Chemical Analysis (Chromatography, Flame Tests, Ion Tests)

Quick-Fire Definitions

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.
R_f value: The ratio of the distance moved by a substance to the distance moved by the solvent front. Unique for each substance in a given solvent.
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.

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
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

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

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).

R_f Values

R_f = distance moved by substance ÷ distance moved by solvent

R_f values are always between 0 and 1. Each substance has a unique R_f value for a specific solvent - this allows identification by comparing with known values.

Worked Example 1: Calculating R_f

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

R_f = 4.2 ÷ 6.0 = 0.70

Worked Example 2: Identifying a substance

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

Step 1: R_f of unknown = 0.55.

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

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

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.

Flame Tests Chemistry Only

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⁺)	Orange-red
Copper (Cu²⁺)	Green

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

Method

Clean a nichrome wire loop by dipping it in hydrochloric acid and holding it in a blue Bunsen flame until no colour is seen.
Dip the clean wire into the sample.
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 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⁺)	White	Ca(OH)₂
Magnesium (Mg²⁺)	White	Mg(OH)₂
Aluminium (Al³⁺)	White (dissolves in excess NaOH)	Al(OH)₃
Copper(II) (Cu²⁺)	Blue	Cu(OH)₂
Iron(II) (Fe²⁺)	Green	Fe(OH)₂
Iron(III) (Fe³⁺)	Brown	Fe(OH)₃

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.

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 = orange-red, Mg = no flame colour).

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⁻ + 2H⁺ → H₂O + CO₂

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⁻)	White	AgCl
Bromide (Br⁻)	Cream	AgBr
Iodide (I⁻)	Yellow	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.

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 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 splint	Squeaky pop
Oxygen (O₂)	Glowing splint	Splint relights
Carbon dioxide (CO₂)	Bubble through limewater	Turns milky (cloudy)
Chlorine (Cl₂)	Damp litmus paper	Bleaches paper 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.

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

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
Sensitivity	Limited	Very high - detect tiny quantities
Speed	Slow	Very fast - rapid results
Sample size	Larger samples needed	Very small samples sufficient
Accuracy	Subjective (e.g. Colour judgement)	Objective - numerical data

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.

Students Also Studied

Previous Topic

Topic 7: Organic Chemistry

Alkanes, alkenes, alcohols, carboxylic acids, polymers and cracking.

Next Topic

Topic 9: Chemistry of the Atmosphere

Greenhouse gases, climate change, carbon footprint and atmospheric pollutants.

Blog Post

All 8 GCSE Required Practicals

Chromatography and ion testing practicals with R_f values and exam tips.

Try These Tools

Interactive Periodic Table

Look up flame test colours and ion properties for chemical analysis.

→

Topic 8 Flashcards

Test yourself with interactive flashcards on purity, chromatography, flame tests and ion identification.

→

Topic 8 Exam Practice

Practice with real AQA past paper questions on chromatography, Rf values, flame tests, ion tests and gas tests. With full mark schemes.

→