Chemical analysis — GCSE Chemistry Revision
Everything you need to revise chemical analysis for GCSE Chemistry: clear notes, the key facts and terms to learn, the mistakes that cost students marks, and practice questions with answers.
Revision notes
Purity and Formulations
A pure substance consists of only one element or compound, not mixed with any other substances. Pure substances melt and boil at specific, fixed temperatures. Impurities lower the melting point and raise the boiling point. Formulations are useful mixtures designed for a specific purpose, where each chemical component is present in a measured quantity to give the product the required properties. Examples include fuels, paints, medicines, and cleaning products. Understanding formulations is crucial for quality control and product development.
Chromatography
Chromatography is a separation technique used to identify substances in a mixture. It involves a stationary phase (like chromatography paper or silica gel) and a mobile phase (a solvent). Substances separate because they have different solubilities in the mobile phase and different affinities for the stationary phase. The more soluble a substance in the mobile phase, and the less it adheres to the stationary phase, the further it travels. This allows for the identification of components based on their unique travel distances.
Rf Values in Chromatography
The Rf value (retardation factor) is a ratio used to identify compounds in chromatography. It is calculated as: Rf = (distance travelled by spot) / (distance travelled by solvent front). Rf values are always less than or equal to 1.0. A pure substance will produce only one spot with a unique Rf value under specific conditions. If a substance produces multiple spots or an Rf value that doesn't match a known pure substance, it indicates an impurity or a mixture. Rf values help in substance identification.
Tests for Cations (Metal Ions)
Cations like Cu²⁺, Fe²⁺, Fe³⁺, and Al³⁺ can be identified using sodium hydroxide solution. Copper(II) ions form a blue precipitate. Iron(II) ions form a green precipitate. Iron(III) ions form a brown precipitate. Aluminium ions form a white precipitate which redissolves in excess sodium hydroxide. Calcium, magnesium, and ammonium ions also have characteristic tests. Flame tests are also used for some metal ions, producing distinct colours.
Key facts
- Pure substances have fixed melting and boiling points.|Formulations are mixtures designed for a specific use.|Chromatography separates mixtures based on solubility and adsorption.|The Rf value helps identify substances in chromatography.|Flame tests identify specific metal ions by their characteristic colours.|Sodium hydroxide solution is used to test for various metal cations.
Key terms
- Pure substance::A single element or compound, not mixed with any other substance.|Formulation::A mixture designed as a useful product, with each component added in a specific proportion.|Chromatography::A technique used to separate and identify components in a mixture.|Rf value::The ratio of the distance moved by the spot to the distance moved by the solvent front in chromatography.|Stationary phase::The non-moving phase in chromatography, e.g., paper or silica.|Mobile phase::The moving phase in chromatography, usually a solvent.
Common mistakes
- Confusing the stationary phase and mobile phase in chromatography.|Incorrectly calculating Rf values (e.g., spot distance / paper length).|Not knowing that impurities lower melting points and raise boiling points.|Forgetting to wear safety goggles during flame tests or chemical tests.
Exam tips
- Memorise the characteristic colours for flame tests and precipitate tests for common ions.|Practice calculating Rf values; remember it's always a fraction ≤ 1.|Understand why different substances separate in chromatography (solubility/adsorption).|Be able to identify common formulations and explain why they are useful mixtures.
Quick quiz
1. Which of the following is a characteristic of a pure substance?
- It boils over a range of temperatures.
- It always forms a solution.
- It has a fixed melting point.
- It is always a compound.
Show answer
It has a fixed melting point. — Pure substances, whether elements or compounds, melt and boil at specific, fixed temperatures. Impurities cause these points to vary over a range.
2. In paper chromatography, what is the mobile phase?
- The chromatography paper.
- The solvent.
- The pencil line.
- The origin.
Show answer
The solvent. — The mobile phase is the solvent that moves up the paper, carrying the dissolved substances with it. The paper itself is the stationary phase.
3. Which of these metal ions would produce a blue precipitate when tested with sodium hydroxide solution?
- Fe²⁺
- Cu²⁺
- Al³⁺
- Fe³⁺
Show answer
Cu²⁺ — Copper(II) ions (Cu²⁺) react with hydroxide ions to form a characteristic blue precipitate of copper(II) hydroxide.
4. A spot travels 4.5 cm up a chromatography paper. The solvent front travels 9.0 cm. What is the Rf value?
- 0.5
- 2.0
- 4.5
- 9.0
Show answer
0.5 — Rf = (distance travelled by spot) / (distance travelled by solvent front) = 4.5 cm / 9.0 cm = 0.5.
5. Which of the following is an example of a formulation?
- Pure water.
- Diamond.
- Paint.
- An element like oxygen.
Show answer
Paint. — Paint is a mixture of pigments, solvents, binders, and additives, all combined in specific proportions to achieve desired properties, making it a formulation. Pure water, diamond, and oxygen are pure substances.
Exam-style questions
Describe how paper chromatography could be used to determine if a food dye is pure or a mixture. [3 marks]
Show mark scheme
- Draw a pencil line near the bottom of chromatography paper and place a spot of the food dye on it.
- Place the paper in a suitable solvent, ensuring the solvent level is below the pencil line.
- If the dye separates into multiple spots, it is a mixture; if only one spot moves (or stays put), it is pure.
A student tests an unknown solution with sodium hydroxide. A green precipitate forms. What ion is likely present? What would be observed if excess sodium hydroxide was added? [3 marks]
Show mark scheme
- The ion present is Iron(II) / Fe²⁺.
- A green precipitate forms (iron(II) hydroxide).
- Adding excess sodium hydroxide would result in no further change / the precipitate would not redissolve.
Explain why a pure substance has a sharp melting point, whereas an impure substance melts over a range of temperatures. [4 marks]
Show mark scheme
- A pure substance consists of only one type of particle.
- It has a specific amount of energy required to overcome intermolecular forces at a fixed temperature.
- An impure substance contains different types of particles / foreign particles.
- These impurities disrupt the regular arrangement of particles and interfere with the forces, requiring different amounts of energy over a range of temperatures to melt.
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