OCR Specification focus:
‘Describe solids, liquids and gases by particle spacing, degree of ordering, and molecular motion.’
The particle model of matter provides a microscopic explanation for the physical properties of solids, liquids, and gases, describing how particle spacing, ordering, and motion determine their observable behaviour.
The Particle Model of Matter
The particle model views all matter as composed of tiny, discrete particles—atoms or molecules—that interact through forces of attraction and repulsion. The differences between solids, liquids, and gases arise from variations in how these particles are arranged and how they move. Despite the immense differences in macroscopic appearance, these states share a common foundation: the same fundamental particles governed by thermal energy and intermolecular forces.
Particle Spacing and Arrangement
Solids
In solids, particles are closely packed in a regular, ordered structure, often forming a crystalline lattice. The strong intermolecular or interatomic forces maintain a fixed separation between neighbouring particles.
Particles are separated by distances of roughly one particle diameter.
The arrangement is rigid and highly ordered.
There is minimal free space between particles.
Solids have a definite shape and volume because their structure resists deformation.
Although solid particles do not move freely, they vibrate about fixed equilibrium positions due to their thermal energy. When energy is added as heat, the amplitude of vibration increases, but the structure remains intact until the melting point is reached.
Liquids
In liquids, particles remain close together but the regular structure of a solid is lost. The particles are still influenced by strong attractive forces, yet they possess enough kinetic energy to move past one another, giving the liquid fluidity.
Spacing is only slightly greater than in solids.
Arrangement is disordered and dynamic—particles constantly rearrange.
Liquids have a fixed volume but no fixed shape, adapting to the container.
The density of a liquid is usually only slightly less than that of the solid form.
The balance between cohesive forces (which pull particles together) and thermal motion (which drives them apart) defines the liquid’s characteristics.
Gases
In gases, particles are widely spaced and move randomly and rapidly in all directions. The intermolecular forces are negligible except during brief collisions.
Average separation is much larger—typically ten or more particle diameters apart.
Arrangement is completely disordered and constantly changing.
Gases have no fixed shape or volume, filling the entire container.
A clear particle-level comparison of a solid (tightly packed, ordered), liquid (close but disordered), and gas (widely spaced, disordered). This directly illustrates how spacing and ordering vary between states. Labels correspond precisely to the three classical phases; no extra phases are shown. Source.
Gases are compressible, unlike solids and liquids, due to the large spaces between particles.
This vast spacing and rapid movement explain why gases have low densities and expand freely when unconfined.
Particle Motion and Kinetic Energy
All particles possess kinetic energy due to their motion, which depends on temperature. As temperature rises, the average kinetic energy of the particles increases, altering their motion and potentially the state of matter.
Solids
Motion is confined to vibrations about fixed points.
Vibrational kinetic energy increases with temperature.
When the vibrations become large enough to overcome the intermolecular forces, the structure breaks down and melting occurs.
Liquids
Particles move with translational, rotational, and vibrational motion.
Their movement is random but constrained by nearby particles.
The energy distribution among particles allows for diffusion, though it is much slower than in gases.
Gases
Motion is random, continuous, and in straight lines between collisions.
An idealised gas with particles travelling in straight paths until collision, emphasising random direction and speed distribution. This supports the kinetic description used to explain low density and compressibility. The schematic is intentionally minimal and matches A-level scope. Source.
The average kinetic energy is directly proportional to the absolute temperature (K).
EQUATION
—-----------------------------------------------------------------
Mean kinetic energy (E) = (3/2) kT
E = average kinetic energy of a particle (J)
k = Boltzmann constant (1.38 × 10⁻²³ J K⁻¹)
T = absolute temperature (K)
—-----------------------------------------------------------------
This relationship shows that as temperature increases, gas particles move faster on average, increasing pressure in a confined space.
Comparison of the Three States of Matter
The following characteristics distinguish the three physical states:
Side-by-side particle sketches for solid, liquid, and gas inside containers. They highlight fixed volume for solid and liquid versus container-filling behaviour of gases arising from large spacing and unrestricted motion. The figure uses generic molecules; no chemical specifics beyond the three states. Source.
Solids
Fixed shape and volume
Particles tightly packed in ordered structure
Strong interparticle forces
Vibrational motion only
Incompressible
Liquids
Fixed volume but no fixed shape
Particles close but disordered
Moderate interparticle forces
Particles move and slide past each other
Slightly compressible
Gases
No fixed shape or volume
Particles widely spaced and disordered
Negligible interparticle forces
Rapid, random motion in all directions
Highly compressible
The gradual change in spacing, order, and motion from solid to gas illustrates how energy input modifies internal energy and state.
