Salt Analysis and Practical Viva: What Is Actually Asked

The order, and why it is fixed

Cation groups in sequence

The role of ammonium chloride before adding ammonium hydroxide is a standard viva question. It suppresses ionisation of the ammonium hydroxide by common ion effect, keeping hydroxide concentration low enough that only the group III hydroxides precipitate and later groups remain in solution.

Preliminary observations worth making

Before any reagent is added, a good deal can be inferred:

• Colour — a coloured salt suggests a transition metal ion, and particular colours narrow it considerably.
• Solubility in water — and whether the solution is acidic, basic or neutral.
• Action of heat — gases evolved, colour change, sublimation.
• Flame test — characteristic colours for several cations.

These take a few minutes and often make the systematic analysis much faster by suggesting where to look.

Common anion tests

The differing solubility of the silver halides in ammonia is what distinguishes them, and being asked to explain that difference is common. It reflects the differing solubility products and the stability of the silver ammine complex.

The brown ring test

Add iron(II) sulphate to the solution, then carefully pour concentrated sulphuric acid down the side so it forms a layer beneath. A brown ring appears at the junction.

Adding the acid slowly down the side is essential — the ring forms at the interface, and mixing destroys it. The brown colour is due to a nitrosyl complex of iron. Both the technique and the reason are asked.

Typical viva questions

• Why is the original solution prepared before analysis, and in what?
• Why must the solution be acidified before passing hydrogen sulphide for group II?
• What is the role of ammonium chloride in group III?
• Why is a flame test performed with a platinum or nichrome wire moistened with acid?
• Why should the precipitate be washed before further testing?
• Which anions interfere with which cation tests, and how are they removed?

Each has a short reason behind it, and the reasons repeat across questions. Learning the six or seven underlying principles covers nearly every viva question, whereas memorising individual answers does not.

Frequently asked questions

Why acidify before passing hydrogen sulphide for group II?

To suppress the ionisation of hydrogen sulphide and keep the sulphide concentration low, so only the least soluble sulphides precipitate and group IV cations remain in solution.

Why is the flame test wire cleaned with acid?

To remove residues from previous samples, which would otherwise give a false colour. Sodium contamination in particular masks other colours easily.

Why must precipitates be washed?

Because adhering mother liquor contains ions from other groups, which would give misleading results in subsequent tests.

What if two cations from different groups are present?

The systematic scheme handles it — that is what it is designed for. Each is precipitated at its own stage, provided the order is followed exactly and each precipitate is properly separated.