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Tests for organic functional groups (AS)

Edexcel IAL Chemistry · WCH13 Practical Skills in Chemistry I

Core Practical 8 includes organic tests. In the exam you must give the full reagent, the conditions, and the observation before and after, and name the group the test shows.

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The AS tests

Group	Reagent and conditions	Positive result
C=C (alkene)	bromine water, shake	orange to colourless
-OH (alcohol or carboxylic acid)	phosphorus(V) chloride, PCl5, dry	steamy fumes of hydrogen chloride
primary or secondary alcohol	acidified potassium dichromate(VI), warm	orange to green (tertiary: no change)
aldehyde	Fehling's or Benedict's solution, warm	blue solution to red precipitate
aldehyde	Tollens' reagent, warm in a water bath	silver mirror
carboxylic acid	sodium hydrogencarbonate solution	effervescence; CO2 turns limewater cloudy
halogenoalkane	warm with NaOH(aq), acidify with dilute HNO3, then add AgNO3(aq) (or warm with ethanolic AgNO3)	white (Cl), cream (Br) or yellow (I) precipitate

Writing answers that score

Give the full reagent name, including 'acidified' for dichromate(VI) or manganate(VII), and the condition (warm, water bath).
Give the colour before and after, in the right order.
Name the functional group, not an ion: the hydroxyl group is -OH, not OH- (June 2025 report).
For PCl5 the gas is hydrogen chloride; the observation is steamy (misty) fumes.
Do not add extra tests that contradict your answer.

Using tests to tell compounds apart

Primary, secondary or tertiary alcohol: tertiary alcohols do not change the colour of acidified dichromate(VI). To separate primary from secondary, oxidise with distillation and test the distillate: an aldehyde (from a primary alcohol) gives a silver mirror with Tollens' or a red precipitate with Fehling's; a ketone does not.
In the halogenoalkane test, acidify with nitric acid before adding silver nitrate, because excess OH- would precipitate silver oxide.

Worked example

How could you distinguish butan-1-ol, butan-2-ol and 2-methylpropan-2-ol?

Warm each with acidified potassium dichromate(VI): 2-methylpropan-2-ol (tertiary) stays orange; the others turn green.
Oxidise the other two with distillation and add Tollens' reagent to each distillate.
Silver mirror: butanal, so the alcohol was butan-1-ol. No mirror: butanone, from butan-2-ol.

Answer: Dichromate picks out the tertiary alcohol; Tollens' on the distillate separates primary from secondary.

Key idea: Bromine water for C=C; PCl5 (steamy HCl fumes) for -OH; acidified dichromate orange to green for primary and secondary alcohols; Fehling's or Tollens' for aldehydes; NaHCO3 fizz for acids; NaOH, HNO3, AgNO3 for halogenoalkanes.

Check yourself

What is observed when PCl5 is added to a dry alcohol?
- A silver mirror
- An orange precipitate
- Steamy fumes
- A colour change from orange to green
Show answer
Steamy fumes. Hydrogen chloride gas forms and fumes in moist air.
What colour change is seen when a primary alcohol is warmed with acidified potassium dichromate(VI)?
- Orange to green
- Green to orange
- Purple to colourless
- Blue to red
Show answer
Orange to green. Cr2O7²- (orange) is reduced to Cr3+ (green). Reversing the colours loses the mark.
Which alcohol gives no colour change when warmed with acidified potassium dichromate(VI)?
- Propan-1-ol
- Propan-2-ol
- 2-methylpropan-2-ol
- Ethanol
Show answer
2-methylpropan-2-ol. Tertiary alcohols have no H on the carbon bearing the OH, so they resist oxidation.
In the test for a halogenoalkane, why is dilute nitric acid added after warming with sodium hydroxide and before adding silver nitrate?
- To oxidise the halide ions
- To neutralise excess hydroxide ions, which would precipitate silver oxide
- To dissolve the halogenoalkane
- To act as a catalyst for the precipitation
Show answer
To neutralise excess hydroxide ions, which would precipitate silver oxide. OH- and Ag+ give a brown precipitate that would hide the result.
What is the positive result for an aldehyde with Tollens' reagent?
- A red precipitate
- Steamy fumes
- Orange to colourless
- A silver mirror
Show answer
A silver mirror. Ag+ is reduced to silver metal on the glass.
Which reagent gives effervescence with a carboxylic acid but not with an alcohol?
- Bromine water
- Sodium hydrogencarbonate solution
- Phosphorus(V) chloride
- Acidified potassium dichromate(VI)
Show answer
Sodium hydrogencarbonate solution. Acids react with hydrogencarbonate to give CO2; alcohols are too weakly acidic.
Describe the test for a carbon-carbon double bond, including the result.
Show answer
Shake with bromine water; it turns from orange to colourless.. Bromine adds across C=C.

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Last updated 8 November 2026. Spotted a mistake? Tell us.