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 Cu2+, Fe2+, Fe3+, Al3+, Zn2+, Ca2+, and Cr3+ as well as the polyatomic NH4+ all form ionic chloride compounds, for example FeCl2 and NHCl4 that are soluble or slightly soluble in water.

The table below shows the color of these cations in aqueous solution.

IonColor in Aqueous Solution
Cu2+Blue
Fe2+Green
Fe3+Yellowish-brown
Al3+Colorless
Zn2+Colorless
Ca2+Colorless
Cr3+Dark green
NH4+Colorless

We can see that some of the cations form similarly colored solutions. For example, Al3+, Zn2+, Ca2+, and NH4+ all form colorless solutions, while the others appear different in color. However, Fe2+ and Cr3+ 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 (NaOH()aq) into an unknown solution and observe any changes. We can test further by adding excess NaOH()aq and recording any changes.
  2. Add a few drops of dilute ammonium hydroxide (NHOH()4aq) into an unknown solution and observe any changes. We can test further by adding excess NHOH()4aq and recording any changes.

There are many cations that form precipitates upon the addition of dilute NaOH()aq or dilute NHOH()4aq. 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 NaOH()aq solution. However, further confirmation often requires the sample to be tested with NHOH()4aq solutions as well.

The solutions used to test the cations, NaOH()aq and NHOH()4aq, 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 NaOH()aq were added to solutions of the ions, as well as the chemical formula for any precipitate that forms.

Notice that all the metal cations react with dilute NaOH()aq to form precipitates, but NH4+ does not.

Most of the precipitates have a unique color helping us to identify the cation. We can identify Cu2+, Fe2+, Fe3+, and Cr3+ by their unique precipitate colors. We can also identify NH4+ because it is the only one of the listed cations that does not form a precipitate. However, Al3+, Zn2+, and Ca2+ all form white precipitates, and so the test with dilute NaOH()aq does not help distinguish between these three cations. Further testing with excess NaOH()aq 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

Metalcation()+hydroxideanion()metalhydroxide()aqaqs

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

CationIonic Equation for the Reaction with Dilute NaOH()aq
Cu2+Cu()+2OH()Cu(OH)()2+2aqaqs
Fe2+Fe()+2OH()Fe(OH)()2+2aqaqs
Fe3+Fe()+3OH()Fe(OH)()3+3aqaqs
Al3+Al()+3OH()Al(OH)()3+3aqaqs
Zn2+Zn()+2OH()Zn(OH)()2+2aqaqs
Ca2+Ca()+2OH()Ca(OH)()2+2aqaqs
Cr3+Cr()+3OH()Cr(OH)()3+3aqaqs

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?

  1. Al3+
  2. Ca2+
  3. Zn2+
  4. Cr3+

Answer

Al3+, Zn2+, and Ca2+ all form white precipitates when a few drops of dilute aqueous sodium hydroxide, NaOH()aq are added. The precipitates, which form Al(OH)3, Zn(OH)2, and Ca(OH)2, are all insoluble hydroxides. These form when the cations react with the hydroxide ion from NaOH()aq: Metalcation()+Hydroxideanion()Metalhydroxide()aqaqs

The only cation of the possible answer options that does not form a white precipitate is Cr3+. When Cr3+ ions react with hydroxide ions from dilute NaOH()aq, the reaction is the following: Cr()+3OH()Cr(OH)()3+3aqaqs

The chromium(III) hydroxide precipitate has a gray-green color. So, the correct answer is option D, Cr3+.

Example 2: Recalling the Observations When a Cation Reacts with Dilute NaOH

What color precipitate is formed when a few drops of dilute NaOH are added to a salt or solution containing the following?

  1. A Fe2+ cation
    1. Gray green
    2. Red brown
    3. White
    4. Pale green
    5. Pale blue
  2. A Fe3+ cation
    1. Pale green
    2. White
    3. Pale blue
    4. Gray green
    5. Red brown

Answer

Part 1

When Fe2+ cations react with the hydroxide ion from dilute NaOH()aq, the following reaction occurs: Fe()+2OH()Fe(OH)()2+2aqaqs

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 Fe3+ cations react with the hydroxide ion from dilute NaOH()aq, the following reaction occurs: Fe()+3OH()Fe(OH)()3+3aqaqs

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, Fe2+ and Fe3+, form different hydroxides, Fe(OH)2 and Fe(OH)3, with different colors.

For some cations, the addition of excess NaOH()aq to each solution can result in changes to the precipitates. The table below shows these observations.

The chemical reaction between the precipitate and excess NaOH()aq is complex, but we can observe that some precipitates redissolve after excess NaOH()aq is added.

