Core practical - Identifying compound (AQA GCSE Chemistry): Revision Notes

Core practical - Identifying compound

What is this practical about?

This practical teaches you how to identify unknown ionic compounds by testing for specific cations (positive ions) and anions (negative ions). You'll use different laboratory tests to work out what ions are present in your mystery compound.

Aim

The goal is to identify an ionic compound by carrying out laboratory tests for both cations and anions.

Safety first

What you'll need (apparatus)

Safety equipment:

• Eye protection and gloves

Basic equipment:

• Sample of unknown solid ionic compound
• Flame test wire loops
• Test tubes and rack
• Bunsen burner
• Distilled water
• Pipettes
• Spatula
• Delivery tube

Chemicals for testing:

• Limewater
• Dilute hydrochloric acid
• Dilute nitric acid
• Silver nitrate solution
• Barium chloride solution
• Sodium hydroxide solution

Method

Testing for cations (positive ions)

Flame tests:

1. Clean a flame test wire loop by dipping it in hydrochloric acid
2. Dip the clean wire into a sample of your compound
3. Hold the wire in the blue part of a Bunsen flame
4. Record the flame colour you observe

Precipitation tests:

1. Make a fresh solution by adding sodium hydroxide solution drop by drop
2. Watch for any precipitate (solid) that forms
3. Note the colour of the precipitate
4. Add more sodium hydroxide to see if the precipitate dissolves in excess

Testing for anions (negative ions)

Testing for carbonates:

1. Add dilute hydrochloric acid to a fresh sample of your compound
2. Bubble any gas through limewater
3. If the limewater goes milky, carbonate ions are present

Testing for halides (chloride, bromide, iodide):

1. Make a fresh solution and add dilute nitric acid
2. Add silver nitrate solution
3. Look for a precipitate and note its colour

Testing for sulphates:

1. Make a fresh solution and add dilute hydrochloric acid
2. Add barium chloride solution
3. Look for a white precipitate

Key ions you need to know

Cations (positive ions)

Group 1 ions (1+ charge):

• Lithium ($\text{Li}^+$) - red flame
• Sodium ($\text{Na}^+$) - yellow flame
• Potassium ($\text{K}^+$) - lilac flame

Group 2 ions (2+ charge):

• Calcium ($\text{Ca}^{2+}$) - red flame
• Magnesium ($\text{Mg}^{2+}$) - no flame colour

Group 3 ions (3+ charge):

• Aluminium ($\text{Al}^{3+}$) - white precipitate that dissolves in excess sodium hydroxide

Transition metals:

• Copper(II) ($\text{Cu}^{2+}$) - blue precipitate, green flame
• Iron(II) ($\text{Fe}^{2+}$) - green precipitate
• Iron(III) ($\text{Fe}^{3+}$) - brown precipitate

Anions (negative ions)

Group 7 ions (halides):

• Chloride ($\text{Cl}^-$) - white precipitate with silver nitrate
• Bromide ($\text{Br}^-$) - cream precipitate with silver nitrate
• Iodide ($\text{I}^-$) - yellow precipitate with silver nitrate

Compound ions:

• Carbonate ($\text{CO}_3^{2-}$) - fizzes with acid, makes limewater milky
• Sulphate ($\text{SO}_4^{2-}$) - white precipitate with barium chloride

How to work out your compound

1. Do the flame test first to identify any Group 1 or Group 2 metals
2. Use sodium hydroxide tests to confirm metal ions and identify transition metals
3. Test for anions using the gas tests and precipitation tests
4. Combine your results to work out the formula of your compound