1.1 Solids, liquids and gases
- Syllabus
- 0620–2026–2027
- Topic
- 1.1
- Level
- —
Learning objectives
1.1.1Distinguishing properties of solids• State the distinguishing properties of solids, liquids and gases1.1.2Structures of solids, liquids and• Describe structures of solids, liquids and gases in terms of particle separation, arrangement and motion1.1.3Changes of state in terms of melting• Describe changes of state in terms of melting, boiling, evaporating, freezing and condensing1.1.4Effects of temperature and pressure on• Describe effects of temperature and pressure on the volume of a gas1.1.5Changes of state using kinetic• Explain changes of state using kinetic particle theory, including the interpretation of heating and cooling curves1.1.6Effects of temperature and pressure on• Explain the effects of temperature and pressure on gas volume using kinetic particle theory
Recognise solids, liquids and gases
A state of matter is distinguished by whether it keeps its own shape and volume and by how readily it flows or is compressed.
| State | Shape | Volume | Flow and compression |
|---|---|---|---|
| solid | fixed | fixed | does not flow; very difficult to compress |
| liquid | takes the container's shape | fixed | flows; very difficult to compress |
| gas | fills the container | not fixed | flows; readily compressed |
At a stated pressure, compare temperature with melting and boiling points: below the melting point the substance is solid, between the two points it is liquid, and above the boiling point it is gas.
A liquid changes shape without changing its fixed volume. A gas changes both shape and volume to fill its container.
Describe particles in each state
The kinetic particle model describes each state using three separate features: particle separation, arrangement and motion.
| State | Separation | Arrangement | Motion |
|---|---|---|---|
| solid | touching or very close | regular, ordered | vibrate about fixed positions |
| liquid | touching or very close | random, disordered | move randomly past one another |
| gas | far apart | random, disordered | move rapidly and randomly in all directions |
Close particles make solids and liquids difficult to compress. Widely separated particles allow a gas to be compressed and to fill the available space.
Heating changes particle motion and average separation; it does not make the particles themselves expand or change size.
Name changes between states
A change of state is named by its starting state and final state. The substance remains the same substance during this physical change.
| From | To | Process |
|---|---|---|
| solid | liquid | melting |
| liquid | solid | freezing |
| liquid | gas | boiling or evaporating |
| gas | liquid | condensing |
Boiling occurs throughout a liquid at its boiling point and forms bubbles. Evaporation occurs only at the surface and can happen below the boiling point.
At a fixed pressure, melting and freezing occur at the melting point; boiling and condensing occur at the boiling point.
This exact objective names melting, boiling, evaporating, freezing and condensing. Direct solid-to-gas changes in some tagged questions are outside its frozen teaching boundary.
Describe how conditions change gas volume
A gas-volume trend is meaningful only when the other condition is controlled. State what remains constant before giving the direction of change.
| Change | Condition held constant | Gas-volume response |
|---|---|---|
| temperature increases | pressure | volume increases |
| temperature decreases | pressure | volume decreases |
| pressure increases | temperature | volume decreases |
| pressure decreases | temperature | volume increases |
A flexible container or movable piston allows volume to respond. A rigid sealed container has fixed volume, so heating changes pressure instead.
If temperature and pressure change together, their effects oppose one another and the volume cannot be predicted without more information.
Explain heating and cooling curves
Temperature tracks the particles' average kinetic energy. A sloping section represents one state changing temperature; a flat section represents a change of state at constant temperature for a pure substance.
| Curve section | Particle-energy explanation |
|---|---|
| heating slope | particles gain kinetic energy, move faster and temperature rises |
| melting/boiling plateau | supplied energy overcomes attractive forces; average kinetic energy and temperature stay constant |
| cooling slope | particles lose kinetic energy, move more slowly and temperature falls |
| condensing/freezing plateau | attractions form and energy is released; average kinetic energy and temperature stay constant |
Read plateaus from high to low temperature on heating as boiling then melting when tracing backwards; on cooling they are condensing then freezing. Before and after each plateau, identify the single state from the direction of the process.
During melting particles leave fixed positions but remain close. During boiling particles become far apart; the reverse changes occur during condensing and freezing.
A flat section does not mean that no energy is transferred. It means transferred energy changes particle attractions rather than average kinetic energy.
Explain gas-volume changes with particles
Gas pressure is caused by particles colliding with the container walls. Temperature changes their average kinetic energy and speed; pressure changes how closely they are confined.
| Change and condition | Particle account | Result |
|---|---|---|
| heat at constant pressure | particles move faster; the gas expands until wall-collision effects return to the imposed pressure | particles become farther apart and volume increases |
| cool at constant pressure | particles move more slowly; external pressure pushes the boundary inward | particles become closer and volume decreases |
| increase pressure at constant temperature | particles keep the same average speed but are forced into less space, giving more frequent wall collisions | volume decreases |
| decrease pressure at constant temperature | the gas expands until its wall-collision effect matches the lower imposed pressure | volume increases |
In a rigid sealed container, volume cannot increase. Heating makes particles move faster and collide with the walls more frequently and with greater force, so pressure increases instead.
Particles do not become larger under pressure or on heating. The changes are in speed, separation, collision frequency and collision force.