In this explainer, we will learn how to identify aqueous positive ions based on their reactivity and the color and solubility of their salts.
A chemist needs to be able to identify unknown substances. This can be done by observing their characteristic properties. Physical properties such as color, physical state at room temperature, density, solubility, melting point, and electrical conductivity are all unique to different substances. Chemical properties, or how substances react, can also indicate the identity of the substance or the family to which a substance belongs.
We can use characteristic properties and reactivity to identify the presence of some cations in aqueous solution. Cations are often found with a corresponding counteranion such as chloride or nitrate ions, but we will not examine the tests for these anions in this explainer. However, some metals such as silver(I), mercury(I), and lead(II) form chloride compounds that are only sparingly soluble, and as such, this can be used as a preliminary test to identify these metals.
Other cations including , , , , , , and as well as the polyatomic cation all form ionic chloride compounds, for example and that are soluble or slightly soluble in water.
The table below shows the color of these cations in aqueous solution.
Cation | Color in Aqueous Solution |
---|---|
Blue | |
Green | |
Yellowish-brown | |
Colorless | |
Colorless | |
Colorless | |
Dark green | |
Colorless |
We can see that some of the cations form similarly colored solutions. For example, , , , and all form colorless solutions, while the others appear different in color. However, and are both green. We can see that color in aqueous solution alone is not enough to identify a cation. We require further tests in order to distinguish between them.
To help confirm the presence of a particular cation in solution we can perform chemical tests. Two such tests are the following:
- Add a few drops of dilute sodium hydroxide () into an unknown
solution and observe any changes. We can test further by adding excess
and recording any changes.
- Add a few drops of dilute ammonium hydroxide
() into an unknown
solution and observe any changes. We can test further by adding excess
and recording any changes.
There are many cations that form precipitates upon the addition of dilute or dilute . The formation (or nonformation) of a precipitate, as well as the precipitate color, can help us to identify the cation.
Some cations can be identified just from the addition of dilute solution. However, further confirmation often requires the sample to be tested with solutions as well.
The solutions used to test the cations, and , are both clear, colorless solutions, and so they will not influence the color of the cations.
The table below shows the observations we would make if a few drops of dilute were added to solutions of the cations, as well as the chemical formula for any precipitate that forms.
Notice that all the metal cations react with dilute to form precipitates, but does not.
Most of the precipitates have a unique color helping us to identify the cation. We can identify , , , and by their unique precipitate colors. We can also identify because it is the only one of the listed cations that does not form a precipitate. However, , , and all form white precipitates, and so the test with dilute does not help distinguish between these three cations. Further testing with excess might help to identify them.
The chemical reaction for the formation of these precipitates is very similar. In general, the metal cation reacts with the hydroxide anion to form a metal hydroxide.
Reaction: The Reaction of Metal Cations with Hydroxide Anions
Let us look at the specific ionic equations for each of the metal cations forming solid hydroxides. The table below shows the reactions.
Cation | Ionic Equation for the Reaction with Dilute |
---|---|
Example 1: Identifying the Cation That Forms a Colored Precipitate
Which of the following metal cations does not produce a white precipitate when a few drops of dilute aqueous sodium hydroxide are added to a salt or solution of that metal cation?
Answer
, , and all form white precipitates when a few drops of dilute aqueous sodium hydroxide, are added. The precipitates, which form , , and , are all insoluble hydroxides. These form when the cations react with the hydroxide ion from :
The only cation of the possible answer options that does not form a white precipitate is . When ions react with hydroxide ions from dilute , the reaction is the following:
The chromium(III) hydroxide precipitate has a gray-green color. So, the correct answer is option D, .
Example 2: Recalling the Observations When a Cation Reacts with Dilute NaOH
What color precipitate is formed when a few drops of dilute are added to a salt or solution containing the following?
- A cation
- Gray green
- Red brown
- White
- Pale green
- Pale blue
- A cation
- Pale green
- White
- Pale blue
- Gray green
- Red brown
Answer
Part 1
When cations react with the hydroxide ion from dilute , the following reaction occurs:
The iron(II) hydroxide precipitate that is formed is pale green in color, and so the correct answer is option D, pale green.
Part 2
When cations react with the hydroxide ion from dilute , the following reaction occurs:
The iron(III) hydroxide precipitate that is formed is a red-brown color. So, the correct answer is option E, red brown.
We can see that the two different iron ions, and , form different hydroxides, and , with different colors.
For some cations, the addition of excess to each solution can result in changes to the precipitates. The table below shows these observations.
The chemical reaction between the precipitate and excess is complex, but we can observe that some precipitates redissolve after excess is added.
