Identification of ions and gases
Tests for anions (negative ions)
- carbonate ($\text{CO}_3^{2-}$): add dilute acid → fizzes, gas turns limewater milky.
- chloride ($\text{Cl}^-$): add nitric acid + silver nitrate → white precipitate.
- bromide ($\text{Br}^-$): same test → cream precipitate.
- sulfate ($\text{SO}_4^{2-}$): add acid + barium ions → white precipitate.
Practice
Adding dilute nitric acid then silver nitrate to a chloride gives a:
Chloride gives a white precipitate; bromide gives a cream one.
Tests for gases
| Gas | Test → result |
|---|---|
| ammonia | damp red litmus → blue |
| carbon dioxide | limewater → milky |
| chlorine | damp litmus → bleached |
| hydrogen | lit splint → squeaky pop |
| oxygen | glowing splint → relights |
Practice
The test for hydrogen gas is:
Hydrogen gives a squeaky pop with a lit splint; oxygen relights a glowing splint.
Flame tests for cations
| Cation | Flame colour |
|---|---|
| lithium | red |
| sodium | yellow |
| potassium | lilac |
| calcium | orange-red |
| copper(II) | blue-green |
Practice
Match each metal ion to its flame colour.
Flame tests: sodium yellow, potassium lilac, copper(II) blue-green (lithium red, calcium orange-red).
You've got it
Key idea
- anions: carbonate fizzes; chloride → white, bromide → cream (silver nitrate)
- gases: hydrogen pops, oxygen relights a glowing splint, CO₂ → milky limewater
- flame tests: sodium yellow, potassium lilac, copper(II) blue-green