Simple Test-tube Reactions (Oxford AQA International A Level Chemistry)

Required Practical 3: Identifying Anions & Cations

Objective

To carry out simple test-tube reactions to identify cations and anions including:

• Group 2 cations

• Ammonium ions

• Group 7 halide ions

• Hydroxide ions

• Carbonate ion

• Sulfate ions

Apparatus

• Test tubes and stopper

• Test-tube racks

• Plastic graduated dropping pipettes

• Deionised / distilled water

• Forceps

• Red litmus paper

• Blue litmus paper

• Kettle

• Water bath

• Petri dish with lid

• Filter paper

• Small spatula

• 0.1 mol dm^–3 solutions of the following:

  • Barium chloride

  • Calcium bromide

  • Magnesium chloride

  • Strontium chloride

  • Ammonium chloride

  • Magnesium sulfate

  • Potassium chloride

  • Potassium bromide

  • Potassium iodide

  • Lead nitrate

  • Nitric acid

• 0.5 mol dm^–3 solutions of the following:

  • Sodium carbonate

  • Acidified potassium dichromate(VI)

  • Hydrochloric acid

• Sodium hydroxide solution:

  • 0.4 mol dm^–3

  • 0.6 mol dm^–3

• Sulfuric acid:

  • 1.0 mol dm^–3

  • Concentrated sulfuric acid in dropping bottles (in a fume cupboard)

• Ammonia solution

  • 1.0 mol dm^–3

  • 2.0 mol dm^–3

  • Concentrated

• 0.02 mol dm^–3 calcium hydroxide solution (limewater)

• 0.05 mol dm^–3 silver nitrate solution

• Solid potassium chloride

• Solid potassium bromide

• Solid potassium iodide

Method

Tests 1 and 2: Testing for Group 2 metal cations

Using NaOH

1. Place 10 drops of 0.1 mol dm^-3 barium chloride solution in a clean test tube

2. Add 10 drops of 0.6 mol dm^–3 sodium hydroxide solution, mixing well

3. Continue to add sodium hydroxide solution, dropwise until in excess

4. Record observations

5. Repeat this test with 0.1 mol dm^-3 solutions of calcium bromide, magnesium chloride and strontium chloride

Using dilute sulfuric acid

1. Place 10 drops of 0.1 mol dm^-3 barium chloride solution in a clean test tube

2. Add 10 drops of 1.0 mol dm^-3 sulfuric acid, mixing well

3. Continue to add the sulfuric acid, dropwise until in excess

4. Record observations

5. Repeat this test with 0.1 mol dm^-3 solutions of calcium bromide, magnesium chloride and strontium chloride

Test 3: Testing for ammonium ions

1. Place 10 drops of ammonium chloride in a clean test tube

2. Add 10 drops of 0.4 mol dm^-3 sodium hydroxide solution and shake the mixture

3. Warm the mixture in the test tube gently using a water bath

4. Test the fumes released from the mixture by using forceps to hold a piece of damp red litmus paper in the mouth of the test tube

5. Record observations

Test 4: Testing for hydroxide ions in aqueous solution

1. Add 1 cm³ of 0.4 mol dm^–3 sodium hydroxide solution in a test tube with red litmus paper or universal indicator paper

2. Record observations

This approach can also be used to test for the alkaline gas, ammonia, which forms hydroxide ions when it comes into contact with water.

1. Take 5 drops of 1.0 mol dm^–3 ammonia solution and place on a filter paper inside a petri dish with lid

2. Dampen a piece of red litmus paper with deionised water and place on the other side of the petri dish

3. Replace the lid and observe for a few minutes

4. Record observations

Test 5: Testing for carbonate ions in aqueous solution

1. Add 2 cm³ of calcium hydroxide (limewater) to a test tube

2. Add 3 cm³ of 0.5 mol dm^-3 sodium carbonate solution to a separate test tube and add an equal volume of dilute hydrochloric acid

3. Immediately connect a delivery tube with the open end into the limewater test tube

4. Record observations

Test 6: Testing for sulfate ions in aqueous solution

1. Add 1 cm³ of 0.1 mol dm^-3 magnesium sulfate solution into a test tube

2. Add an equal volume of dilute hydrochloric acid

3. Add an equal volume of 0.1 mol dm^-3 barium chloride solution

4. Record observations

Test 7: Testing for halide ions in aqueous solution

1. Add 10 drops of 0.1 mol dm^-3 potassium chloride in a clean test tube

2. Add 5 drops of dilute nitric acid and shake

3. Add 10 drops of 0.05 mol dm^-3 silver nitrate solution to the resulting mixture

4. Add an excess of 2.0 mol dm^–3 ammonia solution and shake to mix thoroughly

5. Record observations

6. Repeat steps 1) to 4), but this time add an excess of concentrated ammonia solution

7. Repeat steps 1) to 6) but replacing potassium chloride with potassium bromide and then potassium iodide

Test 8: Testing for halide ions in solid salts

1. Place a small spatula of solid potassium chloride in a clean dry test tube

2. Slowly add a few drops of concentrated sulfuric acid

3. Test the gas evolved with moist blue litmus paper

4. Repeat this experiment with solid potassium bromide, but test the gas produced using a narrow strip of filter paper that has been dipped in acidified potassium dichromate solution

5. Repeat this experiment with potassium iodide, but this time test the gas produced using a narrow strip of filter paper that has been dipped in lead nitrate solution

Practical Tip

• During the test for carbonate ions (test 5) make sure that the end of the delivery tube is below the level of the limewater

• Complete the test for halide ions (test 7) in a fume cupboard as concentrated ammonia solution is corrosive

Results

• Record all of your observations in an appropriate table

• To make it easier, you can set up individual results tables for each test

Evaluation

• These chemical tests show you the expected results for the various anions and cations

• You would be expected to use results of anion and cation tests to identify complete compounds, as shown in the following worked example