Chemical Tests and Identification
Functional Group Tests
Need to identify an organic compound? These tests will help you:
- Alcohols: Mix with sulphuric acid and potassium dichromate - a colour change from orange to green confirms it's an alcohol.
- Alkenes: Add bromine water - if it changes from orange to colourless, you've got an alkene.
- Aldehydes: Use Tollens' reagent (a mixture of silver nitrate, sodium hydroxide and ammonia) - a silver mirror forms with aldehydes. Alternatively, Fehling's solution turns from blue to a brick-red precipitate when heated with aldehydes.
- Carboxylic acids: Add sodium hydrogen carbonate - effervescence (bubbling) of CO₂ indicates a carboxylic acid.
Quick Tip: For practical exams, remember that only aldehydes give positive results with both Tollens' reagent and Fehling's solution - ketones don't react with either!
Anion Tests
Identifying negatively charged ions is straightforward with these reactions:
- Carbonate ions: Add dilute HCl - effervescence occurs. The gas produced turns limewater cloudy (confirming it's CO₂).
- Sulphate ions: Add dilute HCl and barium chloride - a white precipitate forms.
- Halide ions (using silver nitrate followed by ammonia):
- Chloride: White precipitate that dissolves in dilute ammonia
- Bromide: Cream precipitate that remains in dilute ammonia but dissolves in concentrated ammonia
- Iodide: Yellow precipitate that doesn't dissolve in ammonia (dilute or concentrated)
Cation Tests
For positively charged ions, look for these reactions:
- Calcium ions: Give a red flame colour; slight white precipitate with NaOH that dissolves in excess water
- Magnesium ions: Form a white precipitate with sodium hydroxide
- Barium ions: Produce a green flame colour; white precipitate with sulphuric acid
- Strontium ions: Show a red flame colour; white precipitate with sulphuric acid
- Ammonium ions: When mixed with NaOH and heated, turn damp red litmus paper blue
Transition Metal Tests
Transition metal compounds produce distinctive colour changes with reagents:
- Iron(II) ions: Form a green precipitate with NaOH or NH₃ (turns rusty brown in air)
- Iron(III) ions: Produce a rusty brown precipitate with NaOH or NH₃
- Copper(II) ions: Form a blue precipitate with NaOH; with excess NH₃, a royal blue solution forms
- Aluminium ions: Create a white precipitate with NaOH that dissolves in excess NaOH
The vanadium oxidation states also show distinctive colours: VO₂⁻ (yellow), VO²⁺ (blue), V³⁺ (green), and V²⁺ (violet).