Relationship Between Particle Model and Macroscopic Properties
The particle model not only describes microscopic behaviour but also explains macroscopic properties such as density, compressibility, diffusion, and thermal expansion.
Density: Determined by how closely particles are packed; solids usually have the highest density, gases the lowest.
Compressibility: Gases compress easily because particles are far apart; solids resist compression due to lack of free space.
Diffusion: In gases, particles spread rapidly through random motion; in liquids, diffusion occurs more slowly.
Thermal Expansion: Increasing temperature causes greater vibration or motion, slightly increasing spacing between particles, leading to expansion.
Each property links directly to the way particles are spaced, ordered, and in motion—an essential conceptual bridge between microscopic and macroscopic physics.
Role of Intermolecular Forces
Intermolecular forces determine how tightly particles are held together in each state.
Intermolecular Forces: Attractive or repulsive forces between neighbouring particles that determine the physical state and properties of a substance.
In solids, these forces dominate; in gases, they are virtually absent. The transition from one state to another involves a competition between thermal kinetic energy (which tends to separate particles) and intermolecular potential energy (which tends to hold them together).
Summary of Key Relationships
The particle model links microscopic particle behaviour to observable physical properties:
Spacing determines density and compressibility.
Ordering determines rigidity and shape retention.
Motion determines energy content and phase behaviour.
These concepts form the foundation for understanding more complex topics such as internal energy, specific heat capacity, and phase transitions in later parts of thermal physics.
FAQ
In solids, particles are locked into a fixed lattice with strong intermolecular forces that prevent movement beyond vibration.
Liquids have weaker forces, allowing particles to slide past each other while remaining close together, so they can change shape but not volume.
Gases have negligible intermolecular forces and very large particle spacing, allowing free, random motion that fills the available container and gives no fixed shape or volume
Density depends on how closely particles are packed.
In gases, the distance between particles can be tens or hundreds of times their diameter, meaning there are far fewer particles in the same volume compared to a solid or liquid.
Because of this vast spacing, the mass per unit volume is very small, producing the low densities characteristic of gases.
Increasing temperature raises the average kinetic energy of particles.
In solids: vibrations become larger.
In liquids: motion becomes faster, leading to greater fluidity and diffusion.
In gases: particle speed increases significantly, raising pressure if volume is fixed.
At sufficiently high temperatures, added energy can overcome intermolecular forces and change the state of matter.
In liquids, particles are already close together, leaving little empty space to reduce when pressure is applied.
However, because the arrangement is less rigid than in solids, particles can move slightly closer under high pressure.
In gases, there is ample empty space, making them easily compressible, while solids are too rigid and tightly packed to compress meaningfully.
‘Disorder’ describes how randomly the particles are positioned relative to one another.
In solids, particles are ordered and regularly arranged in fixed positions.
In liquids, the structure is disordered because particles are close but constantly changing neighbours.
In gases, the disorder is complete — particle positions and velocities are random at any moment.
This increasing disorder from solid to gas reflects rising energy and mobility within the system.
Practice Questions
Question 1 (2 marks)
Describe how the arrangement and motion of particles differ between a solid and a gas.
Mark scheme:
1 mark: States that particles in a solid are closely packed in a regular, ordered structure and only vibrate about fixed positions.
1 mark: States that particles in a gas are far apart, move randomly and rapidly in all directions, and have negligible intermolecular forces.
Question 2 (5 marks)
Using the particle model, explain how the spacing, ordering, and motion of particles account for the differences in shape, volume, and compressibility between solids, liquids, and gases.
Mark scheme:
1 mark: States that in solids, particles are closely packed in an ordered structure, giving a definite shape and volume and making them incompressible.
1 mark: States that in liquids, particles remain close but are disordered and can slide past one another, giving a definite volume but no fixed shape.
1 mark: States that in gases, particles are far apart and move freely, so gases have no fixed shape or volume and are highly compressible.
1 mark: Correctly links particle spacing to compressibility — large spacing in gases allows compression, minimal spacing in solids prevents it.
1 mark: Refers to particle motion increasing from solid to liquid to gas, explaining increased fluidity and energy with temperature.
