Test-tube tests
Which test, what you see, and the wording examiners pay for — organic functional groups and the required ion tests, on one page.
Reading mode — every observation on show.
Organic functional groups
| Testing for | Test | Positive result |
|---|---|---|
| Alkene (C=C) | Shake with bromine water | orange → colourless electrophilic addition — say “decolourised”, never “clear” |
| Halogenoalkane | Warm with NaOH(aq), acidify with HNO3, add AgNO3(aq) | AgCl white · AgBr cream · AgI yellow ppt AgCl dissolves in dilute NH3, AgBr in conc. NH3, AgI in neither |
| Aldehyde (not ketone) | Warm with Tollens' reagent | silver mirror ammoniacal silver nitrate; ketones do nothing |
| Aldehyde (not ketone) | Warm with Fehling's solution | blue solution → brick-red ppt Cu2+ reduced to Cu2O |
| Carbonyl (aldehyde or ketone) | Add 2,4-DNPH (Brady's reagent) | orange/yellow precipitate melting point of the derivative identifies the exact carbonyl |
| Carboxylic acid | Add NaHCO3 or Na2CO3(aq) | fizzing — CO2 turns limewater milky the only common organic acid strong enough to do this |
| Primary/secondary alcohol | Warm with acidified K2Cr2O7 | orange → green tertiary alcohols stay orange — they resist oxidation |
Inorganic ions
| Testing for | Test | Positive result |
|---|---|---|
| Sulfate SO42− | Acidified BaCl2(aq) | white precipitate (BaSO4) acidify with HCl first to destroy carbonates |
| Halide ions | Acidified AgNO3(aq) | white (Cl−) · cream (Br−) · yellow (I−) ppt confirm with dilute then concentrated NH3 solubility |
| Carbonate CO32− | Add dilute acid | fizzing — CO2 turns limewater milky |
| Ammonium NH4+ | Warm with NaOH(aq) | ammonia gas — damp red litmus turns blue smell it cautiously; litmus is the marked observation |
| Group 2 ion (Mg → Ba) | Add NaOH(aq) / add H2SO4(aq) | hydroxides more soluble down the group; sulfates less Mg(OH)2 white ppt but BaSO4 white ppt — opposite trends |
Observation answers score the change: state the colour before and after, name the precipitate, and never write “clear” when you mean colourless.
Pair it with the colours
Transition-metal and aqua-ion colours have their own page — the other half of the observation marks.
Ion colour tablesStruggling with the test-tube tests? The first tutoring lesson is free.
Free revision tool by Oscar Song Tutoring