Nature of Matter: Elements, Compounds, and Mixtures

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Six ways to name matter

Pure or mixed? One element or many combined? Uniform or patchy? These six terms let you classify any sample of matter. Tap each one to see what it means and an everyday example.

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The three big ideas

A pure substance is made of only one kind of particle, with a fixed composition and fixed properties (such as a sharp melting or boiling point). Gold, distilled water and table salt are pure substances.
A mixture is two or more substances simply mixed together. Its composition can vary, each part keeps its own properties, and the parts can be separated by physical methods. Soil, milk, sea water and tea are mixtures.
Homogeneous mixture — the same throughout; you cannot see the separate parts (salt dissolved in water, clean air, a brass-like alloy).
Heterogeneous mixture — not the same throughout; the parts are visible and unevenly spread (sand in water, oil on water, a fruit salad).

Worked example. Is the air around us a pure substance or a mixture?

It is a mixture — and a homogeneous one — made mainly of nitrogen (about 78%) and oxygen (about 21%), plus small amounts of other gases. Because the proportions can change (more moisture, more smoke), its composition is not fixed.

An element is the simplest pure substance — it cannot be broken into anything simpler by chemical means. More than a hundred are known: metals (iron, copper, gold — shiny, conduct), non-metals (oxygen, carbon, sulphur) and a few in between.
A compound is two or more elements chemically combined in a fixed ratio. It has new properties, different from the elements in it. Water is hydrogen + oxygen (two gases → a liquid that puts out fire); common salt is sodium (a reactive metal) + chlorine (a poisonous gas) → edible salt.
Mixture vs compound: a mixture is a physical mix (variable ratio, parts keep their properties, separated physically); a compound is a chemical union (fixed ratio, new properties, separated only chemically).

Worked example. Iron filings are stirred into sulphur powder. How is this different from heating them until they react?

Stirred together they form a mixture — a magnet still pulls out the iron, because each element keeps its properties. Heated until they react, they form the compound iron sulphide — now a magnet cannot remove the iron, because it is chemically combined.

Mixtures are separated using a difference in some property — size (handpicking, sieving), weight (winnowing, sedimentation & decantation), magnetism (a magnet), solubility (filtration, evaporation), or boiling point (distillation).
Filtration separates an insoluble solid from a liquid. Evaporation recovers a dissolved solid. Distillation collects the pure liquid by boiling and condensing it.
A separating funnel separates two liquids that do not mix (oil and water); a magnet separates magnetic material from a mixture.

Worked example. Separate a mixture of iron filings, salt and sand into three pure samples.

1) Run a magnet through it to pull out the iron filings. 2) Add water and filter — the sand stays on the filter paper while the salt dissolves and passes through. 3) Evaporate the salt solution to leave the salt behind.

Common mistake: thinking filtration removes dissolved salt. Filtration only traps an insoluble solid; to get dissolved salt back you must evaporate (or distil) the water.

Where you'll meet it

Separation in everyday life

Getting clean drinking water

Muddy river water is cleared by letting the heavy dirt settle (sedimentation), pouring off the clear water (decantation) and passing it through a filter. Salty water can be distilled to recover pure water.

Salt from the sea

In coastal salt pans, sea water is let into shallow beds and the Sun slowly evaporates the water. The dissolved salt is left behind — separation by evaporation on a huge scale.

Sorting scrap for recycling

At a recycling yard, a large electromagnet picks iron and steel out of a heap of mixed scrap (magnetic separation), so each material can be recycled on its own.

Check yourself

Competency quiz

Modelled on the competency-based pattern — MCQ, assertion–reason and a case study, testing whether you can use the ideas, not just recall them.

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Nature of Matter: Elements, Compounds & Mixtures