Lesson 9: Chemical Testing Flowchart | CHE 2.5

Lesson at a Glance | He Tirohanga Whakamua

Do NowIdentify 4 unlabelled bottles: Hexane, Hexene, Ethanol, Ethanoic Acid10 min

5 Reagent Diagnostic ToolkitWater, Litmus,

{NaHCO}_{3}

B r_{2}

water,

{Cr}_{2} O_{7}^{2 -}

15 min

Flowchart LogicElimination branching logic for NCEA exam questions15 min

Portfolio EntryWrite Section 9: Master Qualitative Chemical Testing Flowchart10 min

Exit VerificationIdentify unknown X using 2 diagnostic test results5 min

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 $\to$ Solubility / Layer formation.
2. Litmus $\to$ Red (Acid), Blue (Amine), Unchanged (Neutral).
3. ${NaHCO}_{3} (a q) \to$ Carbon dioxide effervescence (Carboxylic Acid).
4. $B r_{2} (a q) \to$ Orange to colorless decolourisation (Alkene).
5. ${Cr}_{2} O_{7}^{2 -} / H^{+} \to$ Orange to green ( $1^{\circ} / 2^{\circ}$ 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 ( $H_{2} O$ )	Polar vs Non-polar	Soluble (1 layer: small alcohols, acids, amines)	Insoluble (2 layers: alkanes, alkenes, haloalkanes)
Litmus Paper	Acid vs Base	Red $\to$ Blue (Amine); Blue $\to$ Red (Carboxylic Acid)	No change (Alcohols, Hydrocarbons)
${NaHCO}_{3} (a q)$	Carboxylic Acid	Vigorous effervescence ( ${CO}_{2}$ gas bubbles)	No fizz / no reaction
Bromine Water ( $B r_{2}$ )	Alkene ( $C = C$ )	Instant orange to colorless decolourisation in dark	Stays orange (Alkane requires UV light)
${Cr}_{2} O_{7}^{2 -} / H^{+}$ heat	$1^{\circ} / 2^{\circ}$ Alcohol	Orange solution turns bright green ( ${Cr}^{3 +}$ )	Stays orange ( $3^{\circ}$ 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.

Order tests by eliminating power

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.

A negative result is information too

Record what a negative rules OUT, not just what a positive confirms. "No effervescence with $N a H C O_{3}$ " 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.

📁 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 5-Unknown Flowchart: Design a step-by-step flowchart to uniquely identify 5 unlabelled bottles: pentane, pent-1-ene, 1-chloropentane, pentan-1-ol, and pentanoic acid.

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 ( ${MnO}_{4}^{-} / H^{+}$ ) cannot distinguish alkenes from primary alcohols (both turn purple to colorless!).

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.

Other teaching approach: Guided Viewing & Practical Chemistry →

Lesson 9: Qualitative Chemical Identification & Testing Flowchart