The results for Al3+ and Zn2+ are the same; their white precipitates redissolve in excess NaOH()aq. If we suspect that a solution contains Al3+ or Zn2+, then the NaOH()aq test will not help us identify these solutions.

However, we can observe that the precipitate of Ca(OH)2 will not redissolve upon the addition of excess NaOH()aq. We can thus conclude that, if we treat a metal cation solution with dilute NaOH()aq and a white precipitate forms, which does not redissolve upon the addition of excess NaOH()aq, then the unknown cation could be Ca2+.

When dilute NaOH()aq is added to a solution of NH4+ ions, a few small bubbles will form. The solution may need to be heated slightly to aid the reaction between NH4+ and the hydroxide ions from the NaOH()aq: NH()+OH()NH()+HO()4+32aqaqgl

The bubbles that form are bubbles of ammonia gas (NH()3g). If enough ammonia gas is formed, we may be able to detect it by its pungent odor. However, NH()3g is toxic in large quantities, so direct sniffing of the test tube is not advisable. A safer way to test for NH()3g 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 NH()3g interacts with water on moist litmus paper, it forms the alkali NHOH()4aq according to the equation below: NH()+HO()NHOH()NH()+OH()3244+glaqaqaq

This is essentially the reverse of the previous reaction.

We can also record the observations when a few drops of dilute NHOH()4aq and then excess NHOH()4aq 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 NHOH()4aq yields the same color precipitate for each cation, except for Ca2+. However, we can use both tests to identify Ca2+ in solution, as only the reaction with dilute NaOH()aq will produce a precipitate.

Only Al3+ and Zn2+ form the same color precipitate with dilute NHOH()4aq solution. We can distinguish between these two cations by treating them with excess NHOH()4aq. If the precipitate does not redissolve, the cation is Al3+. However, if it does redissolve, then the cation is Zn2+.

The equations for the formation of these precipitates are the same as those with dilute NaOH()aq. That is the following: Metalcation()+hydroxideanion()metalhydroxide()aqaqs

However, this time, the source of the hydroxide ions is not NaOH()aq, but NHOH()4aq.

We cannot test for NH4+ by adding NHOH()4aq, 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?

  1. Ammonia; it turns moist litmus paper blue
  2. Nitrogen dioxide; it turns moist litmus paper red
  3. Hydrogen; it causes a lit splint to make a loud popping noise
  4. Oxygen; it relights a glowing splint
  5. Nitrogen; it extinguishes a lit splint

Answer

When the ammonium ion (NH4+) is treated with dilute sodium hydroxide (NaOH()aq), it reacts with the hydroxide ion from NaOH()aq. The reaction is as follows: NH()+OH()NH()+HO()4+32aqaqgl

Ammonia gas (NH()3g) and water are produced and not a precipitate. The NH()3g product can be detected in several ways. Firstly, we would observe bubbles forming in the reaction solution, although this is not enough evidence for NH()3g. Secondly, we might smell a pungent odor being given off from the reaction solution, which could suggest NH()3g. 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 NH()3g, when it reacts with water on the moist litmus paper, will form the alkali ammonium hydroxide (NHOH()4aq) according to the following equation: NH()+HO()NHOH()NH()+OH()3244+glaqaqaq

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 Cr3+ cation?

  1. Orange precipitate
  2. Yellow precipitate
  3. Gray-green precipitate
  4. White precipitate
  5. Pale-blue precipitate

Answer

Cr3+ ions in solution react with hydroxide ions from dilute aqueous ammonia, which are equivalent to ammonium hydroxide (NHOH()4aq), according to the following general equation: Metalcation()+hydroxideanion()metalhydroxide()aqaqs

The specific equation for this reaction is Cr()+3OH()Cr(OH)()3+3aqaqs

An insoluble solid Cr(OH)3 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 (HS2) is bubbled through the acidified solution, and a black precipitate of copper(II) sulfide forms: CuSO()+HS()HSO()+CuS()4224aqgaqs

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: CaCl()+(NH)CO()CaCO()+2NHCl()242334aqaqsaq

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 lime water: CaCO()+2HCl()CaCl()+HO()+CO()3222saqaqlg

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 OH ions to form a MOH()s precipitate according to the equation below: Metalcation()+Hydroxideanion()Metalhydroxide()aqaqs
  • Dilute and excess solutions of NaOH()aq and NHOH()4aq can be used to identify Cu2+, Fe2+, Fe3+, Al3+, Zn2+, Ca2+, and Cr3+ as well as polyatomic NH4+.
  • The results of treatment with dilute and excess NaOH()aq are summarized below.
  • The results of treatment with dilute and excess NHOH()4aq are summarized here:

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