The results for and are the same; their white precipitates redissolve in excess . If we suspect that a solution contains or , then the test will not help us identify these solutions.
However, we can observe that the precipitate of will not redissolve upon the addition of excess . We can thus conclude that, if we treat a metal cation solution with dilute and a white precipitate forms, which does not redissolve upon the addition of excess , then the unknown cation could be .
When dilute is added to a solution of ions, a few small bubbles will form. The solution may need to be heated slightly to aid the reaction between and the hydroxide ions from the :
The bubbles that form are bubbles of ammonia gas (). If enough ammonia gas is formed, we may be able to detect it by its pungent odor. However, is toxic in large quantities, so direct sniffing of the test tube is not advisable. A safer way to test for is to hold a piece of moist red litmus paper near the mouth of the test tube. If the litmus turns blue, then we know that the gas is forming an alkali on the litmus paper. When interacts with water on moist litmus paper, it forms the alkali according to the equation below:
This is essentially the reverse of the previous reaction.
We can also record the observations when a few drops of dilute and then excess are added to an aqueous solution of metal cations. The table below summarizes the expected results.
Notice that treatment of the metal cations with dilute yields the same color precipitate for each cation, except for . However, we can use both tests to identify in solution, as only the reaction with dilute will produce a precipitate.
Only and form the same color precipitate with dilute solution. We can distinguish between these two cations by treating them with excess . If the precipitate does not redissolve, the cation is . However, if it does redissolve, then the cation is .
The equations for the formation of these precipitates are the same as those with dilute . That is the following:
However, this time, the source of the hydroxide ions is not , but .
We cannot test for by adding , hence there is no data for this observation in the table above.
Example 3: Recalling the Correct Product Formed from the Reaction between NH4+ and OH–
When reacted with dilute aqueous sodium hydroxide and heated, the ammonium cation does not produce a precipitate. What product is formed? How could it be detected?
- Ammonia; it turns moist litmus paper blue
- Nitrogen dioxide; it turns moist litmus paper red
- Hydrogen; it causes a lit splint to make a loud popping noise
- Oxygen; it relights a glowing splint
- Nitrogen; it extinguishes a lit splint
Answer
When the ammonium ion () is treated with dilute sodium hydroxide (), it reacts with the hydroxide ion from . The reaction is as follows:
Ammonia gas () and water are produced and not a precipitate. The product can be detected in several ways. Firstly, we would observe bubbles forming in the reaction solution, although this is not enough evidence for . Secondly, we might smell a pungent odor being given off from the reaction solution, which could suggest . Lastly, we could hold a moist piece of red litmus paper near the mouth of the test tube. If it turns blue, then we know that the gas given off is ammonia. This is because , when it reacts with water on the moist litmus paper, will form the alkali ammonium hydroxide () according to the following equation:
This alkali will turn the moist red litmus paper blue. So, the correct answer is option A, ammonia, it turns moist litmus paper blue.
Example 4: Recalling the Observations When a Chromium Cation Reacts with Aqueous Ammonia
What color precipitate is formed when a few drops of aqueous ammonia are added to a salt or solution containing a cation?
- Orange precipitate
- Yellow precipitate
- Gray-green precipitate
- White precipitate
- Pale-blue precipitate
Answer
ions in solution react with hydroxide ions from dilute aqueous ammonia, which are equivalent to ammonium hydroxide (), according to the following general equation:
The specific equation for this reaction is
An insoluble solid precipitate is produced, which is gray-green in color.
The correct answer is option C, gray-green precipitate.
Less common tests also exist for detecting unknown cations. Certain cations can be precipitated in the form of sulfides from acidic solutions.
Using copper as an example, a soluble copper salt is dissolved in water and acidified using dilute acid. At this stage, hydrogen sulfide gas () is bubbled through the acidified solution, and a black precipitate of copper(II) sulfide forms:
This black copper sulfide precipitate is soluble in hot nitric acid.
An additional test for calcium cations is through the addition of ammonium carbonate. When ammonium carbonate is mixed with a soluble calcium salt, such as calcium chloride, a white precipitate of insoluble calcium carbonate is formed:
The identity of the white solid can be confirmed, as calcium carbonate reacts with dilute hydrochloric acid forming carbon dioxide, which can be identified using limewater:
The solid calcium-containing salts can also be used for flame tests, where calcium will give a brick-red color.
Let us summarize the tests that we have learned when identifying unknown cations.
Key Points
- Substances can be identified by their physical and chemical properties.
- Some metal cations can react with ions to form a precipitate according to the equation below:
- Dilute and excess solutions of and can be used to identify , , , , , , and as well as polyatomic cation .
- The results of treatment with dilute and excess are summarized below.
- The results of treatment with dilute and excess are summarized here: