4.2.4Physical Chemistry
Testing for Organic Functional Groups — AQA AS Chemistry Revision Notes & Topical Questions
5 min read 5 parts Lesson 102 of 102
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 5The Four Tests
Alkene
- Alkene Shake with bromine water;
- the orange solution turns colourless, and a saturated compound leaves it orange.
Primary or secondary alcohol
- Primary or secondary alcohol Warm with acidified potassium dichromate(VI) in a water bath at about 60 °C;
- orange turns green.
Aldehyde
- Aldehyde Warm with Fehling's solution to give a brick-red precipitate, or with Tollens' reagent to give a silver mirror.
Carboxylic acid
- Carboxylic acid Add sodium carbonate or hydrogencarbonate solution;
- effervescence is seen and the gas turns limewater milky.
| Functional group | Reagent | Positive observation | Negative observation |
|---|---|---|---|
| Alkene | Bromine water | Orange to colourless | Stays orange |
| Alcohol (1° or 2°) | Acidified K₂Cr₂O₇, warm | Orange to green | Stays orange |
| Aldehyde | Tollens' reagent, warm | Silver mirror | No visible change |
| Carboxylic acid | Na₂CO₃(aq) | Effervescence | No visible change |
Part 2 of 5Method for the Alcohol Test
1
Add about 1 cm³ of the sample to a test tube using a pipette.
2
Add about 1 cm³ of acidified potassium dichromate(VI) solution with a different pipette.
3
Stopper the tube loosely, shake gently and stand it in a hot water bath at about 60 °C for a few minutes.
4
Observe the colour: orange to green shows a primary or secondary alcohol is present.
5
If the solution stays orange, the sample may be a tertiary alcohol or not an alcohol at all.
Part 3 of 5Distinguishing Aldehydes and Ketones
Distinguishing Aldehydes and Ketones
Fehling's
- Fehling's Contains Cu²⁺ ions in alkali;
- an aldehyde reduces them to copper(I) oxide, a brick-red solid, while the blue colour fades.
Tollens'
- Tollens' Contains the diamminesilver(I) ion in alkali;
- an aldehyde reduces Ag⁺ to a silver mirror on the tube.
Ketones
- Ketones Ketones cannot be oxidised easily by these reagents, so they give no visible change.
Safety
- Safety Tollens' reagent should be made fresh and not stored, and the tube should be disposed of promptly.
Part 4 of 5Recording Observations
Recording Observations
Observation versus deduction
- Observation versus deduction Write fizzing or effervescence, not carbon dioxide is given off, because an observation is what you see.
Negative tests
- Negative tests Write no visible change or stays orange; the statement nothing happens does not score.
Colourless
- Colourless Use colourless, not clear, when a coloured solution loses its colour.
Direction
- Direction Write the colour change in order, from the starting colour to the final colour.
Part 5 of 5Exam Checklist & Tips
4.2.4 – Key Points for the Exam
- Observation A statement of what is seen, heard or smelt during a test.
- Deduction A conclusion about the chemistry drawn from an observation.
- Tertiary alcohol An alcohol that is not oxidised by acidified dichromate(VI) because it has no hydrogen on the C–OH carbon.
- Effervescence The formation of bubbles of gas in a liquid.
Common Mistakes
- Reporting the deduction carbon dioxide instead of the observation effervescence.
- Saying the bromine water goes clear; it must go colourless.
- Writing nothing happens for a negative test instead of no visible change.
- Using Tollens' or Fehling's to test for ketones and expecting a positive result.
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