Topic 8 of 10

Chemical Analysis

Be the detective of chemistry — learn how to identify unknown substances through flame tests, precipitate reactions, chromatography, and gas tests.

AQA Hub › Topic 8

Purity & Formulations

What is Purity?

In chemistry, a pure substance contains only one type of element or compound. A pure substance has a sharp, fixed melting point. An impure substance melts over a range of temperatures and has a lower melting point.

If a substance melts sharply at a specific temperature, it's pure. If it melts over a range, it's impure. Use melting point data to determine purity.

Formulations

A formulation is a mixture designed as a useful product. Each component is present in a measured quantity and contributes to the properties of the final product.

  • Paints: pigment, binder, solvent
  • Medicines: active ingredient, stabilisers, flavourings
  • Fuels: blend of hydrocarbons
  • Cleaning agents: surfactants, water, fragrances

Chromatography

Chromatography separates mixtures of dissolved substances. It works because different substances have different attractions to the mobile phase (the solvent) and the stationary phase (the paper).

Paper Chromatography

  1. Draw a pencil line near the bottom of chromatography paper.
  2. Place spots of the samples on the line.
  3. Stand the paper in a solvent (the solvent must be below the pencil line).
  4. As the solvent rises, it carries different substances different distances.

Rf Values

Rf = distance moved by substance ÷ distance moved by solvent

Rf values are always between 0 and 1. Each substance has a unique Rf value for a specific solvent — this allows identification.

A substance travels 4.2 cm. The solvent front travels 6.0 cm. Calculate Rf.

Rf = 4.2 ÷ 6.0 = 0.70

The pencil line is drawn in pencil (not pen) because pencil is insoluble and won't dissolve in the solvent. The solvent level must be below the spots so they don't dissolve directly.

Flame Tests

Different metal ions produce characteristic flame colours when heated in a Bunsen flame. This allows identification of the metal present.

  • Lithium (Li⁺): Crimson red
  • Sodium (Na⁺): Yellow
  • Potassium (K⁺): Lilac
  • Calcium (Ca²⁺): Orange-red
  • Copper (Cu²⁺): Green

Method

  1. Clean a nichrome wire loop by dipping it in hydrochloric acid and holding it in a blue Bunsen flame until no colour is seen.
  2. Dip the clean wire into the sample.
  3. Hold the sample in the flame and observe the colour.
The wire must be cleaned thoroughly between tests to avoid contamination from previous samples.

Metal Hydroxide Precipitates

Adding sodium hydroxide (NaOH) solution to solutions containing metal ions produces coloured precipitates that identify the ion.

  • Calcium (Ca²⁺): White precipitate — Ca(OH)₂
  • Magnesium (Mg²⁺): White precipitate — Mg(OH)₂
  • Aluminium (Al³⁺): White precipitate — Al(OH)₃ (dissolves in excess NaOH)
  • Copper(II) (Cu²⁺): Blue precipitate — Cu(OH)₂
  • Iron(II) (Fe²⁺): Green precipitate — Fe(OH)₂
  • Iron(III) (Fe³⁺): Brown precipitate — Fe(OH)₃
Aluminium and magnesium both give white precipitates. To distinguish them, add excess NaOH: if it dissolves, it's aluminium (amphoteric hydroxide).

Testing for Carbonates

To test for carbonate ions (CO₃²⁻), add dilute hydrochloric acid. Carbonates fizz (effervesce) as they decompose to produce carbon dioxide gas.

CO₃²⁻ + 2H⁺ → H₂O + CO₂

Confirm CO₂ by passing it through limewater — it turns milky (cloudy).

Testing for Halides

Add dilute nitric acid then silver nitrate solution (AgNO₃).

  • Chloride (Cl⁻): White precipitate — AgCl
  • Bromide (Br⁻): Cream precipitate — AgBr
  • Iodide (I⁻): Yellow precipitate — AgI
You must add dilute nitric acid first to remove carbonate and sulfate ions that would also form precipitates and interfere with the test.

Testing for Sulfates

Add dilute hydrochloric acid then barium chloride solution (BaCl₂).

A white precipitate of barium sulfate (BaSO₄) confirms sulfate ions are present.

Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)

Tests for Gases

  • Hydrogen (H₂): Hold a burning splint near the gas. It burns with a squeaky pop.
  • Oxygen (O₂): Insert a glowing splint. It relights.
  • Carbon dioxide (CO₂): Bubble through limewater. It turns milky (cloudy).
  • Chlorine (Cl₂): Hold damp litmus paper near the gas. It bleaches the paper white.
These four gas tests are required practical knowledge — you must know them for the exam.

Instrumental Methods (HT)

Modern instrumental methods are fast, accurate, and can detect very small quantities. They include:

  • Flame emission spectroscopy: Identifies metal ions by the wavelength of light emitted when heated.
  • Mass spectrometry: Measures the relative mass of molecules.

Advantages over traditional methods: more sensitive, more accurate, faster, require smaller samples.