Lesson Explainer: Tests for Cations Chemistry

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.

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:

1. 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.

   

2. 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.

Let us look at the specific ionic equations for each of the metal cations forming solid hydroxides. The table below shows the reactions.

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.

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.