Chemical Analysis
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Purity
In chemistry, a pure substance contains only one element or one compound — not a mixture. This differs from the everyday meaning of "pure".
A sharp melting point is the key test for purity — impurities broaden the range.
Purity — Key Knowledge
- Pure substance single element or single compound only
- Melting/boiling points pure substances have sharp, fixed points
- Mixtures melt and boil over a range of temperatures
Formulations
A formulation is a mixture designed with precise proportions of components, each serving a specific purpose.
Most commercial products are formulations, not pure substances.
Formulations — Key Knowledge
- Formulation useful mixture with exact proportions
- Examples paints, medicines, alloys, cleaning products, fuels
Chromatography
Chromatography separates and identifies substances in a mixture. Different substances travel different distances through the stationary phase.
Works with any mixture of soluble substances, not just inks and dyes.
Chromatography — Key Knowledge
- Mobile phase solvent that carries substances
- Stationary phase paper or thin-layer plate
- Single spot indicates a pure substance
- Multiple spots indicates a mixture
Rf values
The Rf value identifies a substance by how far it travels relative to the solvent. Each substance has a characteristic Rf under specific conditions.
Rf = distance moved by substance ÷ distance moved by solvent
both measured from the baseline
Compare a sample's Rf to known values to identify the substance.
Rf values — Key Knowledge
- Rf value unique to a substance under given conditions
- Conditions Rf depends on the solvent used and the temperature
Identification of common gases
Four common gases each have a specific test for identification.
Each gas has its own distinct test — not all gases are tested with a burning splint.
Identification of common gases — Key Knowledge
- Hydrogen squeaky pop with a burning splint
- Oxygen relights a glowing splint
- Carbon dioxide turns limewater milky/cloudy
- Chlorine bleaches damp litmus paper white
Flame tests for metal ions
A sample is held in a Bunsen flame on a cleaned wire loop. The flame colour identifies the metal ion present.
The colour comes from metal ion electrons releasing energy as light at specific wavelengths.
Flame tests for metal ions — Key Knowledge
- Lithium crimson/red
- Sodium yellow
- Potassium lilac
- Calcium orange-red
- Copper green
Metal hydroxide precipitates
Adding sodium hydroxide solution to a sample produces a coloured precipitate that identifies the metal ion.
The precipitate colour identifies the specific ion, not the whole compound.
Metal hydroxide precipitates — Key Knowledge
- Copper II) (blue precipitate
- Iron II) (green precipitate
- Iron III) (brown precipitate
- Aluminium/calcium/magnesium white precipitate
Carbonate and sulfate tests
Carbonates are identified by adding dilute acid and testing the gas with limewater. Sulfates are identified using barium chloride solution.
Both tests rely on producing a visible result — gas or precipitate — from a specific ion.
Carbonate and sulfate tests — Key Knowledge
- Carbonate test add dilute acid — gas turns limewater milky confirms CO₂ and carbonate present
- Sulfate test add dilute HCl then barium chloride solution — white precipitate confirms sulfate present
Halide tests
Halide ions are identified by adding dilute nitric acid followed by silver nitrate solution. The precipitate colour indicates which halide is present.
The silver nitrate test distinguishes between the three common halide ions by colour alone.
Halide tests — Key Knowledge
- Chloride white precipitate
- Bromide cream precipitate
- Iodide yellow precipitate
- Dilute nitric acid added first to remove interfering ions
Instrumental methods and flame emission spectroscopy
Modern laboratories use instrumental methods instead of chemical tests. Flame emission spectroscopy identifies metals by the wavelengths of light they emit.
Instruments give objective numerical data rather than relying on human judgement of colours.
Instrumental methods and flame emission spectroscopy — Key Knowledge
- Advantages of instrumental methods more sensitive, faster, more accurate, can test very small samples
- Flame emission spectroscopy produces a line spectrum unique to each element
- Line spectrum can distinguish metals with similar flame colours