Pure Substances & Mixtures

A pure substance is one kind of matter (element or compound); a mixture combines two or more. Pure substances have fixed melting and boiling points; mixtures do not. Mixtures can be homogeneous or heterogeneous and are separated by filtration, evaporation, distillation, or chromatography using property differences. Slide through particle models and separation methods to see the contrasts.

Pure Substance vs Mixture

Type0

💡 Pure Substance vs Mixture

①Pure substance: one kind of material (element or compound)

②Mixture: two or more pure substances mixed

③Pure substances: constant boiling/melting points

④Mixtures: variable boiling/melting points

Classification of Matter

🔍 Identification Tips

①Elements: cannot be broken down further (e.g. gold, iron, oxygen)

②Compounds: two or more elements bonded (water = H + O)

③Homogeneous mixtures: uniform throughout

④Heterogeneous mixtures: composition varies by spot

Methods of Separation

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Separation Principle

Use property differences → separate mixtures

Particle size, boiling point, solubility, migration speed, etc.

🧪 Separation Methods

①Filtration: particle-size difference (sand + water)

②Evaporation: boiling-point difference removes solvent (salt water → salt)

③Distillation: separates liquids by boiling point (water + ethanol)

④Chromatography: migration-speed difference (separates ink dyes)

Exam Key Points

Core Summary

Pure: constant properties | Mixture: variable

Pure substances have fixed boiling/melting points; mixtures do not

🎯 Exam Key Points

①Pure = element + compound

②Mixture = homogeneous + heterogeneous

③Test for pure: are boiling/melting points constant?

④Filtration (size), evaporation/distillation (boiling), chromatography (migration)

⑤Air is a mixture! (nitrogen + oxygen + others)

Examples and Unit Test

Example 1

Among water (H₂O), sugar water, and gold, which is a mixture of two or more substances?

A pure substance is a single kind of substance (element or compound), while a mixture is two or more pure substances mixed together.

Water and gold are pure substances, while sugar water is a mixture of water and sugar.

▸ Sugar water (a mixture of water and sugar)

A single kind is a pure substance; two or more mixed is a mixture.

Example 2

One substance kept a constant temperature while boiling; another kept changing temperature while boiling. Which is the pure substance and which is the mixture?

A pure substance has a constant boiling point and melting point, but a mixture does not.

So the one with a constant temperature while boiling is the pure substance, and the one whose temperature changed is the mixture.

▸ Constant temperature = pure substance / changing temperature = mixture

A pure substance has a constant boiling point; a mixture does not.

Unit-test style

Which statement about pure substances and mixtures is correct?

①Sugar water is a pure substance

②A pure substance has a constant boiling point and melting point

③A mixture is made of only one kind of substance

④Water (H₂O) is a mixture

⑤A heterogeneous mixture has the same composition everywhere

▸ ② A pure substance has a constant boiling point and melting point

A pure substance is a single kind of substance, so its boiling point and melting point are constant.

Sugar water is a mixture (①), a mixture is two or more substances mixed (③), water is a compound and a pure substance (④), and a heterogeneous mixture has a different composition by location (⑤).

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