Knowledge Organiser: Purity and Formulations
Key Definitions
- Pure substance: one type of element/compound only
- Formulation: designed mixture with measured component quantities
- Melting point depression: impurities lower melting point
- Boiling point elevation: impurities raise boiling point
Pure Substance Properties
- Sharp, fixed melting point
- Sharp, fixed boiling point
- Temperature constant during state change
- One spot on chromatogram
Effect of Impurities
- Lower the melting point
- Raise the boiling point
- Cause melting/boiling over a range
- Broader range = more impurities
Formulation Examples
- Paint: pigment + binder + solvent
- Medicines: active ingredient + binder + filler
- Petrol: hydrocarbons + octane improvers
- Alloys: e.g. steel = iron + carbon
Key Equations
- No calculation equations — descriptive topic
- Pure substance: melting point is sharp (single temperature, not a range)
- Impure substance: melts over a range; melting point is LOWER than pure form
Common Mistakes
- Saying impurities raise the melting point: Impurities LOWER the melting point and cause melting to occur over a range of temperatures — not at a sharp fixed point
- Confusing pure (chemistry) and pure (everyday): In chemistry, "pure" means a single element or compound — fruit juice is NOT pure in the chemistry sense
- Saying formulations are pure substances: Formulations are designed MIXTURES — they contain more than one component in specific proportions
- Forgetting that impurities also raise boiling point: Impurities lower melting point AND raise boiling point — examiners ask about both
Memory Aid
PURE = one type of substance ONLY, SHARP melting point To remember what impurities do to melting and boiling points: "IMPURITIES: Melt LOW, Boil HIGH — they spread the range WIDE" Think of salting icy roads (lowers freezing/melting point) and salting cooking water (raises boiling point) — both effects in one real-world example. Quick Check: State TWO ways you could test whether a sample of an organic compound is pure. Show Answer 1. Measure the melting point — if it melts sharply at the known literature value, it is likely pure. 2. Run paper or thin-layer chromatography — a single spot confirms a single compound (pure substance).