Chemical Tests for Ions
Testing for Ions
In separate chemistry, you need to know how to identify unknown substances using a range of chemical tests. These tests are based on characteristic reactions that produce observable results — colour changes, precipitates, or gas production.
Flame Tests for Metal Ions
A flame test identifies metal ions by the colour they produce when heated in a Bunsen flame. A clean nichrome wire loop is dipped in the sample, then held in the blue flame.
Flame test colours: Lithium = crimson red. Sodium = yellow. Potassium = lilac. Calcium = orange-red. Copper = green.
Testing with Sodium Hydroxide Solution
Adding a few drops of sodium hydroxide solution to a solution containing metal ions produces a coloured precipitate (an insoluble solid). The colour of the precipitate identifies the metal ion:
- Calcium (Ca²+): white precipitate.
- Magnesium (Mg²+): white precipitate.
- Aluminium (Al³+): white precipitate — but dissolves in excess NaOH to form a colourless solution.
- Copper(II) (Cu²+): blue precipitate.
- Iron(II) (Fe²+): green precipitate.
- Iron(III) (Fe³+): brown precipitate.
Calcium, magnesium, and aluminium all produce white precipitates with NaOH. To distinguish between them:
- Aluminium dissolves in excess NaOH (the white precipitate disappears). Calcium and magnesium do not.
- Calcium can be confirmed by a flame test (orange-red flame). Magnesium does not produce a visible flame colour.
Testing for Negative Ions (Anions)
Testing for carbonates (\(\text{CO}_3^{2-}\)): Add dilute acid to the sample. If carbonate ions are present, the mixture fizzes and produces carbon dioxide gas, which turns limewater milky (cloudy).
Testing for halide ions (\(\text{Cl}^-\), \(\text{Br}^-\), \(\text{I}^-\)): Add dilute nitric acid, then silver nitrate solution. The colour of the precipitate identifies the halide:
- Chloride: white precipitate (silver chloride).
- Bromide: cream precipitate (silver bromide).
- Iodide: yellow precipitate (silver iodide).
Testing for sulfate ions (\(\text{SO}_4^{2-}\)): Add dilute hydrochloric acid, then barium chloride solution. A white precipitate of barium sulfate forms if sulfate ions are present.
The acid is added first in halide and sulfate tests to remove any carbonate ions that might otherwise interfere with the results by forming precipitates of their own.
Tests for Gases
You also need to know how to test for common gases:
- Hydrogen (\(\text{H}_2\)): Hold a burning splint near the gas. It burns with a squeaky pop.
- Oxygen (\(\text{O}_2\)): Insert a glowing splint into the gas. It relights.
- Carbon dioxide (\(\text{CO}_2\)): Bubble through limewater. It turns milky/cloudy.
- Chlorine (\(\text{Cl}_2\)): Hold damp litmus paper near the gas. It bleaches the litmus paper white.
Modern laboratories use instrumental methods of analysis that are more sensitive, faster, and can handle very small samples. Examples include spectroscopy and chromatography. These methods are particularly useful for detecting trace amounts of substances or for analysing complex mixtures.
Instrumental methods of analysis are preferred over chemical tests in industry because they are more accurate, more sensitive (can detect smaller amounts), faster, and require smaller sample sizes.