Kinetic Particle Model of Matter
Section: Physics | Syllabus: Cambridge IGCSE Combined Science (0653)
States of Matter
Matter exists in three states: solid, liquid and gas. Each state has its own distinguishing shape, volume, flow and compressibility.
- Solids
- have a definite shape
- have a definite volume
- cannot flow
- are not compressible
- Liquids
- have no definite shape - they take the shape of their container
- have a definite volume
- are able to flow
- are not compressible
- Gases
- have no definite shape - they take the shape of their container
- have no definite (fixed) volume - they fill their container
- are able to flow
- are highly compressible
| Property | Solid | Liquid | Gas |
|---|---|---|---|
| Shape | Definite | No definite shape | No definite shape |
| Volume | Definite | Definite | No fixed volume |
| Flow | Cannot flow | Can flow | Can flow |
| Compressibility | Not compressible | Not compressible | Highly compressible |
Changes of State
- When a substance changes state, the number of particles in it does not change, so its mass stays the same - only the energy of the particles changes
- Changes of state are physical changes, so they are reversible
Melting and Freezing
- Melting - a solid changes into a liquid (e.g. ice → water)
- Freezing - a liquid changes into a solid (e.g. water → ice)
Boiling and Condensing
- Boiling - a liquid changes into a gas (also called evaporating)
- Condensing - a gas changes into a liquid
The four changes of state: melting, freezing, boiling and condensing
- Only these four changes of state are required - a direct change between a solid and a gas is not required
Particle Arrangement, Separation and Motion
- Solids
- arrangement: particles are arranged in a regular, fixed pattern
- separation: particles are close together, touching their neighbours
- motion: particles vibrate about a fixed position but cannot move from place to place
- Liquids
- arrangement: particles are arranged randomly, with no fixed pattern
- separation: particles are close together, about as close as in a solid
- motion: particles move around each other, in random directions, at moderate speed
- Gases
- arrangement: particles are arranged randomly
- separation: particles are far apart, with large spaces between them
- motion: particles move quickly in random directions, colliding with each other and with the walls of their container
Simple particle diagrams for a solid, a liquid and a gas
Forces, Separation and Properties
Extended Only
- the forces of attraction between particles, their separation, and their motion together determine the properties of a solid, liquid or gas
- in a solid, strong forces hold closely-spaced particles in fixed positions, giving a fixed shape and fixed volume
- in a liquid, weaker forces allow the closely-spaced particles to move around each other, giving a fixed volume but no fixed shape
- in a gas, forces between the widely-spaced particles are very weak, so they move independently and fill their container, giving no fixed shape and no fixed volume
Particle Motion and Temperature
- the higher the temperature of a substance, the greater the average kinetic energy of its particles, and the faster they move
- the lower the temperature, the less kinetic energy the particles have, and the slower they move
- there is a lowest possible temperature, called absolute zero, at which particles have the least possible kinetic energy
- absolute zero = -273,^
Gas Pressure
- gas particles are in constant, random motion, travelling in straight lines until they collide with another particle or with the walls of their container
- each time a particle collides with a surface, it exerts a tiny force on that surface
- billions of collisions happen every second - the combined effect of all these collisions on the container walls is the pressure of the gas
Gas particles colliding with the walls of a sealed container produce gas pressure
Pressure as Force per Unit Area
Extended Only
- gas pressure can be described in terms of the forces exerted by particles colliding with a surface, creating a force per unit area
- p = (F/A)
Gas Pressure, Temperature and Volume
For a fixed mass of gas, the pressure changes if the temperature or the volume changes.
Effect of Temperature at Constant Volume
- heating a fixed mass of gas at constant volume:
- the particles gain kinetic energy and move faster
- the particles collide with the walls of the container more frequently
- the particles also collide with the walls with greater force
- the pressure of the gas increases
- cooling a fixed mass of gas at constant volume has the opposite effect: the particles move slower, and collide with the walls less frequently and with less force, so the pressure decreases
Effect of Volume at Constant Temperature
- decreasing the volume of a fixed mass of gas at constant temperature (compressing it):
- the particles still move at the same average speed, since the temperature has not changed
- the particles are confined to a smaller space, so they reach the walls of the container more often
- the more frequent collisions increase the pressure of the gas
- increasing the volume at constant temperature has the opposite effect: collisions with the walls become less frequent, so the pressure decreases
| Change | Particle speed | Collision frequency | Pressure |
|---|---|---|---|
| Increase temperature (constant volume) | Increases | Increases | Increases |
| Decrease temperature (constant volume) | Decreases | Decreases | Decreases |
| Decrease volume (constant temperature) | No change | Increases | Increases |
| Increase volume (constant temperature) | No change | Decreases | Decreases |
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