Chemical analysis
Learning outcomes
After studying this section, you should be able to describe how to test for:
- metal ions using flame tests
- metal ions and ammonium ions using sodium hydroxide solution
- carbonate ions, sulfate ions, and halide ions
- common gases.
Summaries
Flame tests
| Metal ion | Flame test colour |
|---|---|
| lithium, Li+ | red |
| sodium, Na+ | yellow |
| potassium, K+ | lilac |
| calcium, Ca2+ | orange-red |
| copper(II), Cu2+ | blue-green |
| barium, Ba2+ | light green |
Tests using sodium hydroxide solution
Metal ions
Metal ions may form precipitates when they react with hydroxide ions. The table shows the colours of these precipitates in sodium hydroxide solution.
| Metal ion | Colour of precipitate |
|---|---|
| copper(II), Cu2+ | Light blue |
| iron(II), Fe2+ | Green (turns brown after a while) |
| iron(III), Fe3+ | Red-brown |
| chromium(II), Cr3+ | Green, pale green solution in excess NaOH |
| aluminium, Al3+ | White, colourless solution in excess NaOH |
| calcium, Ca2+ | White |
| zinc, Zn2+ | White, colourless solution in excess NaOH |
Left to right: Cu2+, Fe3+, Fe2+
Ammonium ions
Ammonia is given off when a solution containing ammonium ions mixed with aqueous sodium hydroxide, then warmed. The ammonia then turns damp red litmus paper blue.
Tests for negatively charged ions (anions)
Carbonate ions
Bubbles of gas are given off when dilute acids react with substances containing carbonate ions. Limewater is used to confirm that the gas is carbon dioxide.
Sulfate ions
A white precipitate forms when aqueous barium chloride or aqueous barium nitrate is added to a solution containing sulfate ions. The solution must be appropriately acidified.
Halide ions
Precipitates form when aqueous silver nitrate is added to solutions containing halide ions. The solution must be acidified with dilute nitric acid. The table shows the colours of the silver halide precipitates.
| Halide ion | Colour of silver halide precipitate |
|---|---|
| chloride, Cl‒ | white |
| bromide, Br‒ | cream |
| iodide, I‒ | yellow |
Common gases
- acidify the sample with dilute aqueous potassium manganate(VII) solution with dilute sulfuric acid
- bubble the gas through the acidified potassium manganate(VII) solution
- sulfur dioxide causes a colour change from purple to colourless.
| Gas | What you do | What you see |
|---|---|---|
| ammonia, NH3 | Hold damp red litmus paper in the gas | Paper turns blue |
| carbon dioxide, CO2 | Bubble through limewater | Limewater turns milky |
| chlorine, Cl2 | Hold damp litmus paper in the gas | Paper is bleached white |
| hydrogen, H2 | Put a lighted splint in the gas | Gas ignites with a 'pop' |
| oxygen, O2 | Put a glowing splint in the gas | Splint relights |
