Everything around you is matter. Understanding how matter is classified, how it changes state, and how elements, compounds and mixtures differ is foundational to all science from Sec 1 through to O-Level Chemistry and Physics.
All matter can be classified as a pure substance or a mixture. Pure substances are either elements or compounds.
| Category | Definition | Examples |
|---|---|---|
| Element | Made of only one type of atom; cannot be broken down into simpler substances by chemical means | Oxygen (O), Iron (Fe), Gold (Au), Carbon (C) |
| Compound | Made of two or more different elements chemically joined in a fixed ratio; has different properties from its elements | Water (H₂O), Salt (NaCl), Carbon dioxide (CO₂) |
| Mixture | Two or more substances physically combined; components keep their own properties and can be separated | Air, seawater, soil, brass |
A compound has different properties from the elements that formed it. Water (H₂O) is a liquid at room temperature, but hydrogen and oxygen are both gases. Salt (NaCl) is a white solid, but sodium is a reactive metal and chlorine is a toxic yellow-green gas.
Exam trap: Do not say a compound is a "mixture" of elements. In a compound, elements are chemically bonded; in a mixture, they are physically combined.
| Property | Solid | Liquid | Gas |
|---|---|---|---|
| Shape | Fixed | Takes shape of container | Takes shape of container |
| Volume | Fixed | Fixed | Not fixed — fills the container |
| Compressibility | Cannot be compressed | Very slightly (essentially no) | Easily compressed |
| Ability to flow | Cannot flow | Flows | Flows |
| Density | Usually high | Usually medium | Very low |
| Change of state | Direction | Energy change | Example |
|---|---|---|---|
| Melting | Solid → Liquid | Absorbs (energy in) | Ice melting at 0°C |
| Freezing | Liquid → Solid | Releases (energy out) | Water freezing at 0°C |
| Evaporation | Liquid → Gas | Absorbs | Wet clothes drying; puddles disappearing |
| Condensation | Gas → Liquid | Releases | Water droplets on a cold glass |
| Boiling | Liquid → Gas (throughout) | Absorbs | Water boiling at 100°C |
| Sublimation | Solid → Gas (directly) | Absorbs | Dry ice (solid CO₂) at room temperature; iodine crystals when heated |
Pure substances have sharp, fixed melting and boiling points. Mixtures melt and boil over a range of temperatures — this is used to test for purity.
| Evaporation | Boiling | |
|---|---|---|
| Temperature | Occurs at any temperature below boiling point | Occurs at the boiling point only |
| Location | Surface only | Throughout the liquid |
| Speed | Slow and gradual | Rapid |
| Bubbles | No bubbles | Bubbles throughout |
Trap: Dissolving sugar or salt is a physical change — no new substance forms and the solid can be recovered by evaporation. Students often wrongly call dissolving a chemical change.
| Method | What it separates | Principle | Example |
|---|---|---|---|
| Filtration | Insoluble solid from liquid | Difference in particle size | Sand from water |
| Evaporation | Soluble solid from solution | Liquid evaporates; solid remains | Salt from seawater |
| Distillation | Solvent from solution; liquids with different b.p. | Difference in boiling points | Pure water from salt solution; ethanol from water |
| Chromatography | Dissolved substances of different solubilities | Components move at different rates through a medium | Inks in a felt-tip pen; food colouring |
| Magnetic separation | Magnetic material from non-magnetic mixture | Magnetism | Iron filings from sand |
| Sieving | Solids of different sizes | Size difference | Gravel from sand |
Use chromatography to check whether a substance is pure (one spot = pure) or a mixture (multiple spots at different heights).
Classify each of the following as element, compound or mixture: (a) copper wire, (b) air, (c) carbon dioxide, (d) seawater, (e) pure water.
A student places an ice cube on a warm surface. The ice melts and then the puddle slowly disappears over several hours. (a) Name the change of state that occurs when ice melts. (b) Name the change of state that causes the puddle to disappear. (c) For each change, state whether energy is absorbed or released.
State whether each change below is physical or chemical. Give one reason for each answer. (a) Burning magnesium in oxygen to form magnesium oxide. (b) Dissolving sodium chloride in water. (c) Melting butter in a frying pan. (d) Iron rusting when left in moist air.