Tests for Ions, Gases and Chromatography
Identifying Substances in Chemistry
Chemists use a range of analytical techniques to identify unknown substances. These range from simple laboratory tests — such as flame tests and gas tests — to instrumental methods such as the flame photometer. This lesson covers the key tests you need to know, many of which are Triple-only content that goes beyond the Combined Science course.
Flame Tests
A flame test is used to detect the presence of certain metal ions. A clean nichrome wire loop is dipped in the sample and held in a Bunsen flame. Different metal ions produce characteristic flame colours:
- Lithium (Li+): crimson / red
- Sodium (Na+): bright yellow
- Potassium (K+): lilac
- Calcium (Ca2+): orange-red
- Copper (Cu2+): blue-green
Flame test colours: lithium = crimson, sodium = yellow, potassium = lilac, calcium = orange-red, copper = blue-green. Sodium’s yellow is so intense it can mask other colours — the wire must be thoroughly cleaned between tests.
Tests for Metal Ions Using Sodium Hydroxide
Many metal ions form insoluble hydroxide precipitates when sodium hydroxide solution is added. The colour of the precipitate identifies the metal ion:
- Copper (Cu2+): blue precipitate
- Iron(II) (Fe2+): green precipitate
- Iron(III) (Fe3+): brown (rust-coloured) precipitate
- Calcium (Ca2+): white precipitate
- Aluminium (Al3+): white precipitate that dissolves in excess NaOH
Adding sodium hydroxide solution to an ammonium salt and gently warming releases ammonia gas (NH3). Ammonia can be detected by holding damp red litmus paper near the mouth of the tube — it turns blue because ammonia is an alkaline gas. This is the definitive test for the ammonium ion (NH4+).
Gas Tests
Four gases are commonly tested in GCSE chemistry. Each has a specific test and result:
Hydrogen (H2): hold a lit splint to the mouth of the test tube — a squeaky pop confirms hydrogen.
Oxygen (O2): hold a glowing splint to the mouth of the tube — oxygen relights (re-ignites) the splint.
Carbon dioxide (CO2): bubble the gas through limewater (calcium hydroxide solution) — it turns milky white (cloudy) as calcium carbonate precipitates.
Chlorine (Cl2): hold damp litmus paper in the gas — it is first turned red (acid) then bleached white. Chlorine is a pale yellow-green gas with a pungent smell.
Gas tests: hydrogen — squeaky pop with lit splint. Oxygen — relights glowing splint. Carbon dioxide — turns limewater milky. Chlorine — bleaches damp litmus paper.
Chromatography
Chromatography separates mixtures based on the relative solubility of each component in a solvent. In paper chromatography, a spot of mixture is placed on a pencil baseline on chromatography paper. The paper is placed in a small volume of solvent (below the baseline), which travels up the paper by capillary action, carrying dissolved components with it. Components that are more soluble in the solvent travel further.
Calculating Rf Values
The Rf value (retention factor) is calculated as:
$$R_f = \frac{\text{distance moved by substance}}{\text{distance moved by solvent front}}$$
Rf values are always between 0 and 1. Each substance has a characteristic Rf value in a specific solvent at a specific temperature. A pure substance produces a single spot; a mixture produces multiple spots. Unknown substances can be identified by comparing their Rf values with known reference values run on the same chromatogram.
The solvent front travels 8.0 cm. A dye spot travels 6.4 cm. Calculate the Rf value.
$$R_f = \frac{6.4}{8.0} = 0.80$$
The dye has an Rf value of 0.80 in this solvent. Always measure to the centre of the spot from the pencil baseline, not from the bottom of the paper.
Instrumental Analysis: The Flame Photometer
The flame photometer is an instrumental technique that identifies metal ions by analysing the specific wavelengths of light they emit when placed in a flame. It is far more sensitive than a manual flame test and can detect and measure the concentration of metal ions in very dilute solutions. Flame photometry is used in hospitals to measure sodium and potassium ion concentrations in blood and urine samples, and in agriculture to analyse soil mineral content.
For the exam, learn flame test colours precisely: lithium gives crimson, sodium gives yellow, potassium gives lilac, calcium gives orange-red, and copper gives blue-green. A common error is confusing the white precipitate from aluminium or calcium ions with that from magnesium: adding excess sodium hydroxide dissolves the aluminium hydroxide precipitate but not the magnesium hydroxide, which lets you distinguish them reliably in an exam question.