Qualitative tests for anions and cations

The different chemical properties and reactions of various cations and anions enable you to distinguish between them using simple laboratory chemicals. It’s common to separate out these tests into cation tests (group 1, group 2 and the ammonium ion), anion tests (halides, nitrates, sulfates, etc.) and transition metal ion tests (iron(II), iron(III), copper, etc.)

A common test to distinguish group 1 and group 2 ions is the flame test, where the metal compound or its solution is heated in a roaring blue Bunsen flame. The resulting coloured flame can be used to identify the cation present. A number of experimental techniques can be used as shown in the videos below.

A non-contaminated nichrome wire can be dipped into hydrochloric acid and then a solid sample of the metal compound before being transferred to the Bunsen flame.

An alternative technique is to soak one end of a wooden splint into a solution of the metal compound as demonstrated in the first part of this video from our practical videos for 14–16 collection. From 07:55 you can look at some tests for non-metal ions (sulfate, carbonate, chloride, bromide and iodide), which are explored further down this page.

If you’re looking for something a bit more special, the following video demonstration from our Exhibition chemistry series highlights a beautiful demonstration (as well as an updated risk assessment, 2018) that can be used at the end of a lesson after students have experimented with flame tests.

One further cation that needs to be identified is the ammonium ion. The following video by Yeo Yong Kiat demonstrates the sodium hydroxide test for the ammonium ion.

Questions you could ask your students:

• Why is it important to heat the nichrome wire before dipping it in the solution?
• How does cobalt blue glass help you to visualise the potassium lilac flame?
• Why do different cations give rise to different flame colours?
• Why might flame tests be insufficient to identify some cations?
• Why does the damp red litmus paper turn blue in the test ammonia?

When it comes to identifying anions in solution, there are a lot of examples and the following videos work through some of the most common tests that can be carried out in a test tube. You can also watch the second half of the RSC Identifying ions video above to see the experiments carried out one after another.

Bromide and iodide ions give similarly coloured precipitates. It’s helpful for students to know how to ‘extend’ this test to differentiate between the two silver halides precipitates; silver bromide dissolves in concentrated ammonia solution while silver iodide does not.

• Testing for nitrate ions
• Testing for nitrite ion
• Testing for carbonate ions
• Testing for sulfate ions

The following video from Pdst will walk your students through the vast majority of the anion tests. In particular, at 7:32, you can observe a solubility test to distinguish between carbonate and hydrogencarbonate ions, both of which react with an acid to evolve carbon dioxide. You can also watch the test for phosphate ions at 12:27.

A useful way to help students conceptualise these tests is to ask them to draw simple flow diagrams with ‘yes/no’ answers that will enable them to identify a given unknown or proceed to the next experiment. For example, if the acid test for carbonate/hydrogencarbonate ions yields a negative result, there is no need to add further hydrochloric acid before adding barium chloride to test for the sulfate ion. Careful consideration of the chemicals needed for each test and the order in which they are carried out can reduce the number of steps required.

With so many chemical test options and reactants in use, it’s important that students take the idea of contamination seriously, as well as the potential for chemical spills. Discuss with students how they can set up the classroom and their desks to minimise accidents and best practice when working with bottles of chemicals and droppers. In addition, students will benefit from working in an orderly fashion and labelling test tubes in advance.

The final series of tests look to identify the presence of transition metal ions. While not all transition metals are shown in the videos below, they provide a good overview of the different tests and the particular precipitates of iron(II), iron (III) and copper, as well as aluminium in Group 3. It’s important to note that the Fe(II) hydroxide green precipitate turns orange/brown when left standing in air as it is oxidised to Fe(III).

• Transition metal ions with sodium hydroxide
• Testing metal ions with ammonia solution
• Testing iron(II) and iron (III) with carbonates
• Sodium hydroxide and ammonia test for aluminium