2.1.2 Particle model
- Syllabus
- 0625–2026–2027
- Topic
- 2.1.2
- Level
- —
Learning objectives
2.1.2.1Particle structure of solids, liquids• Describe the particle structure of solids, liquids and gases in terms of the arrangement, separation and motion of the particles and represent these states using simple particle diagrams2.1.2.2Relationship between the motion of• Describe the relationship between the motion of particles and temperature, including the idea that there is a lowest possible temperature (−273 °C), known as absolute zero, where the particles have least kinetic energy2.1.2.3Pressure and the changes in pressure• Describe the pressure and the changes in pressure of a gas in terms of the motion of its particles and their collisions with a surface2.1.2.4Random motion of microscopic particles• Know: the random motion of microscopic particles in a suspension is evidence for the kinetic particle model of matter2.1.2.5And explain this motion (sometimes• Describe and explain this motion (sometimes known as Brownian motion) in terms of random collisions between the microscopic particles in a suspension and the particles of the gas or liquid2.1.2.6Forces and distances between particles• Know: the forces and distances between particles (atoms, molecules, ions and electrons) and the motion of the particles affects the properties of solids, liquids and gases2.1.2.7Pressure and the changes in pressure• Describe the pressure and the changes in pressure of a gas in terms of the forces exerted by particles colliding with surfaces, creating a force per unit area2.1.2.8Microscopic particles may be moved by• Know: microscopic particles may be moved by collisions with light fast-moving molecules and correctly use the terms atoms or molecules as distinct from microscopic particles
Model solids, liquids and gases with particles
A simple particle model represents matter as tiny particles. To identify a state, compare three features: arrangement, average separation and motion.
| State | Arrangement and separation | Motion |
|---|---|---|
| solid | closely packed in an ordered pattern | vibrate about fixed positions |
| liquid | closely packed but irregular, with no fixed pattern | move around and slide past one another |
| gas | widely separated and randomly arranged | move rapidly and randomly in all directions |
In a simple diagram, use equal-sized circles. Draw a solid as close ordered rows, a liquid as close irregular circles, and a gas as circles spread far apart at random. The spaces are part of the model: a gas is easy to compress because its particles are far apart, whereas solids and liquids have little empty space between particles.
Do not draw larger particles when a substance changes state. The particle size and identity stay the same; it is their arrangement, separation and motion that change. The circles are schematic, so their exact number and drawn size are not measurements.
Link temperature to particle motion
A higher temperature means that particles have a greater average kinetic energy and move faster on average; a lower temperature means less average kinetic energy and slower motion.
Heating transfers energy to the particles, so their average speed increases. Cooling removes energy, so their average speed decreases. In a sealed rigid container, heating changes speed but not the average separation, because the volume and number of particles stay fixed.
Absolute zero is the lowest possible temperature: −273 °C. At this temperature particles have their least kinetic energy. This is the syllabus statement to use when describing the lower limit of temperature.
Absolute zero is not 0 °C, and ‘least kinetic energy’ must not be replaced by ‘zero gravitational potential energy’. Do not claim that every particle has exactly the same speed: temperature describes average particle motion.
Explain gas pressure through collisions
Gas pressure is produced because rapidly moving particles repeatedly collide with the surfaces of their container.
Each particle changes direction when it hits a surface. Across an enormous number of collisions, the surface experiences a continuous push. If a fixed amount of gas is heated in a rigid container, the particles move faster, hit the walls more often and make stronger impacts, so the pressure rises. Cooling reverses these changes and lowers the pressure.
The pressure does not rise because each particle expands or because new particles appear. Compare pressure changes only after stating what is held fixed; detailed force-per-area and momentum reasoning is developed in the Supplement card later in this Topic.
Use random motion as evidence for the particle model
Microscopic particles suspended in a still liquid or gas can be seen moving continually in irregular, random directions. This observed motion is evidence for the kinetic particle model of matter.
| Stage | What it says |
|---|---|
| observation | a visible speck follows a jagged path with sudden changes of direction and unequal step lengths |
| evidence | the motion continues even though the suspension as a whole is still |
| conclusion | matter contains particles that are in continual random motion |
Record what is observed before explaining it: the path is random and jerky, not a smooth curve or motion in one common direction. The visible speck is a microscopic suspended particle; it is not an individual molecule of the liquid or gas.
Explain Brownian motion by uneven collisions
Brownian motion is the continual random motion of microscopic particles in a suspension, caused by collisions with the much smaller particles of the surrounding liquid or gas.
The liquid or gas particles move rapidly and randomly. They strike every side of a suspended smoke particle or pollen grain, but the impacts are not perfectly balanced at each instant. The resulting unbalanced force changes the microscopic particle’s speed or direction. A new unequal set of impacts then changes its motion again, producing the observed irregular path.
The bright light makes the suspended particles visible; it does not drive their motion. Convection currents, collisions between the suspended particles, and the suspended particle’s own atoms are not the Brownian-motion mechanism described here.
Connect particle behaviour to state properties
The properties of a solid, liquid or gas result from the combined effects of particle separation, forces between particles and particle motion.
| State | Separation and forces | Motion | Resulting properties |
|---|---|---|---|
| solid | particles are close; attractive forces are strong | particles vibrate about fixed positions | fixed shape and volume; difficult to compress |
| liquid | particles are close; forces keep them together but not in fixed positions | particles move and slide past one another | fixed volume but no fixed shape; flows; difficult to compress |
| gas | particles are far apart; forces are negligible except during collisions | particles move rapidly and randomly | fills its container; no fixed volume; easy to compress |
‘Particle’ is a general model word. Depending on the substance, the relevant particles may be atoms, molecules, ions or electrons. Do not assume that every material is made of molecules.
No single feature explains every property. Close spacing helps explain low compressibility, while forces and freedom of motion distinguish a rigid solid from a flowing liquid. Strong forces alone do not mean that particles are motionless.
Turn particle impacts into force per unit area
When a gas particle strikes and rebounds from a surface, its momentum changes. That change requires a force, and the particle exerts an equal force on the surface.
A huge number of impacts produces a total force on the wall. More collisions each second or a greater momentum change in each collision increases that total force. Gas pressure is the total normal force from these impacts divided by the area over which it acts.
p=AF
Here p is pressure, F is the total force perpendicular to the surface, and A is the surface area. For the same area, a larger collision force gives a larger pressure; for the same force, spreading it over a larger area gives a smaller pressure.
Pressure is not the same quantity as force. A correct particle explanation links collision and rebound → change of momentum → force on the surface → force per unit area.
Distinguish molecules from microscopic specks
In a Brownian-motion experiment, the visible microscopic particle and the invisible atoms or molecules of the surrounding fluid are different objects.
| Object | Relative scale and motion | Correct description |
|---|---|---|
| atom or molecule of the liquid or gas | much lighter, fast-moving and not seen individually in this experiment | collides with the microscopic particle and may rebound |
| microscopic smoke particle or pollen grain | much larger and heavier; visible as a speck through the microscope | is moved by the changing, unequal total effect of many molecular collisions |
For example, a fast nitrogen molecule can strike a stationary smoke particle: the smoke particle moves and the nitrogen molecule rebounds. Many such impacts from changing directions create the smoke particle’s irregular motion.
Say ‘smoke particle’ or ‘pollen grain’, not ‘smoke molecule’ or ‘pollen molecule’. Microscopic means small enough to require a microscope; it does not mean the same size as an atom or molecule.