What Are Pure Substances and Mixtures?
A pure substance has constant composition and uniform properties throughout. A mixture contains two or more substances that are physically combined but not chemically bonded. Mixtures can be homogeneous (uniform) or heterogeneous (non-uniform).
A pure substance is a single chemical compound or element with fixed properties (melting/boiling points, density). A mixture blends two or more substances and is either homogeneous (one phase, e.g., salt water) or heterogeneous (multiple phases, e.g., sand and water).
- •Fixed composition
- •Constant properties
- •Single melting/boiling point
- •Cannot be separated by physical methods
- •Variable composition
- •Properties depend on ratio
- •No fixed melting/boiling point
- •Can be separated by physical methods
Step-by-step worked examples
Is salt water (sodium chloride + water) a pure substance or mixture?
Salt water is a MIXTURE (homogeneous). It contains two substances: NaCl and H₂O physically combined. Chemical bonds do not form between them. Salt and water can be separated by evaporation.
Is pure oxygen gas (O₂) a pure substance or mixture?
Pure O₂ is a PURE SUBSTANCE (element). All particles are identical O₂ molecules. It has constant properties: boiling point −183°C, density 1.43 g/L. No separation needed; already pure.
Is air (N₂, O₂, Ar, CO₂, etc.) a pure substance or mixture?
Air is a MIXTURE (homogeneous). It contains multiple gases: ~78% N₂, ~21% O₂, ~1% Ar and trace gases. Composition varies slightly by location and altitude. These gases are not chemically bonded.
Flashcards
Quick quiz
Q1.Which is a pure substance?
Q2.Which is a homogeneous mixture?
Q3.How can salt be separated from salt water?
Q4.Which has a fixed melting point?
The full card deck, worked steps and AI-tutor support for “What Are Pure Substances and Mixtures?” are in Notek — study by hand before your exam.
Common mistakes
Confusing homogeneous mixtures with pure substances. — Correct: A homogeneous mixture looks uniform but contains multiple substances; a pure substance is a single compound.
Thinking dissolved salt in water means they bonded chemically. — Correct: Salt dissolves in water physically — no new chemical bonds form. It's still a mixture.
Assuming all visible mixtures are heterogeneous. — Correct: Homogeneous mixtures can look uniform visually but still contain multiple substances (e.g., air, saltwater).
Believing pure substances cannot be separated. — Correct: They can't be separated by physical methods. Chemical methods (like electrolysis) can decompose them.
FAQ
Is an alloy (e.g., bronze) a pure substance or mixture?
It depends — if copper and tin are chemically bonded, it's a compound (pure). If just physically mixed, it's a mixture. Most alloys are considered mixtures.
Can a mixture change to a pure substance?
Yes — by physical separation methods. Evaporate salt water to get pure salt and pure water.
Is milk a homogeneous or heterogeneous mixture?
Milk appears homogeneous to the naked eye, but under a microscope, it is a colloidal mixture (tiny fat particles suspended in water). Often classified as homogeneous for simplicity.
What is a colloid?
A special mixture where one substance is dispersed as fine particles throughout another. Examples: milk, fog, smoke, paint.