Pure Substances and Mixtures

This lesson covers the distinction between pure substances and mixtures, how to use melting point to determine purity, and the concept of formulations, as required by the Edexcel GCSE Chemistry specification (1CH0, Topic 1). Understanding what "pure" means in chemistry — which is different from everyday language — is essential for chemical analysis and separation techniques.

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Pure Substances in Chemistry

In chemistry, a pure substance contains only one type of element or one type of compound. This is very different from the everyday meaning of "pure."

Term	Everyday Meaning	Chemistry Meaning
Pure	Clean, natural, uncontaminated	Contains only one element or compound
Pure water	Clean drinking water	Contains only H\u2082O molecules (nothing else)
Pure orange juice	Juice with no additives	Would mean only one chemical compound (not realistic)
Pure gold	High-quality gold	Contains only gold atoms (24 carat)

Examples of Pure Substances

• Distilled water — contains only H\u2082O molecules.
• Pure iron — contains only Fe atoms.
• Pure sodium chloride — contains only Na\u207a and Cl\u207b ions.
• Diamond — contains only carbon atoms bonded in a giant covalent structure.

Exam Tip: If the exam refers to a "pure substance," use the chemistry definition. A pure substance is a single element or a single compound. Tap water is NOT pure in the chemistry sense because it contains dissolved minerals and other substances, even though we might call it "pure" in everyday life.

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Mixtures

A mixture consists of two or more substances that are not chemically combined. The substances in a mixture:

• Retain their own individual chemical properties.
• Are not bonded to each other.
• Can be separated by physical methods (such as filtration, distillation, chromatography or evaporation).
• Are present in variable proportions — you can change the amounts of each component.

Examples of Mixtures

Mixture	Components
Air	Nitrogen, oxygen, argon, carbon dioxide, water vapour
Sea water	Water, sodium chloride, other dissolved salts
Crude oil	Many different hydrocarbons
Bronze	Copper and tin (an alloy)
Ink	Dyes and solvents

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Melting Point and Purity

One of the most important ways to determine whether a substance is pure is to measure its melting point.

Pure Substances

A pure substance has a sharp, fixed melting point. This means it melts at one specific temperature. For example:

• Pure water melts at exactly 0 \u00b0C.
• Pure aspirin melts at exactly 136 \u00b0C.

Impure Substances (Mixtures)

When an impurity is added to a pure substance:

• The melting point is lowered (it starts melting at a lower temperature).
• The substance melts over a range of temperatures rather than at a single, sharp temperature.

For example, if you add salt to ice:

• The ice begins to melt below 0 \u00b0C.
• It melts over a range (e.g., −5 \u00b0C to −1 \u00b0C) rather than sharply at 0 \u00b0C.

Boiling Points and Impurities

Impurities have the opposite effect on boiling points:

• The boiling point is raised (the substance boils at a higher temperature).
• The substance boils over a range rather than at a single temperature.

How to Test for Purity

To determine whether a substance is pure:

1. Measure the melting point of the substance.
2. Compare it to the known melting point from a data book or reference table.
3. If the melting point is sharp and matches the known value, the substance is pure.
4. If the melting point is lower than expected and occurs over a range, the substance is impure.

Observation	Conclusion
Sharp melting point that matches the known value	Substance is pure
Melting point over a range, below the expected value	Substance is impure (contains impurities)
Sharp melting point but does not match any known value	Substance is pure but is a different substance

Exam Tip: When describing how to test for purity, always include all three steps: measure the melting point, compare it with a known value from a data book, and state whether it is sharp or over a range. This structured approach gains full marks.

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Worked Example: Interpreting Melting Point Data

A student has synthesised aspirin and wants to check its purity. The known melting point of pure aspirin is 136 \u00b0C.

Sample	Observed Melting Point
A	136 \u00b0C (sharp)
B	129–134 \u00b0C (range)
C	118 \u00b0C (sharp)

Analysis:

• Sample A is likely pure aspirin — sharp melting point that matches the known value.
• Sample B is impure aspirin — it melts over a range and below the known value, indicating the presence of impurities.
• Sample C is a pure substance (sharp melting point) but it is not aspirin — the melting point does not match 136 \u00b0C. It is a different substance entirely.

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Formulations

A formulation is a mixture that has been designed as a useful product. Each component in a formulation is present in a carefully measured quantity and contributes specific properties to the final product.

Key Features of Formulations

• Each component is present in a precise, measured quantity.
• Changing the proportions would change the properties of the product.
• They are designed for a specific purpose.

Examples of Formulations

Formulation	Components and Their Purposes
Medicines	Active ingredient (treats condition), binding agent (holds tablet together), bulking agent (makes tablet manageable size), coating (helps swallowing)
Paints	Pigment (colour), solvent (makes it spreadable), binder (sticks to surface), additives (e.g. anti-mould)
Cleaning products	Detergent (removes grease), water (solvent), fragrance (pleasant smell), antibacterial agent
Alloys	Two or more metals in specific proportions (e.g. brass: 70% copper, 30% zinc)
Fertilisers	Specific ratios of nitrogen, phosphorus and potassium (NPK)

Exam Tip: The key phrase for formulations is "each component is present in a measured quantity." This phrase must appear in your definition. Do not confuse a formulation with just any mixture — a formulation is specifically designed with controlled proportions for a particular use.

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Everyday Formulations in Detail

Paints