Purity and Formulations

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CognitoPurity and formulations

Key facts & methods

What does pure mean in chemistry?

In chemistry, pure means a single element or compound, not mixed with any other substance.
Pure water = H2O only. Tap water is impure (contains dissolved salts, chlorine etc.).
Note: everyday meaning of pure (e.g. pure orange juice) is different from the chemistry definition.
Test for purity — melting point: pure substance has a sharp, precise melting point.
Test for purity — boiling point: pure substance boils at a precise temperature.
Impurities: lower the melting point AND widen the melting range. Raise the boiling point.

Formulations

A formulation is a mixture designed to have specific useful properties.
Exact proportions of components are important — the ratio matters.
Examples: medicines (active drug + excipients in precise amounts), fuels (with additives), cleaning products, cosmetics, alloys, food products.
Each component serves a purpose: solubility, stability, taste, effectiveness.
Compare to ordinary mixtures: formulations are carefully designed, not arbitrary.

Practical and exam relevance

Know how to test purity using melting/boiling point — narrow precise range = pure; wide range = impure.
Know what a formulation is and be able to give examples with reasons for the exact composition.
Know that chromatography distinguishes pure (one spot) from impure (multiple spots).

Exam questions — 4 questions · 11 marks · AQA 8462 style

1 markPure substance melting pointAQA 8462 style

A pure substance is heated slowly. What will you observe at its melting point?

A It melts over a range of 10-15°C

B It melts sharply at one precise temperature

C It cannot melt — pure substances remain solid

D Its melting point changes depending on the mass of sample

1 markEffect of impuritiesAQA 8462 style

How do impurities affect the melting point of a substance?

A They increase the melting point and sharpen it

B They have no effect on the melting point

C They lower the melting point and widen the melting range

D They increase the melting point but widen the range

4 marksFormulation exampleAQA 8462 style

A pharmaceutical company produces an aspirin tablet. The tablet contains aspirin, starch, magnesium stearate and a coating. Explain why this is a formulation rather than just a mixture, and what role two of the additional components might play. (3 marks)

Hint: Define formulation (precisely designed mixture). Explain the role of two named components — be specific about what each does.

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Purity and Formulations

What does pure mean in chemistry?

Formulations

Practical and exam relevance

Module complete! 🎉