To compare evidence from two analysis methods, list what each test shows, find the ions that fit both, and say how confident you are. Agreement between independent tests is the strongest kind of support.
This is the last lesson of integrated chemical reasoning, and it builds on linking structure to a stated transformation by asking you to weigh two sets of observations rather than one.
What does each method tell you?
Different tests measure different properties, so each one rules some ions in and others out.
- Flame test: some metal ions give a characteristic colour, for example lithium crimson, sodium yellow, potassium lilac, calcium orange-red and copper blue-green. Others give no colour.
- Sodium hydroxide test: a precipitate may form, and its colour and whether it dissolves in excess sodium hydroxide help to identify the ion.
These are demonstrations done by trained people with eye protection and under supervision. Here you only interpret the results.
Worked example
All results below are invented for practice. Three unlabelled solutions, S, T and U, each contain one metal ion.
| Solution | Flame test | Sodium hydroxide, a few drops | Sodium hydroxide, excess |
|---|---|---|---|
| S | blue-green | light blue precipitate | precipitate stays |
| T | orange-red | white precipitate | precipitate stays |
| U | yellow | no precipitate | no change |
Solution S: blue-green flame points to copper. A light blue precipitate that does not dissolve in excess also fits copper(II). The two tests agree, so Cu²⁺ is strongly supported.
Solution T: orange-red flame points to calcium. A white precipitate that stays in excess fits calcium and also magnesium. Magnesium gives no flame colour, and T does give one, so the flame test rules magnesium out. Ca²⁺ is the most consistent explanation. The precipitate result alone could not have decided between them.
Solution U: yellow flame points to sodium, and sodium hydroxide gives no precipitate, which fits an ion such as Na⁺. But yellow can mask other flame colours, so say sodium is likely and that a further test would increase confidence.
The mistake to watch for
Mistaken answer: “T gives a white precipitate, so it contains calcium.”
This conclusion rests on one test, and several ions give a white precipitate with sodium hydroxide. Aluminium also gives one, but it dissolves in excess. Magnesium gives one that stays.
The correct approach uses both tests: the flame colour separates calcium from magnesium, and the behaviour in excess separates both from aluminium. Name the second piece of evidence in the answer.
Check yourself
All results are invented.
1. A solution gives no flame colour. With sodium hydroxide it forms a white precipitate that dissolves when more is added. Which ion is likely, and why does the second test matter?
Show answer
Aluminium (Al³⁺) is likely, since its white precipitate dissolves in excess. Calcium and magnesium also give white precipitates, but theirs stay. The behaviour in excess is what separates aluminium from them.
2. A solution gives a lilac flame and no precipitate with sodium hydroxide. What does it probably contain, and is the evidence consistent?
Show answer
Potassium (K⁺). A lilac flame fits potassium, and potassium does not form a precipitate with sodium hydroxide. The two results agree.
3. A solution shows a faint orange-red tinge in the flame but a strong yellow flame overall. Why is this result hard to interpret?
Show answer
Sodium gives an intense yellow flame that can hide weaker colours. The orange-red may be from calcium, but you cannot be sure, so a second test is needed to confirm.
Where this leads next
You have now covered the whole route through this module. Try the mixed practice set to bring every lesson together. The scientific investigation critic is useful when you must judge whether a pair of results is strong enough to justify your conclusion.
Students often accept a single clue because it fits. A teacher in online one-to-one Co-ordinated Sciences tuition can set up pairs of results that almost agree and train you to explain why they do or do not.