Lesson 8: Qualitative Analysis & Unknown Organic Identification
Design an efficient test sequence that identifies unknown organic liquids from observed reactions, solubility and phase behaviour.
🎯 Ngā Whāinga Akoranga | Learning Intentions
Qualitative chemical tests used to identify unknown organic liquids (litmus test, NaHCO3, Br2 water, MnO4-/H+, Cr2O72-/H+, water solubility/phase separation).
Design a systematic elimination flowchart to distinguish between 5 unknown organic bottles based on experimental observations and chemical reasoning.
🎥 Media Anchor & Pedagogical Scaffold
Qualitative Identification of Unknown Organics
Video Clip: Qualitative Analysis — chemical tests to identify unknown compounds | GCSE, CSEC, and CAPE Chemistry (gSquared Academy) (Runtime: 11m 30s).
🧠 1. Before Viewing (Activate & Predict)
If you are given 5 unlabelled clear liquids in a chemistry lab, what experimental steps allow you to safely identify each compound?
👁️ 2. During Viewing (Watch With a Job)
- Test table: as you watch, create a table with columns: Functional group tested | Reagent used | Positive result | What it indicates.
- Bromine water test: when would you add bromine water to an unknown organic compound, and what does a colour change (or no change) tell you?
- Unknown identification: the video shows how to build an argument from tests. If a compound turns bromine water colourless but does NOT react with sodium carbonate, what functional group can you confirm or eliminate?
🗣️ 3. After Viewing & Kaiako Move (Process & Apply)
Kaiako Move: Conduct the ‘5 Unknowns’ qualitative analysis practical under timed exam-style conditions. The five are the Do Now’s four — a cyclohexene, a primary alcohol, a carboxylic acid and an alkane — plus one unseen bottle you add. The Do Now is planned on paper; this is the same reasoning run at the bench.
Immediate Task: Complete Section 8 of your Organic Portfolio: Unknown Identification Qualitative Flowchart & Justification Log.
⚡ Whakaoho | Do Now: Organic Recall Challenge (10 mins)
Four bottles, no labels. A cyclohexene, a primary alcohol, a carboxylic acid and an alkane. You have litmus, sodium hydrogencarbonate, bromine water and acidified dichromate.
Two minutes, before touching anything: write the order you would run the tests in, and why that order. A good sequence eliminates the most possibilities with the fewest tests — that is the actual skill being assessed.
⚠️ Hazard
Bromine water and acidified dichromate are both toxic and irritant to skin, eyes and airways; dichromate is also a known human carcinogen. Wear safety glasses, work in the fume hood as briefed, and tell your kaiako immediately if either contacts skin — rinse with water for at least 10 minutes. Dispose of dichromate waste only as your kaiako directs, never down the sink.
📖 Activity 1: Core Reaction Mechanism & Structure Analysis (25 mins)
Run your scheme (20 min). Following the safety brief, work through your test order on the four unknowns. Record every observation, including the negatives — a test that does nothing is evidence. Identify each bottle and state which single observation was decisive for each one.
Defend it (5 min). Swap identifications with another pair. Where you disagree, settle it with observations rather than opinion, and if neither has decisive evidence, name the one extra test that would.
Kaiako answer key — the four bottles. The efficient order is hydrogencarbonate, then bromine water, then dichromate, and the reason is worth drawing out with the class: each test in that order removes one candidate outright.
- NaHCO3(aq). Steady effervescence with only one bottle — that is the carboxylic acid, and the gas is CO2. Nothing from the other three. Blue litmus turning red on the same bottle confirms it.
- Bromine water, in the dark. Orange to colourless within seconds with one of the remaining three — that is the cyclohexene. The other two are unchanged. "In the dark" matters: in UV light the alkane will slowly substitute and fade too, which would fake an alkene.
- Acidified dichromate, warm. Orange to green with one of the last two — that is the primary alcohol. The remaining bottle, negative to all three tests, is the alkane.
The three things to press on. First, the alkane is identified by three negatives, not by a positive — students routinely leave it unnamed because "nothing happened", when nothing happening is the evidence. Second, bromine water must be run in the dark. In UV light the alkane slowly substitutes and the orange fades too, which fakes an alkene positive — that is the one way this scheme can be made to give a confidently wrong answer, and it is worth saying before students start. Third, a student who proposes acidified permanganate in place of dichromate has proposed a test that cannot separate these bottles: MnO4−/H+ goes purple to colourless with the alkene and with the primary alcohol. Dichromate is the right choice here precisely because it reports on the alcohol and leaves the alkene alone.
📝 Activity 2: Level 2 Chemistry Portfolio Task & Merit/Excellence Scaffolding (20 mins)
Organic Portfolio — Section 8. Submit: (1) your test sequence with the reasoning for the order; (2) a full observation table including negative results; (3) four identifications, each justified by a named decisive observation and the structural reason that observation occurs.
🏫 Kaiako Planning & Pedagogy Notes
Where this sits in the standard. This lesson has no new chemistry in it. Every test used here comes from an earlier lesson; what is new is the reasoning — using the reactions in the standard's list as evidence, and giving an account of observed properties, which is the wording of the Achieved criterion.
Exit ticket (3 min, on a slip, collected). Your unknown gave no result with hydrogencarbonate, no result with bromine water, and no result with warm acidified dichromate. What is it, and how do you know?
What to watch for while they work. Watch the order pairs choose before they touch anything. Running dichromate first is not wrong so much as wasteful — the carboxylic acid is already oxidised and gives nothing, so the test spends itself separating one bottle instead of eliminating a whole branch. Catching that at the planning stage is worth more than letting them discover it, because a sequence is judged on its reasoning, not on whether it eventually arrives.
Differentiation. Support: Give the four candidate compounds and the four reagents as cards and have students physically arrange a decision tree before writing anything. The sequencing is the assessed skill and it is easier to see than to write. Extension: Add a fifth unlabelled bottle that gives a positive result to two tests, and ask what single extra observation would resolve it.
The error to head off. Students leave the alkane unnamed because "nothing happened". Three negative results is a complete identification, and saying so out loud early changes how the whole class records their observations.