GCSE Chemistry Required Practical: Identifying Unknown Compounds

Key Stage 4

Meaning

Use a variety of chemical tests to identify the ions in ionic compounds.

Method

Flame Test

1. Dip the end of an Inoculation Loop into concentrated Hydrochloric Acid.
2. Place the end of the Inoculation Loop into the powdered metal compound.
3. Hold the end of the Inoculation Loop in a roaring blue Bunsen Burner flame.
4. Observe and record the colour of the flame.
5. Use the following table to identify the metal ion.

Metal Ion	Flame Colour
Lithium	Red
Sodium	Orange
Potassium	Lilac (Purple)
Calcium	Orange-red
Barium	Brown-green
Copper	Green-blue
Lead	Blue

Halide Test

The test for Halide anions begins by adding dilute Nitric Acid to remove any possible Carbonates. Then Silver Nitrate is added to form a precipitate of Silver Halide.

1. Add four spatulas of Sodium Halide to a 50ml beaker with 40ml water and stir with a glass rod to dissolve.
2. Measure roughly 2ml of Sodium Halide solution using a pipette.
3. Add the solution to a test tube.
4. Add 4 drops of 0.4M Nitric Acid to remove any Carbonates.
5. Measure roughly 2ml 0.05M Silver Nitrate solution using a clean pipette.
6. Add the Silver Nitrate to the test tube.
7. Use the following information to identify the Halide present.

\(NaCl(aq) + AgNO3(aq) \rightarrow NaNO3(aq) + AgCl(s)\)

A white precipitate is formed.

\(NaBr(aq) + AgNO3(aq) \rightarrow NaNO3(aq) + AgBr(s)\)

A cream coloured precipitate is formed.

\(NaI(aq) + AgNO3(aq) \rightarrow NaNO3(aq) + AgI(s)\)

A yellow precipitate is formed.

Sulphate Test

The test for Sulphate anions begins by adding dilute Hydrochloric Acid to remove any possible Carbonates. Then Barium Chloride is added to form a precipitate of Barium Sulphate.

1. Add four spatulas of Metal Sulphate to a 50ml beaker with 40ml water and stir with a glass rod to dissolve.
2. Measure roughly 2ml of Metal Sulphate solution using a pipette.
3. Add the solution to a test tube.
4. Add 4 drops of 0.4M Hydrochloric Acid to remove any Carbonates.
5. Measure roughly 2ml 0.1M Barium Chloride solution using a clean pipette.
6. Add the Barium Chloride solution to the test tube.
7. If a Sulphate is present a white precipitate is formed.

\(Na2SO4(aq) + BaCl2(aq) \rightarrow 2NaCl(aq) + BaSO4(s)\)

Carbonate Test

The test for carbonate anions begins by adding Hydrochloric Acid to the solution. This produces a gas which is collected and bubbled through Limewater to identify if Carbon Dioxide has been produced. If Carbon Dioxide has been produced then the Limewater will turn from transparent to cloudy.

1. Add two spatulas of Metal Carbonate solution to a test tube.
2. Measure roughly 2ml of Limewater solution using a pipette.
3. Add the Limewater to a second test tube.
4. Measure 2ml 0.4M Hydrochloric Acid solution using a clean pipette.
5. Add the Hydrochloric Acid solution to the first test tube containing the Metal Carbonate Solution.
6. Immediately attach a bung with a delivery tube to pass the gas through the Limewater in the second test tube.

\(Na2CO3(aq) + 2HCl(aq) \rightarrow 2NaCl(aq) + H2O(l) + CO2(g)\)

A gas is produced and bubbled through Limewater.

\(CO2(g) + CaO(aq) \rightarrow CaCO3(s)\)

The Calcium Carbonate makes the Limewater appear cloudy.

Unknown Compound Test

To identify an unknown ionic compound use each experiment mentioned above and record the results to compare with the expected results for each Metal Compound.