# T.1(a) Nature of Matter

F.3 Chemistry · Topic I(a) · Lesson 1

Classifying matter into elements, compounds and mixtures, and physical versus chemical change.

## Key points

- A **pure substance ／ 純物質** is a single substance that has no other substances mixed with it. A **mixture ／ 混合物** consists of two or more pure substances which **have not chemically combined together**.
- An **element ／ 元素** is a substance which cannot be broken down into anything simpler by **chemical methods**. A **compound ／ 化合物** is composed of two or more elements **chemically joined together**: $\text{water}\xrightarrow{\ \text{electricity}\ }\text{hydrogen}+\text{oxygen}$.
- In an iron/sulphur **mixture** each substance keeps its own properties: only the iron is attracted by a magnet, and only the iron gives hydrogen with dilute hydrochloric acid. Heated together they form **iron(II) sulphide**, a compound with new properties.
- A pure substance has a **sharp** melting point and boiling point; a mixture does not — liquid air has no single boiling point.
- **Physical properties** (colour, state, odour, hardness, density, solubility, m.p., b.p., malleability, conductivity) can be found **without the substance changing into another substance**. **Chemical properties** (flammability, rusting, reactivity, toxicity) describe how it changes into another substance.
- In a **physical change** no new substance is formed: melting, boiling, dissolving, mixing, breaking glass. In a **chemical change** one or more new substances are formed: burning, electrolysis, photosynthesis, heating copper(II) sulphate crystals.

## Must know

| | Element | Compound | Mixture |
|---|---|---|---|
| **Composition** | Fixed | Elements in a fixed ratio by mass | Any proportion |
| **Properties** | Its own | New — different from its elements | Same as its constituents |
| **Separating** | Cannot be split further | Chemical (heating, electrolysis) | Physical (evaporation, distillation, magnet) |
| **M.p. / b.p.** | Sharp | Sharp | Not sharp |
| **Examples** | oxygen, iron, sulphur, diamond, ozone | water, dry ice, sugar, iron(II) sulphide | air, sea water, milk, brass |

## Common mistakes

- ✗ "Dry ice and diamond are elements." ✓ Dry ice is solid carbon dioxide, a **compound**. Diamond, graphite and ozone are **elements** — one kind of atom only.
- ✗ "Air is a compound." ✓ Air is a **mixture**: its gases are not chemically joined and can be present in any proportion.
- ✗ "Insoluble in water is a chemical property." ✓ Solubility, melting point and conductivity are **physical**. A chemical property always involves changing into another substance.
- ✗ "Dissolving sugar forms a new substance." ✓ The sugar can be recovered by evaporating the water — a **physical** change. But heating copper(II) sulphate crystals **is** chemical.
- ✗ "Mineral water is pure, so it is a pure substance." ✓ In chemistry **pure** means one substance only. Mineral water contains dissolved mineral salts, so it is a **mixture**.

## Quick check

1. Classify as element, compound or mixture: ozone, dry ice, brass, distilled water.
2. Iron and sulphur are mixed, then heated. Give one test that shows a compound has formed.
3. Is dissolving sugar in water physical or chemical? Explain in one sentence.

## Answers

1. Ozone — element. Dry ice — compound. Brass — mixture. Distilled water — compound.
2. If the reaction is complete, a magnet no longer attracts the product; or dilute hydrochloric acid gives a bad-egg-smelling gas instead of hydrogen.
3. Physical — no new substance is formed, and the sugar can be recovered by evaporating the water.

## Did you know?

Air is a mixture, so it has no single boiling point. When liquid air is warmed, its gases boil off one after another: nitrogen at about −196 °C and oxygen at about −183 °C. This is how oxygen is separated from air in industry.