Pure Substances and Formulations

SPEC 4.8.1.1 & 4.8.1.2 • PAPER 2

Two short-answer definitions worth easy marks — if you avoid the ‘pure water’ trap and learn what makes a mixture a formulation.

Why This Lesson Matters

This is a small spec section, but a reliable one for the exam. Two ideas you must lock down:

Idea 1

The chemistry meaning of pure is not the everyday meaning. Examiners deliberately give you contexts (mineral water, ground water) where the everyday word and the chemistry word disagree.

Idea 2

A formulation is a mixture — but a designed one. Just saying “a mixture” will not score the mark.

Both ideas appear as 1–2 mark questions, often inside larger questions on water treatment, alloys or paints.

Pure: The Chemistry Definition

Spec definition

A pure substance is a single element or a single compound, not mixed with any other substance.

Two cases only:

Pure element

One type of atom only. Examples: copper (Cu), oxygen gas (O₂), diamond (C).

Pure compound

Two or more elements chemically bonded in fixed ratio. Examples: water (H₂O), sodium chloride (NaCl), carbon dioxide (CO₂).

Note

Air is a mixture of nitrogen, oxygen and argon — not a pure substance, even though it’s ‘just gases’.

Pure Substances Have Fixed Melting and Boiling Points

The diagnostic test for purity is the melting or boiling point.

Pure substance

Melts at a specific (sharp, fixed) temperature.

Boils at a specific temperature.

The value matches the data-book value for that compound (e.g. pure water boils at 100 °C at 1 atm).

Mixture (impure sample)

Melts or boils over a range of temperatures.

The value is shifted from the pure-substance value (e.g. salt water boils above 100 °C).

The wider the range, the more impure the sample.

Diagnostic

Sharp melting/boiling point at the expected value → pure. Range or shifted value → impure / mixture.

Heating Curves: Pure vs Impure

Plot temperature against time as you heat a solid. The shape of the melting plateau tells you everything.

Read this graph

The flat purple plateau is a pure substance melting at one fixed temperature. The dashed mixture line melts over a temperature range below the pure m.p. — that range is the diagnostic sign of an impure substance.

Worked Example: Reading a Data Table

A student tests three solid samples. The data-book value for the pure compound is melting point 80 °C.

SampleMelting point recordedPure or impure?

A80 °C (sharp)Pure — matches the data-book value at a single, fixed temperature

B72–78 °CImpure — melts over a 6 °C range and below the pure value

C80–84 °CImpure — melts over a range and above the pure value

Mark scheme

To score, you must reference both features — the deviation from the pure value and the range. Either feature alone is usually only worth 1 of 2 marks.

‘Pure’ in Everyday English

The spec deliberately points out a second meaning of the word ‘pure’:

Everyday meaning

A pure substance can mean a substance that has had nothing added to it, so it is unadulterated and in its natural state, e.g. ‘pure milk’.

The two meanings clash on real-world labels:

ProductEveryday ‘pure’?Chemistry ‘pure’?

‘Pure milk’ (spec example)YesNo — mixture of water, fats, proteins, sugars, salts

Bottled mineral waterYesNo — contains dissolved Ca²⁺, Mg²⁺, etc.

‘100% pure’ orange juiceYesNo — mixture of water, sugars, acids, vitamins

Distilled water (lab)YesYes — effectively only H₂O

Copper wire (electrical grade)YesYes — essentially one element

Examiner Alert: The ‘Pure Water’ Trap

Examiner Alert P2 Jun22 Q05.5

Question gave ground water containing sodium ions and chloride ions, sold by a company as ‘pure water’. The stem stated this was not chemically pure. Students were asked what the company meant by ‘pure’.

Examiner verdict: Roughly a third of candidates scored. The rest typically claimed the water was ‘chemically pure’ — even though the stem had already ruled that out.

Mark scheme — accepted answers

One mark for any of: ‘unadulterated’, ‘in its natural state’, ‘nothing is added’, or ‘contains no microbes’.

Lesson

If the question stem says the water contains dissolved ions, do not claim it is chemically pure. Switch to the everyday meaning of pure.

Practice 1: Pure Boiling Point

The data book lists the boiling point of pure ethanol as 78 °C. A student tests a colourless liquid in the lab. It boils between 79 °C and 86 °C.

Explain what this tells you about the liquid. 2 marks

PThe liquid is not pure ethanol; it is a mixture / impure.

EPure ethanol would boil at a single fixed temperature of 78 °C. The sample boils over a range (79–86 °C) and the boiling point is shifted from the pure value.

EA range of boiling temperatures is the diagnostic sign of a mixture, because different components boil at different temperatures.

Formulation: Definition

Spec definition

A formulation is a mixture that has been designed as a useful product.

Each chemical in the mixture has a particular purpose. Formulations are made by mixing the components in carefully measured quantities to ensure the product has the required properties.

