Experiment: Qualitative analysis of cations

Qualitative analysis means finding out which ions are present in a salt, without measuring how much. Identifying the cation (the positive metal ion) is a core Form 4 skill and a favourite Paper 3 practical, because the observations follow a clear, testable pattern. This guide sets out how to use sodium hydroxide solution and ammonia solution to identify a cation, the observations to expect, the confirmatory tests, and the science process skills the practical papers reward.

Aim

To identify the cation present in a given salt solution by adding sodium hydroxide solution and ammonia solution, and by carrying out confirmatory tests.

Apparatus and materials

• Test tubes and a test tube rack
• Dropper (teat pipette)
• Bunsen burner (for confirmatory heating, where needed)
• Sodium hydroxide solution
• Ammonia solution
• The unknown salt solution
• Reagents for confirmatory tests, for example dilute acids and other named reagents

Procedure

1. Put a small volume of the unknown solution into a clean test tube.
2. Add sodium hydroxide solution slowly, drop by drop, and record the colour of any precipitate that forms.
3. Continue adding sodium hydroxide solution until it is in excess, and record whether the precipitate dissolves or stays.
4. Repeat steps 1 to 3 with a fresh portion of the unknown solution, this time using ammonia solution in place of sodium hydroxide, adding it dropwise then in excess.
5. Compare the two sets of observations, because some hydroxides dissolve in excess sodium hydroxide, some in excess ammonia, and some in neither.
6. Carry out the appropriate confirmatory test for the cation you suspect, and record the result.

Expected observations

The colour of the precipitate and its behaviour in excess are the key clues. A blue precipitate that does not dissolve in excess sodium hydroxide points to copper(II). A green precipitate that turns brown on standing points to iron(II) oxidising to iron(III), while a brown precipitate points to iron(III). A white precipitate that dissolves in excess sodium hydroxide points to a cation such as aluminium, lead(II) or zinc, and the behaviour with ammonia helps to separate these. Recording exactly what is seen at each step, colour formed, and dissolved or not in excess, is what identifies the ion.

Inference and conclusion

Each cation forms a hydroxide with a characteristic colour, and each hydroxide has a characteristic solubility in excess alkali. By matching the observed colour and solubility to the known pattern, the cation can be identified, and a confirmatory test then verifies it. The conclusion names the cation present and states the observations that support it. The reasoning is that a metal ion reacts with hydroxide ions to form an insoluble metal hydroxide, whose colour and further behaviour are a reliable fingerprint for that ion.

Science process skills (Paper 3 style)

Observing and recording systematically. Record observations in a table with a row for each test (sodium hydroxide dropwise, sodium hydroxide in excess, ammonia dropwise, ammonia in excess) and a column for the colour and solubility seen.

Controlling variables. Use the same small volume of unknown solution for each test, and add each reagent the same way, so that differences in observation are due to the cation and not to the method.

Making an operational definition. Define “soluble in excess” operationally as the precipitate disappearing and the mixture becoming clear when more of the reagent is added.

Inferring and communicating. From the pattern of colours and solubilities, infer the cation, then write a conclusion that states the ion and quotes the observations that identify it.

Safety precautions

• Wear safety goggles, because sodium hydroxide and ammonia solutions are corrosive or irritant to the eyes.
• Smell ammonia only by wafting the vapour gently towards the nose, so that the concentrated fumes are not inhaled.
• Handle solutions of lead and other toxic metals with care and wash your hands afterwards, because their compounds are toxic.
• Add reagents slowly and point the test tube away from people, so that no solution is splashed onto anyone.

Common errors

• Adding the reagent too fast. Pouring in sodium hydroxide misses the colour of the precipitate that forms first; add it drop by drop and watch.
• Not testing with excess. Stopping too early misses whether the precipitate dissolves, which is often the deciding observation; always continue to excess.
• Using a dirty test tube. Traces of a previous solution can give a misleading precipitate; use a clean tube and a fresh portion each time.
• Confusing the reagents. Keep the sodium hydroxide and ammonia tests separate, because the two can give different behaviour for the same cation.
• Vague recording. “A precipitate formed” is not enough; state the colour and whether it dissolved in excess.

How our teachers use this experiment

In one-to-one SPM Chemistry lessons, taught in English from RM50 per hour, we build a clear observation table so that identifying a cation becomes a matter of matching a pattern, not guessing. Because qualitative analysis rewards precise, well-recorded observation, and it is a standard Paper 3 practical (Paper 3 is a practical test assessing science process skills) within SPM Chemistry, the habit of writing exactly what is seen, the colour, and the behaviour in excess, is where the marks are won or lost.