Analysis
What you need to know
Reflections and Exam tips
Analysis
Flame tests
Identifying Group 1 and Group 2 metals is carried out using flame tests, as they tend to have unique flames which we can associate with the elements.
Explanation of flame colour
The energy of the flame causes electrons in the metal ion to rise to higher energy levels, and as they fall back to their original level they give out specific frequencies of light.
- Lithium will burn with a bright red flame
- Sodium will burn with a golden yellow flame
- Potassium will burn with a lilac flame
- Calcium will burn with a brick red flame
- Barium will burn with a green flame
- Copper will burn with a blue-green flame
A nichrome wire is dipped into concentrated hydrochloric acid and put onto the upper part of a blue flame. This is repeated until no colour is observed.
Sodium hydroxide tests for cations
Some metal ions form white or coloured precipitates when they react with sodium hydroxide.
| Cation | Positive result with sodium hydroxide |
|---|---|
Iron(II), Fe4- Iron(III), Fe3+ Copper(II), Cu2+ Aluminium, Al3+ Calcium, Ca2+ Magnesium, Mg2+ Ammonium, NH4+ |
Grey green solid is formed Orange solid is formed Blue solid is formed White solid is formed which dissolves in excess sodium hydroxide White solid is formed White solid is formed A gas is evolved which turns damp red litmus paper blue |
Chlorides
Add dilute nitric acid followed by silver nitrate. Chlorides will produce a white precipitate, bromide a cream precipitate and iodide a yellow precipitate. To distinguish between white and cream, dilute ammonia is added and the white precipitate dissolves. When concentrated ammonia is added to the cream and yellow precipitate, the cream dissolves while the yellow does not.
Carbonates
Add hydrochoric acid and if substance fizzes and the gas produced turns limewater milky/cloudy its a carbonate.
Alternatively, substance is put in a boiling tube and heated strongly with the gas produced bubbled through limewater. Presence of carbonate - limewater turns cloudy.
Sulphates
Add hydrochloric acid followed by Barium Chloride and if white precipitate is produced its a sulphate.
Nitrates and Ammonium ion
Warm with aqueous sodium hydroxide if a gas is given off that turns red litmus blue and has a pungent smell AMMONIUM ions present. If no gas is produced add aluminium powder and warm and a pungent smell produced and red litmus turns blue then its a NITRATE.