Lesson at a Glance | He Tirohanga Whakamua
Ngā Whāinga Ako | Learning Intentions
Students will know
- How to select reagents, state exact visual observations, and write balanced chemical equations to identify unknown organic compounds.
- The standard diagnostic toolkit:
1. Water Solubility / Layer formation.
2. Litmus Red (Acid), Blue (Amine), Unchanged (Neutral).
3. Carbon dioxide effervescence (Carboxylic Acid).
4. Orange to colourless decolourisation (Alkene).
5. Orange to green ( Alcohol).
Students will demonstrate
- By designing a complete, error-free identification flowchart for 5 unknown liquids.
- By completing Section 9 of their Level 2 Organic Chemistry Mastery Portfolio.
Curriculum alignment
- NZC (2007) · Science · Level 7 · Material World: “Investigate and measure the chemical and physical properties of a range of groups of substances, for example, acids and bases, oxidants and reductants, and selected organic and inorganic compounds.”
Do Now | Tīmatanga Whakaaro (10 min)
Unlabelled Bottle Mystery Prompt:
"You are given 4 unlabelled bottles containing hexane, hex-1-ene, ethanoic acid, and propan-1-amine. What is the single test you should run FIRST to split them into two distinct groups?"
Unpack: Add Water! Hexane and hex-1-ene are non-polar hydrocarbons and will form two separate layers (insoluble). Ethanoic acid and propan-1-amine are polar and will dissolve completely (one layer). Now test each pair!
Master Qualitative Testing Reagents (15 min)
| Reagent / Test | Target Functional Group | Positive Observation | Negative Observation |
|---|---|---|---|
| Water () | Polar vs Non-polar | Soluble (1 layer: small alcohols, acids, amines) | Insoluble (2 layers: alkanes, alkenes, haloalkanes) |
| Litmus Paper | Acid vs Base | Red Blue (Amine); Blue Red (Carboxylic Acid) | No change (Alcohols, Hydrocarbons) |
| Carboxylic Acid | Vigorous effervescence ( gas bubbles) | No fizz / no reaction | |
| Bromine Water () | Alkene () | Instant orange to colourless decolourisation in dark | Stays orange (Alkane requires UV light) |
| heat | Alcohol | Orange solution turns bright green () | Stays orange ( Alcohol, Alkane) |
Flowchart Logic (15 min)
Where this usually goes wrong: students test in a random order and run out of sample. A good flowchart is ordered by how much each test eliminates, not by which reagent is nearest.
Put the test that splits the set most evenly first. Bromine water separates unsaturated from saturated in one step, halving the candidates, so it earns an early branch. A test that only ever confirms one compound belongs near the end.
Record what a negative rules OUT, not just what a positive confirms. "No effervescence with " eliminates every carboxylic acid at once. Students who only note positives end up unable to justify their final identification.
Do this now: build a branching flowchart that separates an alkene, an alcohol, a carboxylic acid and an amine using the fewest tests. State the maximum number of tests any one unknown requires.
Answer (kaiako)
One valid three-test route: ① NaHCO₃ — fizz → carboxylic acid. ② red litmus — turns blue → amine. ③ Br₂ water — decolourises → alkene; no change → alcohol. Maximum tests for any one unknown: 3 (the alcohol is identified by elimination on the third test). Accept any order that separates uniquely in ≤3 tests.
🛑 Identifying an unknown reverses the usual safety order
A risk assessment normally starts from knowing what is in the bottle. Here the identity is the question, so the hazards are unknown until the work is finished — an unlabelled organic liquid is treated as flammable, toxic and corrosive until shown otherwise. The reagents themselves carry risk: bromine water is toxic and corrosive, acidified dichromate is toxic, is a known human carcinogen (chromium(VI) compounds are IARC Group 1) and has controlled disposal. This lesson is built to be completed from supplied observations, and a well-ordered flowchart is stronger evidence of chemical reasoning than a wet test would be.
📁 Organic Chemistry Portfolio — Section 9: Master Qualitative Testing Flowchart
Students open their Level 2 Chemistry Portfolio and complete Section 9:
Section 9 Requirements:
1. Master 4-Unknown Flowchart: Design a step-by-step flowchart to uniquely identify 4 unlabelled bottles: pentane, pent-1-ene, pentan-1-ol and pentanoic acid. Then answer the question that decides whether you have understood your own flowchart: if a fifth bottle held 1-chloropentane, could you find it with the reagents in this unit?
2. Observations & Reagents Table: For each step, state Reagent name, Exact visual observation for positive/negative results, and organic product structural formula.
3. Excellence Diagnostic Justification: 1-paragraph explanation of why acidified potassium permanganate () cannot distinguish alkenes from primary alcohols (both turn purple to colourless!).
Answers & marking notes (kaiako) — screen only, does not print
- One valid flowchart: ① NaHCO₃ — fizzes → pentanoic acid. ② Br₂ water — decolourises fast, in the dark → pent-1-ene. ③ Cr₂O₇²⁻/H⁺ warm — orange → green → pentan-1-ol. ④ remainder, no positive test → pentane. Accept any order that still separates uniquely; the observations column carries the marks, and the alkane is named by three negatives rather than by a positive.
The fifth-bottle question: no — and that is the answer, not a failure. Every reagent in this unit reports on a C=C, an oxidisable –OH or an acidic proton, and 1-chloropentane has none of the three, so it would behave exactly like pentane and the two would be indistinguishable. Separating them needs a halide test, which this standard's reaction list does not contain. If your class has already met silver nitrate elsewhere you may restore the fifth bottle, but it cannot be required of them here. - Excellence — the question actually set: acidified permanganate cannot distinguish an alkene from a primary alcohol because it oxidises both, and in both cases the species that changes colour is the manganese, not the organic compound. MnO₄⁻ attacks the C=C of the alkene, and it oxidises the primary alcohol on through to the carboxylic acid; either way the purple MnO₄⁻ is reduced to colourless Mn²⁺. So one observation — purple to colourless — is produced by two different reactions, and an observation that two candidates share carries no diagnostic information. The fix is to choose reagents each of which reacts with only one of them: bromine water responds to the C=C and leaves the alcohol alone, and acidified dichromate oxidises the alcohol and leaves the alkene orange. That is the shape of the full-marks answer: name what each reagent reacts with, then say why a shared observation cannot separate two compounds.
- Also worth credit — ordering: order matters where reagents cross-react. Running dichromate before the acid is eliminated wastes a test, because the acid is already oxidised; and Br₂ in UV light would slowly substitute the ALKANE, faking a positive, which is why the bromine test is run in the dark. A justified order is real evidence of chemical reasoning, but it answers a different question from the one set above.
Exit Verification | Ka Mutu Hoki (5 min)
Exit Check:
"My Section 9 flowchart uniquely identifies all 5 unknown liquids stating exact reagent names and colour changes."
Teacher Planning & NCEA Alignment
NCEA Level 2 Chemistry Alignment (4 Credits External):
- Qualitative Analysis: Design chemical flowcharts and state observations (colour changes, solubility, effervescence) to identify organic compounds.
- Chemical Reasoning: Write equations for positive diagnostic chemical tests.
Vocabulary: Qualitative analysis, diagnostic test, flowchart, water solubility, litmus test, effervescence, decolourisation, dichromate, permanganate, Lucas reagent.