States of Matter & Phase Changes
Solids, liquids and gases, and how adding or removing heat changes one state into another.
Lesson objectives
By the end of this lesson, you should be able to:
- Describe solids, liquids and gases by their shape, volume and particles.
- Name the phase changes: melting, freezing, evaporation, boiling, condensation, sublimation and deposition.
- Explain that adding or removing heat causes a change of state.
- Give everyday examples of each phase change.
An ice cube left on the hot ground slowly disappears. Where does it go? To answer, you need to know the three states of matter and how heat changes them.
1. The three states of matter
All matter is made of tiny particles. How tightly they are packed and how freely they move decide whether matter is a solid, a liquid or a gas.
| State | Shape | Volume | Particles | Examples |
|---|---|---|---|---|
| Solid | Definite (keeps its own shape) | Definite | Tightly packed in fixed positions; they only vibrate in place. | Book, banana, basket, ice |
| Liquid | No definite shape: takes the shape of its container | Definite | Close together but free to slide past one another, so liquids flow. | Water, oil, juice |
| Gas | No definite shape | No definite volume: spreads out to fill any container | Far apart and moving quickly in all directions. | Air, water vapour, the air inside a soap bubble |
Gas takes up space and pushes on things. When Mang Pedro pumps air into a flat tyre, the tyre expands and becomes firm because air occupies space and exerts pressure. A soap bubble is mostly air wrapped in a thin film of soapy water, so it is not a solid.
2. Heat changes the state of matter
A phase change happens when heat is added to or removed from matter. Adding heat makes particles move faster and spread apart. Removing heat slows them down and pulls them closer.
| Phase change | From → To | Heat | Everyday example |
|---|---|---|---|
| Melting | Solid → Liquid | Added | Ice melts into water at its melting point. |
| Freezing (solidification) | Liquid → Solid | Removed (released) | Water in the freezer turns into ice at its freezing point. |
| Evaporation | Liquid → Gas, slowly, below the boiling point | Added | Wet clothes dry on the clothesline; rubbing alcohol dries quickly and cools your skin. |
| Boiling | Liquid → Gas, quickly, all through the liquid | Added | Water boiling in a kettle at its boiling point (100 °C). |
| Condensation | Gas → Liquid | Removed | Droplets form outside a glass of ice water; clouds form from water vapour high in the sky. |
| Sublimation | Solid → Gas, skipping the liquid | Added | Dry ice (solid carbon dioxide) and mothballs slowly disappear. |
| Deposition | Gas → Solid, skipping the liquid | Removed | Frost forms on leaves on a freezing morning. |
On the hot ground the ice cube first melts (solid → liquid), then the water evaporates (liquid → gas). So it goes solid → liquid → gas, and the water vapour mixes into the air.
3. Water droplets on a cold glass
The water on the outside of a glass of ice water does not leak through the glass. Water vapour in the warm air around the glass touches the cold surface, loses heat and condenses into droplets.
Review at a glance
The big ideas
- Solids have a definite shape and volume; liquids have a definite volume only; gases have neither.
- Liquids flow because their particles can slide past one another.
- Adding heat causes melting, evaporation, boiling and sublimation.
- Removing heat causes freezing, condensation and deposition.
- Heat (thermal energy) is what must be added or removed to change a state.
Key terms & meanings
- Melting point
- The temperature at which a solid becomes a liquid.
- Freezing point
- The temperature at which a liquid becomes a solid.
- Boiling point
- The temperature at which a liquid quickly turns into vapour throughout.
- Evaporation
- A liquid slowly changing into a gas below its boiling point.
- Condensation
- A gas cooling and changing into a liquid.
- Sublimation
- A solid changing straight into a gas.
- Deposition
- A gas changing straight into a solid.
- Dry ice
- Solid carbon dioxide, which sublimes at room temperature.
Add heat: solid → liquid → gas. Remove heat: gas → liquid → solid. Sublimation and deposition skip the liquid.
Study tip: Cover the meanings, explain each term in your own words, then try the practice. Use Reshuffle and practise again until you can get every item right.