Pure Substances and Mixtures

This lesson covers pure substances, mixtures and formulations as required by the Edexcel GCSE Combined Science specification (1SC0). You need to understand how purity is defined in chemistry, how melting and boiling points can be used to assess purity, and what a formulation is.

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Purity in Chemistry

In everyday language, "pure" usually means "natural" or "nothing added" — for example, "pure orange juice." However, in chemistry, pure has a very specific meaning:

A pure substance in chemistry is one that consists of only a single element or a single compound. Nothing else is mixed with it.

Examples

Substance	Pure or Mixture?	Reason
Distilled water (H₂O)	Pure	Contains only water molecules
Tap water	Mixture	Contains dissolved minerals and salts
24-carat gold	Pure	Contains only gold atoms
18-carat gold	Mixture (alloy)	Contains gold, silver and copper
Clean air	Mixture	Contains nitrogen, oxygen, argon and other gases

Exam Tip: "Pure" in chemistry means only one substance — not "clean" or "healthy." Pure hydrochloric acid is a pure substance even though it is dangerous. Mineral water is a mixture even though it is "pure" in everyday language.

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Testing for Purity Using Melting and Boiling Points

A pure substance has a sharp, fixed melting point and a sharp, fixed boiling point that match the known literature values.

An impure substance (a mixture):

• Has a lower melting point than expected.
• Has a higher boiling point than expected.
• Melts and boils over a range of temperatures rather than at a single fixed value.

Using Data to Determine Purity

Sample	Melting Point (°C)	Known MP of Pure Substance	Conclusion
A	0	0 (water)	Likely pure
B	−3 to 1	0 (water)	Impure — melts over a range and below 0 °C
C	801	801 (NaCl)	Likely pure
D	785–795	801 (NaCl)	Impure — melts over a range below 801 °C

graph TD
    A["Measure melting point<br/>of sample"] --> B{"Does it match the<br/>known literature<br/>value exactly?"}
    B -->|"Yes, sharp<br/>fixed MP"| C["Substance is<br/>likely PURE"]
    B -->|"No, lower and/or<br/>melts over a range"| D["Substance is<br/>IMPURE (mixture)"]

    style A fill:#2c3e50,color:#fff
    style B fill:#2980b9,color:#fff
    style C fill:#27ae60,color:#fff
    style D fill:#e67e22,color:#fff

Exam Tip: If given melting point data in the exam, check two things: (1) Does the value match the expected value? (2) Is it a single value or a range? A range always indicates a mixture.

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Mixtures

A mixture contains two or more substances that are not chemically bonded together. The components of a mixture:

• Retain their individual properties.
• Can be present in any proportion (the ratio is variable).
• Can be separated by physical methods — no chemical reaction is needed.

Comparing Compounds and Mixtures

Feature	Compound	Mixture
Bonding	Elements are chemically bonded	Substances are NOT chemically bonded
Proportions	Fixed ratio of atoms	Any proportion
Properties	Different from constituent elements	Each substance retains its own properties
Separation	Chemical reactions only	Physical methods (filtration, distillation, etc.)

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Formulations

A formulation is a mixture that has been designed as a useful product by carefully choosing specific quantities of each component.

Examples of Formulations

Formulation	Components
Paint	Pigment (colour), solvent (liquid), binder (holds pigment), additives
Medicine	Active ingredient (drug), bulking agent, flavourings, coatings
Fertiliser	Specific proportions of nitrogen, phosphorus and potassium compounds
Cleaning products	Surfactants, solvents, fragrances, colourants
Alloys	Specific metals in set proportions (e.g. steel = iron + carbon)
Fuels	Blends of hydrocarbons with additives

Key Features of Formulations

• Each component has a specific purpose.
• The proportions of each component are carefully controlled.
• The formulation is designed to have specific properties for its intended use.
• Changing the proportions changes the properties of the product.

Exam Tip: A formulation is a mixture — it is not a pure substance and it is not a compound. The key idea is that each component is present in a precise quantity to give the product the desired properties.

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Why Are Formulations Important?

Formulations are used across many industries:

Industry	Example	Why Formulation Matters
Pharmaceutical	Paracetamol tablets	Correct dose of active ingredient, with binding agents and coatings
Agriculture	NPK fertiliser	Correct ratio of nutrients for plant growth
Cosmetics	Moisturiser	Balance of water, oils and active ingredients
Food	Ready meals	Correct balance of flavour, preservation and nutrition

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Summary

• In chemistry, a pure substance contains only a single element or compound.
• Pure substances have sharp, fixed melting and boiling points matching known values.
• Impure substances (mixtures) melt at lower temperatures, boil at higher temperatures, and change state over a range.
• A mixture contains two or more substances not chemically bonded, which can be separated by physical methods.
• A formulation is a carefully designed mixture where each component is present in a precise quantity for a specific purpose.
• Examples of formulations include paints, medicines, fertilisers, cleaning products and alloys.

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Worked Example — Identifying Purity from Melting Point Data

A student is given four samples labelled A, B, C and D. They believe each could be either pure water (MP = 0 °C) or pure aspirin (MP = 135 °C). The student records:

Sample	Observed melting behaviour
A	Melts sharply at 0 °C
B	Melts from −2 °C to 2 °C
C	Melts sharply at 135 °C
D	Melts from 128 °C to 133 °C

Interpretation