← AQA GCSE Chemistry/Topic 8

TURN OBSERVATIONS INTO EVIDENCE.

Chemical Analysis

Separate mixtures, identify substances and choose the right chemical test.

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Quick reference: key definitions
Key Definitions

Topic Core Terms

Pure substance
Contains only one type of element or compound. Has a sharp, fixed melting point.
Formulation
A mixture designed as a useful product, with each component in a measured quantity.
Chromatography
A technique for separating dissolved substances based on how they distribute between a mobile phase and a stationary phase.
Rf value
The ratio of the distance moved by a substance to the distance moved by the solvent front. Comparing values under identical conditions aids identification, though different substances can share similar Rf values.
Precipitate
An insoluble solid formed when two solutions are mixed. Used to identify metal ions.
Flame test
A test where a sample is held in a Bunsen flame to identify metal ions by the characteristic colour produced.
CHAPTER 01 · 2 sectionsPurity and separationFormulations and chromatography

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.

Exam Tip

Exam Technique

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
  • Alloys: specific proportions of metals

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.
Paper chromatography and calculating the Rf value a b solvent front separated dye spots pencil start line solvent chromatography paper Calculating Rf Rf = distance moved by substance (a) distance moved by solvent (b) always between 0 and 1, with no units compare Rf values with known substances to identify them

Paper chromatography separates dissolved substances. Different substances travel different distances up the paper, depending on their attraction to the mobile phase (solvent) and stationary phase (paper).

Rf Values

Rf = distance moved by substance ÷ distance moved by solvent

Rf values are always between 0 and 1. Comparing Rf values under identical conditions helps identify substances by comparison with known values, though different substances can share similar Rf values in a given solvent.

Worked Example

Worked Example 1: Calculating Rf

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

Rf = 4.2 ÷ 6.0 = 0.70

Worked Example

Worked Example 2: Identifying a substance

An unknown dye has an Rf of 0.55 in ethanol. Known dyes: Red (0.32), Blue (0.55), Green (0.78). Identify the unknown.

Step 1: Rf of unknown = 0.55.

Step 2: Compare with known values. Rf matches Blue dye (0.55).

Note: If the unknown produced multiple spots, it would be a mixture. A pure substance gives only one spot.

Exam Tip

Exam Technique

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 into the solvent.

CHAPTER 02 · 2 sectionsIdentifying metal ionsFlame colours and hydroxide precipitates

Flame Tests Chemistry Only

This section is only required for Separate Science (Chemistry GCSE) students, not Combined Science.

Different metal ions produce characteristic flame colours when heated in a Bunsen flame.

Metal ion Flame colour
Lithium (Li⁺)Crimson red
Sodium (Na⁺)Yellow
Potassium (K⁺)Lilac
Calcium (Ca²⁺)Brick-red
Copper (Cu²⁺)Green
Flame Test Colours Li⁺ crimson Na⁺ yellow K⁺ lilac Ca²⁺ orange-red Cu²⁺ green nichrome wire loop dipped in the sample, held in a blue Bunsen flame

Each metal ion produces a unique, characteristic flame colour when heated. This allows identification of unknown metal ions in a sample.

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.
Key Concept

Key Fact

The wire must be cleaned thoroughly between tests to avoid contamination from previous samples.

Exam Tip - Masking of Flame Colours

If a sample contains a mixture of metal ions, flame test colours can be masked or obscured. For example, sodium's intense yellow flame will easily hide the lilac flame of potassium or the crimson flame of lithium. Flame tests only work reliably for single pure metal compounds!

Metal Hydroxide Precipitates Chemistry Only

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

Metal ion Precipitate colour Formula
Calcium (Ca²⁺)WhiteCa(OH)₂
Magnesium (Mg²⁺)WhiteMg(OH)₂
Aluminium (Al³⁺)White (dissolves in excess NaOH)Al(OH)₃
Copper(II) (Cu²⁺)BlueCu(OH)₂
Iron(II) (Fe²⁺)GreenFe(OH)₂
Iron(III) (Fe³⁺)BrownFe(OH)₃
Metal hydroxide precipitates with sodium hydroxide Add a few drops of sodium hydroxide solution to a solution of the metal ion Ca²⁺ white Ca(OH)₂ Mg²⁺ white Mg(OH)₂ Al³⁺ white Al(OH)₃ Cu²⁺ blue Cu(OH)₂ Fe²⁺ green Fe(OH)₂ Fe³⁺ brown Fe(OH)₃ dissolves in excess NaOH

Adding sodium hydroxide solution produces coloured precipitates that can identify the metal ion present. Three ions give white precipitates, but aluminium's dissolves in excess NaOH.

