An unknown salt needs both of its ions identified
- Every salt contains a positive ion and a negative ion.
- Naming the salt means identifying both, not just one.
- The tests are chosen so that each gives a unique result.
- Results are recorded as observations first, then interpreted.
- The name of the salt is the conclusion, put together at the end.
Key Idea
The two halves of the answer are found by separate tests and combined afterwards.
A workable order for the tests
- Start with a flame test on the solid, which points to the metal ion.
- Dissolve a fresh sample and add sodium hydroxide solution, then add it in excess.
- Those two together usually settle the positive ion.
- For the negative ion, add dilute acid first to check for a carbonate.
- Then test a fresh portion with barium chloride or with silver nitrate as appropriate.
Practical
- Flame tests: clean the wire in dilute hydrochloric acid and hold it in a blue flame until it gives no colour, then dip it in the solid and return it to the edge of the flame.
- Cation tests: add sodium hydroxide solution to the solution of the salt a little at a time, noting the colour of any precipitate, then continue to excess to see whether it dissolves.
- Ammonium: warm the mixture gently and hold damp red litmus paper at the mouth of the tube.
- Sulfate: add dilute hydrochloric acid first, then barium chloride solution, and a white precipitate confirms a sulfate.
- Halides: add dilute nitric acid first, then silver nitrate solution, and white, cream or yellow identifies chloride, bromide or iodide.
- A fresh portion is used for every test, and the acid always goes in before the barium chloride or the silver nitrate.
- Avoid contamination: use clean apparatus and a fresh sample every time, since a trace of the previous test ruins the next.
Combining the results
- Each observation narrows the possibilities rather than giving the answer outright.
- A white precipitate with sodium hydroxide leaves two candidates until excess is added.
- A result that rules an ion out is as useful as one that rules it in.
- The two ions are then written together with the correct charges.
- The formula follows once the charges are balanced.
Example
- Lilac flame, white precipitate with silver nitrate: potassium chloride.
- Green precipitate with sodium hydroxide, fizzing with acid: iron(II) carbonate.
- No precipitate but ammonia on warming, white precipitate with barium chloride: ammonium sulfate.
Where an identification goes wrong
- A contaminated wire or tube carries an ion from the previous test.
- Skipping the excess step leaves aluminium and calcium unseparated.
- Skipping the acid step lets a carbonate give a false precipitate.
- Reading a colour in poor light, or against a coloured background, misleads.
- Stating a conclusion without the observation behind it leaves the answer unsupported.
Self review
- Why must both ions in a salt be identified?
- What order of tests would you use on an unknown solid?
- A sample gives a lilac flame and a yellow precipitate with silver nitrate. Name the salt.
- Why is a fresh portion used for each test?
- Give two mistakes that lead to a wrong identification.
