Observation first
Write exactly what changes: colour, gas, precipitate, pH or no visible change. Do not write the conclusion in the observation column.
Chemistry • Senior secondary • Evidence interpretation
Organic Chemistry Identification Tests Guide
The goal is not to memorise colours. The goal is to move carefully from test → observation → chemical inference → remaining alternatives. One observation rarely proves the complete identity of an unknown.
The evidence chain
1. Test supplied: What chemical probe or measurement is being used?
2. Observation: What was actually seen or measured?
3. Inference: Which structural feature is consistent with that result?
4. Alternative: What other compound class could still fit the evidence?
5. Next evidence: What independent test or structural information would distinguish the remaining possibilities?
Common interpretation patterns
Kaiako note: These rows describe commonly taught observation patterns only. They deliberately omit concentrations, quantities, preparation and operational steps.
| Test / supplied evidence | Typical observation | What it supports | What it does NOT prove alone |
|---|---|---|---|
| Bromine water supplied to test for C=C | orange/brown colour disappears | An alkene / carbon–carbon double bond is consistent with the observation. | The complete carbon skeleton or exact alkene isomer. |
| Carbonate or hydrogencarbonate test supplied for an acidic organic sample | effervescence / gas produced | A carboxylic acid is consistent when the gas behaviour and other evidence agree. | Which particular carboxylic acid is present. |
| Acidified dichromate oxidation evidence supplied | orange → green for an oxidisable sample | A primary or secondary alcohol can be consistent with oxidation under the stated conditions. | Primary versus secondary alcohol by colour change alone; both may give the same visible change. |
| Acidified dichromate oxidation evidence supplied | no visible orange → green change under the stated test conditions | A tertiary alcohol may be consistent if the sample is known to be an alcohol and other evidence supports it. | That the unknown must be a tertiary alcohol; many substances can give no visible change. |
| pH / indicator evidence | acidic reading | An acidic functional group may be present. | Functional-group identity by itself; pH is supporting evidence, not a fingerprint. |
Do not overclaim an observation
Weak conclusion: “It turned bromine water colourless, so the unknown is but-1-ene.”
Stronger conclusion: “Decolourising bromine water supports the presence of a C=C bond. The result is consistent with an alkene, but additional structural evidence is needed to distinguish alkene isomers.”
Rewrite this overclaim:
“The dichromate changed from orange to green, so the sample is propan-1-ol.”
Practice unknowns — no wet practical required
Unknown A
Supplied data: bromine water decolourises rapidly; carbonate test shows no gas.
- Which functional group is supported?
- Which functional group is not supported by the carbonate result?
- What is one thing you still cannot know?
Unknown B
Supplied data: carbonate test gives effervescence; bromine water shows no decolourisation.
- What class is consistent with the gas-producing result?
- Why is the bromine result useful but not enough to identify the exact molecule?
Unknown C
Supplied data: the sample is known independently to be an alcohol. Acidified dichromate evidence shows orange → green.
- Which broad alcohol possibilities remain?
- Which alcohol class is less consistent with the observation under the stated test conditions?
- What further structural information would you seek?
Evidence table for your own unknown
| Test or supplied evidence | Exact observation | Inference | Alternative still possible? |
|---|---|---|---|
Best-supported identification:
Evidence sentence: “I identify the unknown as / containing __________ because __________ and __________. This rules out __________ because __________.”
Remaining uncertainty or limitation:
Kaiako quick key for supplied-data practice
- A: C=C / alkene supported; carboxylic acid not supported by the no-gas carbonate result; exact structure/isomer remains unknown.
- B: carboxylic acid is consistent with effervescence in the supplied carbonate test; the negative bromine result does not identify the exact acid.
- C: primary or secondary alcohol remain consistent with oxidation evidence; tertiary alcohol is less consistent under the stated test conditions; more structural/product evidence is needed to distinguish primary from secondary.