Testing for cations (positive ions)
Two main tests identify metal ions. In a flame test, a clean wire dipped in the sample is held in a Bunsen flame: sodium (Na+) gives yellow, potassium (K+) lilac, copper(II) (Cu2+) blue-green, lithium (Li+) red and calcium (Ca2+) orange-red. Adding sodium hydroxide solution forms coloured hydroxide precipitates:
| Ion | With NaOH |
| Cu2+ | light blue ppt |
| Fe2+ | green ppt |
| Fe3+ | red-brown ppt |
| Al3+, Zn2+, Ca2+ | white ppt |
Al3+ and Zn2+ hydroxides dissolve in excess NaOH; Ca2+ does not. The ammonium ion (NH4+) is found by warming with NaOH: ammonia gas turns damp red litmus blue.
Key idea / 要点
Anion tests: carbonate + dilute acid → CO2 (turns limewater milky). Halides + acidified silver nitrate: chloride = white ppt, bromide = cream, iodide = yellow. Sulfate + acidified barium chloride → white ppt.
Testing for gases
Remember / 记住
- Carbon dioxide: turns limewater milky.
- Hydrogen: lighted splint gives a squeaky pop.
- Oxygen: relights a glowing splint.
- Chlorine: bleaches damp litmus paper.
- Ammonia: turns damp red litmus paper blue.
Always add dilute acid first when testing for a sulfate or halide, to remove carbonate ions that would otherwise interfere. Use dilute nitric acid with silver nitrate for halides, and dilute hydrochloric or nitric acid with barium chloride for sulfates. Take care with the flame test: the wire must be cleaned in acid and heated until it gives no colour of its own, otherwise sodium contamination masks other colours. These simple, reliable tests let you identify the ions and gases present in an unknown compound step by step, building a full picture of its composition.