To a chemist a pure substance is a single element or a single compound with nothing else mixed in. Distilled water and copper wire are pure. Orange juice sold as "100% pure" is a mixture of water, sugars, acids and much more: pure on the label only means nothing was added.

A pure substance has a fixed melting point and boiling point. Impurities change them:

• a pure solid melts sharply at its own melting point
• an impure solid melts lower, over a range of temperatures
• dissolved impurities raise the boiling point of a liquid

1. Pure stearic acid melts at 69 °C
2. Sample A melts sharply at 69 °C: pure
3. Sample B melts over 61 to 66 °C: impure (lower, and over a range)

A sample that melts sharply at the wrong temperature is not your substance: it may be a different pure substance.

A formulation is a mixture designed as a useful product. Each component has a job and is measured out carefully: paints, medicines, fuels, cleaning products, alloys, fertilisers and foods are all formulations.

percentage of a component = mass of component ÷ total mass × 100

Not on any equation sheet: it is the usual percentage of a whole.

1. Paint: pigment 120 g, binder 180 g, water 240 g, additives 10 g
2. total = 550 g
3. pigment = 120 ÷ 550 × 100 = 21.8% (3 s.f.)
4. In a 2.5 kg tin: 21.8 ÷ 100 × 2.5 = 0.545 kg of pigment

Divide by the total of every component, including the one you are asked about.