Topic 4 of 4
Turn observations into a justified conclusion
Use independent evidence and distinguish uncertainty from absence.
O-Level 6092 (2026) / SEC G3 K324 (2027)
Turn observations into a justified conclusion
Use independent evidence and distinguish uncertainty from absence.
- Observe before adding reagents
Record colour, state and whether the sample is already cloudy.
- Test fresh portions
Keep cation, anion and gas tests independent.
- Record sequence
A few drops, excess reagent and warming may give different results.
- Infer only what the evidence supports
Use a second test when several ions fit. Negative results also eliminate possibilities.
Worked example
Identify a dissolved salt
Fresh portions of a solution give a green precipitate with NaOH, insoluble in excess, and a white precipitate with acidified barium nitrate.
- The cation result supports Fe2+, not Fe3+ which gives red-brown hydroxide.
- The acidified barium test supports SO42-.
- Combining ion charges gives FeSO4.
Iron(II) sulfate is consistent with the evidence. State the observations separately from this inferred identity.
"No change" needs context: no precipitate with ammonia may distinguish calcium from zinc, but a very dilute sample could also make a precipitate hard to see. In an unfamiliar test, report what occurred and draw a general conclusion supported by the instructions; do not invent an ion name from an unfamiliar colour.