Identifying Ions » Required Practical: Identifying Ions

The aim of the practical

In this practical you are given an unknown single ionic compound. It contains one positive ion and one negative ion. Your job is to work out which two ions it contains, using the tests from the last three lessons. You do not learn any new test here. You learn how to put the tests together into a sensible plan.

The ions you can be asked about are the ones from the flame tests through to the sulfate test:

This practical covers two apparatus and techniques skills. The first is the safe use of a Bunsen burner. The second is the use of the right test reagents and techniques to identify unknown samples, including gas tests, flame tests and precipitation reactions.

Equipment

• The unknown solid, and a small amount of it dissolved in distilled water to make a solution
• Bunsen burner, heat-proof mat, and a clean flame test wire or loop
• Test tubes, a test tube rack, dropping pipettes and a spatula
• Sodium hydroxide solution, dilute hydrochloric acid, dilute nitric acid, silver nitrate solution, barium chloride solution and limewater
• Safety glasses

Method: finding the positive ion

1. Put on safety glasses and tie back long hair. Set up the Bunsen burner on the heat-proof mat.
2. Do a flame test on a small amount of the solid, using the method from the Flame Tests lesson. Use the blue flame and write down the colour.
3. Dissolve a small amount of the solid in distilled water in a test tube.
4. Add a few drops of sodium hydroxide solution. Write down whether a precipitate forms and what colour it is.
5. If the precipitate is white, add more sodium hydroxide solution until it is in excess. Write down whether the precipitate dissolves.

If the solid does not dissolve in water, do the flame test and the acid and limewater test on the solid itself, because those do not need a solution.

Method: finding the negative ion

1. Put a little of the solid, or its solution, in a clean test tube and add dilute hydrochloric acid. If it fizzes, bubble the gas through limewater. Limewater turning milky means a carbonate.
2. For the halide test, add dilute nitric acid to a fresh sample of the solution, then add silver nitrate solution. Write down the colour of any precipitate.
3. For the sulfate test, add dilute hydrochloric acid to a fresh sample of the solution, then add barium chloride solution. A white precipitate means a sulfate.

Use a clean test tube for each test, so that one test cannot spoil the next. Record every result in a table as you go.

Planning the order of the tests

You can do the tests in any order, but a good plan saves time and stops mistakes.

• Look at the solid first. A coloured solid or solution gives you a clue. Blue is a hint of copper(II), for example. It is only a hint, so confirm it with a test.
• Use a fresh sample for every test. Never pour one test back into the main sample.
• Test for carbonate before the halide and sulfate tests. The acid reacts with any carbonate ions, so they cannot give a misleading precipitate with silver nitrate or barium chloride.
• Use the right acid. Use nitric acid with silver nitrate, and hydrochloric acid with barium chloride. Adding hydrochloric acid to the silver nitrate test would put chloride ions in and give a white precipitate you did not earn.
• Make the results fit together. Your answer must be a sensible compound made of one positive ion and one negative ion. If your tests suggest two different metal ions, something has gone wrong.

Safety

Making your results reliable

A good scientist repeats a test if the result is unclear. A pale precipitate can be hard to see, so hold the test tube against a white background. If you are unsure, compare your result with a known sample tested the same way. Use equal small amounts each time so that the tests can be compared.

Exam-style question

A student is given a white solid, Z. The student carries out tests on Z.

Flame test: orange-red flame. Z dissolved in water, then sodium hydroxide solution added: white precipitate that does not dissolve in excess. Z solution with dilute nitric acid then silver nitrate solution: white precipitate.

(a) Name the compound Z. (2 marks) (b) Describe how the student should carry out the flame test safely and correctly. (3 marks) (c) The student wants to test another sample for a sulfate ion. Name the two reagents needed. (2 marks)