Mixture

A mixture combines two or more substances physically without chemical bonding, separable by physical means.

Definition

A mixture is a combination of two or more substances that are physically combined but not chemically bonded. Each substance in a mixture retains its own chemical properties, and the substances can be separated again by physical methods such as filtration, evaporation, distillation, or magnetism.

Key Characteristics

The components of a mixture are not chemically bonded and retain their individual properties.
Mixtures have no fixed composition — the ratio of components can vary.
Mixtures can be homogeneous (uniform throughout, e.g., salt water) or heterogeneous (non-uniform, e.g., sand and gravel).
Mixtures can be separated by physical methods without a chemical reaction.
The boiling point, melting point, and density of a mixture depend on its composition, unlike pure substances which have fixed values.
Air is a mixture of gases (mainly nitrogen 78%, oxygen 21%, argon ~1%, carbon dioxide ~0.04%).

Real-World Example

Sea water is a mixture of water and dissolved salts (mainly sodium chloride). The salt and water are not chemically bonded — sailors and engineers can obtain fresh water from sea water by distillation (heating to evaporate the water and then condensing it), leaving the salt behind. The salinity (saltiness) of sea water varies by location, confirming it has no fixed composition.

Note

A mixture contains two or more substances combined physically but not chemically. The substances keep their own properties and can be separated by physical methods. Homogeneous mixtures (like salt water) look uniform throughout, while heterogeneous mixtures (like muddy water or a salad) have visibly different parts. Unlike compounds, mixtures have variable composition.

Separation Techniques

Filtration — separates an insoluble solid from a liquid (e.g., sand from water).
Evaporation — removes a solvent to recover a dissolved solid (e.g., salt from salt water).
Distillation — separates liquids with different boiling points (e.g., alcohol from water).
Chromatography — separates dissolved substances based on how far they travel in a solvent (e.g., ink colours).
Magnetism — separates a magnetic material from a non-magnetic one (e.g., iron filings from sand).

Practice Problems

Is air a pure substance or a mixture? Give one piece of evidence for your answer.

Show answer

Air is a mixture. Evidence: its composition varies — different locations have slightly different concentrations of gases, and the amount of water vapour changes with humidity. A pure substance would have a fixed, invariable composition.

A student has a mixture of salt and sand. Describe a step-by-step method to obtain pure dry salt from this mixture.

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Step 1 — Add water to dissolve the salt (sand does not dissolve). Step 2 — Filter the mixture to remove the sand. Step 3 — Evaporate the water from the filtrate by gentle heating. The solid left behind is pure salt. The sand is retained in the filter paper.

What is the difference between a homogeneous and a heterogeneous mixture? Give one example of each.

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A homogeneous mixture has a uniform composition throughout with no visible boundaries between components (e.g., salt dissolved in water — you cannot see the salt). A heterogeneous mixture has a non-uniform composition and you can see distinct regions or particles (e.g., oil and water — you can see the two separate layers).

A mixture of ethanol (boiling point 78 °C) and water (boiling point 100 °C) needs to be separated. Which technique should be used, and how does it work?

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Distillation should be used. The mixture is heated until it reaches 78 °C, at which point ethanol evaporates first. The vapour is passed into a condenser where it cools and turns back into liquid ethanol, which is collected separately. Water remains in the original flask as it needs a higher temperature (100 °C) to evaporate.