns a darker
blue. Our ionic equation for the
production of the initial precipitate is simply Cu2+ plus 2OH− react to form
CuOH2. Again, we’re not going to focus on
the nature of the product when the precipitate dissolves.
Next up, we have the last of our
metal cations. Solutions of zinc chloride are
colorless. Adding a dilute solution of sodium
hydroxide to a solution of Zn2+ ions will produce a white precipitate of zinc
hydroxide. With excess sodium hydroxide
solution, the precipitate will react and dissolve. We get exactly the same result with
ammonium hydroxide solution and excess ammonium hydroxide solution, with the white
precipitate of zinc hydroxide dissolving in excess ammonium hydroxide. And in our balanced ionic equation,
the zinc two plus ions and OH− ions are in a ratio of one to two.
Now let’s move on to the only
nonmetal cation in this video. Ammonium chloride forms colorless
solutions. We can begin to detect ammonium
ions by adding sodium hydroxide either in solid or solution form. What happens is essentially an acid
base reaction, where the acidic ammonium ion reacts with the basic hydroxide ion
producing ammonia gas and liquid water. You may see bubbles produced, but
ammonia doesn’t have a color, so how do we know its ammonia? The most obvious way is via the
smell because the smell of ammonia is very distinctive. However, if you have the option,
you should not inhale ammonia. Instead, the best option is to
check whether the gas is basic using wet, red litmus paper. If the gas is ammonia, the ammonia
will dissolve into the water, turning the red litmus paper blue. In some cases, you may need to
provide some heat to produce enough ammonia for the test.
Now that’s all the ions we’re going
to look at in this video. Let’s look at them all at once. Here’re the cations and here’re the
tests; I’ve isolated ammonium for now. If we get a precipitate for a
particular test, we’ll put a tick; otherwise, we’ll put a cross. For all the metal ions in this
video, we get a precipitate when we treat their solutions with dilute sodium
hydroxide solution. For aluminum, calcium, and zinc,
the precipitate is white, while the precipitates for chromium three, iron two, iron
three, and copper two are all different colors. For the aluminum, chromium three,
and zinc ions, adding an excess of sodium hydroxide will cause the precipitate to
dissolve. For the rest, adding more sodium
hydroxide solution does not affect the precipitates.
If instead of using sodium
hydroxide solution we use ammonium hydroxide solution, we get exactly the same
results for all the ions with dilute ammonium hydroxide apart from with calcium,
where we don’t get a precipitate at all. And there’s still no precipitate if
we add an excess. For copper two and zinc ions, when
we add an excess of ammonium hydroxide, the precipitate will react and
redissolve. In the case of the copper two ions,
the solution will turn a much darker blue than before. For the remaining metal ions,
adding an excess ammonium hydroxide will have no further effect on the
precipitate. And finally, for the ammonium ion,
all we need to do is add sodium hydroxide and a little heat. We can then treat any gas given off
with wet, red litmus paper, which should turn blue. After all that, it’s time for some
practice.
Which of the following metal
cations does not produce a precipitate when a few drops of aqueous ammonia are added
to a salt or solution of that metal cation? (A) Zn2+, (B) Al3+, (C) Ca2+, (D)
Cr3+, or (E) Cu2+.
The question describes these five
options as metal cations. We’re dealing with cations of zinc,
aluminum, calcium, chromium, and copper. And we’re being asked which of
these metal cations as a salt or in solution would not respond to aqueous ammonia
and produce a precipitate. The formula for ammonia is NH3. Ammonia can react with water to
produce ammonium hydroxide, a good source of the hydroxide ion. The question doesn’t tell us
directly if the ammonium hydroxide is dilute, to an excess, or concentrated. But since it’s only a few drops, we
can safely assume it’s dilute.
The question is, what is going to
happen when we treat each of these ions with dilute aqueous ammonia? Zn2+ ions will react to produce a
white precipitate of zinc hydroxide, and we get a similar reaction with Al3+ ions,
producing aluminum hydroxide. However, calcium two plus ions do
not react with aqueous ammonium hydroxide no matter the concentration. There isn’t an easy way to
understand why; you’ll just need to remember it. Meanwhile, chromium three ions do
react, forming a gray-green precipitate of chromium three hydroxide, while the
precipitate we get from copper two plus is pale blue copper two hydroxide.
Now, there is one possible point of
confusion with this question if we’re not quite sure if we’re dealing with dilute or
excess ammonium hydroxide. In excess ammonium hydroxide, the
precipitates formed from Zn2+ and Cu2+ ions dissolve. But since we are only dealing with
a few drops, we can be fairly confident that we aren’t dealing with dilute
conditions. Therefore, out of the five options,
the only metal cation that does not produce precipitate when treated with a few
drops of aqueous ammonia is calcium two plus.
Now, let’s finish up with the key
points. Different cations can be detected
based on the precipitates they do or do not form. And these are the ions and tests
we’ve looked at. These are the colors of the
precipitates we get when we treat each of these ions with dilute sodium hydroxide
solution. If we treat these with excess
sodium hydroxide solution, the precipitates for aluminum three plus, chromium three
plus, and zinc two plus will redissolve. If instead of using dilute sodium
hydroxide we use dilute ammonium hydroxide instead, otherwise known as aqueous
ammonia, we get exactly the same precipitates apart from calcium, where we don’t get
a precipitate at all.
If we continue to treat with
ammonium hydroxide to the point of excess, the precipitates for copper two plus and
zinc two plus redissolve. The rest stay as they were. And the last cation, ammonium, can
be detected by treatment with sodium hydroxide and potentially a little heat. We can smell the ammonia gas given
off or detected using wet, red litmus paper, which should turn blue.