Three things a formulation IS

1. A mixture (more than one substance).

2. Designed for a specific purpose.

3. Components present in precise, measured proportions.

Lose any of those three and it is not a formulation. A random mix of substances is not a formulation; a single pure substance is never a formulation.

Examples of Formulations

The spec lists seven categories you should recognise:

Fuels

Petrol = mix of hydrocarbons + additives, blended to give the right combustion properties.

Cleaning agents

Surfactants, solvents, fragrances and preservatives in measured ratios.

Paints

Pigment + binder + solvent + additives. Each gives a specific property.

Medicines

Active drug + binder + filler + coating, in precise doses.

Alloys

Metal mixed with other metals or non-metals (e.g. brass, steel, solder).

Fertilisers

NPK fertilisers blend nitrogen, phosphorus and potassium compounds.

Foods

Many processed foods (e.g. bread, sauces) are formulations of carefully measured ingredients.

Spec note

You do not need to memorise the names of components in any specific brand or product.

How to Identify a Formulation

Exam questions often give you a list of substances or a data table and ask which is a formulation. Apply the three-part test:

Test 1

Is it a mixture? (If it’s a single element or compound, it is not a formulation.)

Test 2

Are the components present in specific proportions? (Look for percentages or named amounts.)

Test 3

Is it designed for a useful purpose? (Crude oil straight from the ground is a mixture but not designed — not a formulation. Petrol blended for cars is.)

Quick filter

Pure substance → not a formulation. Random natural mixture (sea water, crude oil) → not a formulation. Designed mixture with measured proportions → formulation.

Worked Example: Alloys Are Formulations

Two lead-free solders are widely used in electronics. Both are tin-rich alloys, but they differ in how much copper and silver they contain — and that changes the melting point:

SolderTin (%)Copper (%)Silver (%)Melting point

Tin–copper99.30.70227 °C

SAC30596.50.53.0217 °C

Why are these formulations? Each is a mixture of metals (test 1 ✓), the percentages are precisely measured (test 2 ✓), and each is designed for a specific job: tin–copper is cheaper but melts hotter; SAC305 melts lower so it’s gentler on heat-sensitive components (test 3 ✓).

Cross-link

Alloys reappear in Module 10.07. The same logic underlies P2 Nov21 Q08.2: when two solders contain the same metals but melt at different temperatures, the mark scheme accepts ‘the proportions (of metals) are different’.

Practice 2: Spot the Formulation

Look at the four substances below. State which one is a formulation and explain why the others are not. 3 marks

A. Pure copper wire B. Sea water C. Brass: 70% copper, 30% zinc, used for door fittings D. Carbon dioxide gas

PC (brass) is the formulation.

EIt is a mixture (copper + zinc) in precise proportions (70:30), designed for a useful purpose (door fittings).

EA and D are pure substances (one element / one compound), so cannot be formulations. B is a mixture, but it is not designed — sea water occurs naturally with variable composition, so its components are not in carefully measured quantities.

Common Errors

Wrong answerCorrect answerWhy it matters

“Tap water is pure because it’s drinkable”“Tap water is a mixture; pure means a single substance”Confuses everyday and chemistry meanings

“A formulation is a mixture”“A formulation is a mixture designed as a useful product, with components in precise proportions”“Mixture” alone is not enough for the mark

“A mixture has a fixed boiling point”“A mixture boils over a range; pure substances have a fixed boiling point”Reverses the diagnostic test

“A pure substance must be an element”“A pure substance is a single element or a single compound”Compounds (e.g. pure water) count as pure

Phrases the Mark Scheme Rewards

Pure (chem)

“A single element or compound, not mixed with any other substance.” (spec 4.8.1.1)

Pure (everyday)

“Unadulterated” / “in its natural state” / “nothing is added” / “contains no microbes”. (MS Jun22 P2 Q05.5)

Purity test

“A pure substance has a sharp/fixed melting (or boiling) point; a mixture melts (or boils) over a range.” (typical MS pattern)

Formulation

“A mixture designed as a useful product, with components in carefully measured quantities, each having a specific purpose.” (spec 4.8.1.2)

Proportions

“The proportions (of components) are different.” (MS Nov21 P2 Q08.2 — alloys)

Lesson Recap

Pure

A single element or compound. Sharp, fixed melting/boiling point at the data-book value. Mixtures show a range and shifted values.

Two meanings

‘Pure’ in chemistry ≠ ‘pure’ in everyday English (= unadulterated, natural state). Read the question stem before deciding which meaning is being tested.

Formulation

A mixture designed as a useful product, with components in precise measured quantities, each having a specific purpose. Examples: fuels, cleaning agents, paints, medicines, alloys, fertilisers, foods.

Up next

Lesson 8.02: Chromatography (RP6) — the separation method that uses purity ideas directly, including the Rf calculation that examiners report students inverting.