Exam Tip

Exam Technique

Aluminium, calcium and magnesium all give white precipitates. To distinguish Al³⁺, add excess NaOH: if the precipitate dissolves, it’s aluminium (amphoteric hydroxide). Ca²⁺ and Mg²⁺ can be distinguished by flame test (Ca = brick-red, Mg = no flame colour).

CHAPTER 03 · 4 sectionsTesting ions and gasesUse observations to identify substances

Testing for Carbonates Chemistry Only

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

CO₃²⁻(aq) + 2H⁺(aq) → H₂O(l) + CO₂(g)

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

Testing for Halides Chemistry Only

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

Halide ion Precipitate colour Formula
Chloride (Cl⁻)WhiteAgCl
Bromide (Br⁻)CreamAgBr
Iodide (I⁻)YellowAgI

Ionic Equations for Halide Precipitation:

Ag⁺(aq) + Cl⁻(aq) → AgCl(s)   (white precipitate)
Ag⁺(aq) + Br⁻(aq) → AgBr(s)   (cream precipitate)
Ag⁺(aq) + I⁻(aq) → AgI(s)   (yellow precipitate)
Testing for halide ions with silver nitrate Add dilute nitric acid, then silver nitrate solution Cl⁻ (chloride) white precipitate AgCl Br⁻ (bromide) cream precipitate AgBr I⁻ (iodide) yellow precipitate AgI

Adding silver nitrate solution (after acidifying with dilute nitric acid) produces coloured precipitates to identify halide ions: white for chloride, cream for bromide, yellow for iodide.

Exam Tip

Exam Technique

You must add dilute nitric acid first to remove carbonate (or sulfite) ions that would also form precipitates with silver and interfere with the test.

Testing for Sulfates Chemistry Only

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

Gas Test Positive result
Hydrogen (H₂)Burning splintSqueaky pop
Oxygen (O₂)Glowing splintSplint relights
Carbon dioxide (CO₂)Bubble through limewaterTurns milky (cloudy)
Chlorine (Cl₂)Damp litmus paperBleaches paper white
Tests for hydrogen, oxygen, carbon dioxide and chlorine Hydrogen, H₂testlit splint heldat the mouthresultsqueaky pop Oxygen, O₂testglowing splintput into the gasresultsplint relights Carbon dioxide, CO₂limewatertestbubble the gasthrough limewaterresultlimewater turnsmilky (cloudy) Chlorine, Cl₂damp blue litmustestdamp litmus paperheld in the gasresultbleached white

The four required gas tests: hydrogen produces a squeaky pop, oxygen relights a glowing splint, carbon dioxide turns limewater milky, and chlorine bleaches damp litmus paper white.

Key Concept

Key Fact

These four gas tests are required practical knowledge - you must know them for the exam.

CHAPTER 04 · 1 sectionInstrumental analysisFast, sensitive methods for separate chemistry

Instrumental Methods Chemistry Only Higher Tier

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

Flame Emission Spectroscopy

A sample is heated in a flame. The light emitted is passed through a spectroscope, which separates it into a line spectrum. Each element produces a unique pattern of spectral lines - this acts like a "fingerprint" for identification.

Flame emission spectroscopy can also measure the concentration of metal ions by comparing the intensity of emitted light with standard reference samples.

Advantages over Traditional Methods

Feature Traditional tests Instrumental methods
SensitivityLimitedVery high - detect tiny quantities
SpeedSlowVery fast - rapid results
Sample sizeLarger samples neededVery small samples sufficient
AccuracySubjective (e.g. Colour judgement)Objective - numerical data
Exam Tip

Exam Technique

A common exam question asks why instrumental methods are preferred. Key answer: they are faster, more sensitive, more accurate, and can handle smaller samples. However, they are expensive and require specialist training.

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