Kinetic model of matter

You must have learnt about the states of matter before. In this guide, we shall use the kinetic model of matter to explain ‘why’ solids, liquids and gasses have particular properties that define their existence.

States of matter

Matter exists in three basic states: solids, liquids and gasses.

Here is a table demonstrating comparisons between the general properties of solids, liquids and gases:

  Solids Liquids Gasses
Description Fixed volume.

Fixed shape.

Fixed volume.

Takes the shape of its container.

Variable volume.

Takes the shape of its container.

Arrangement of particles Arranged in a regular pattern called a lattice. Random. Random.
Separation of particles (intermolecular gap) Close together.

Tightly packed.

A little close together

Slightly further apart than in solid phase.


Far apart.

Movement of particles Vibration about a fixed position. Slow movement in a random way from place to place with molecules sliding past each other. Fast, random movement.
Attractive forces between particles Stronger than in the liquid phase. Slightly weaker than in the solid phase. No attractive forces between the particles.
Image <image for a solid here> <image for a liquid here>


<image for a gas here>


Changes in state

Heat a cube of ice (a solid) and it changes to a runny liquid. Continue heating and the liquid vanishes! Sounds like a magic trick, but the real fact is that the ice cube has simply changed states!

Here is a table expressing the changes of state:

Melting A solid to a liquid
Boiling A liquid to a gas
Condensation A gas to a liquid
Freezing A liquid to a solid
Sublimation A solid to a gas
De-sublimation A gas to a solid


The kinetic model of matter

The kinetic model of matter is a model in which matter consist of molecules in motion.

The world ‘kinetic’ means ‘related to movement’ (see our guide to energy transformations for more information). In the kinetic model of matter, the things that are moving are the particles (which could be atoms, ions and molecules), and thus it is also known as the particle model of matter.

Properties of solids and the kinetic model of matter

Property Reasoning
Solids have a fixed shape and do not flow As the particles of a solid are packed closely together, they are unable to flow; however they can vibrate in their positions.
Solids cannot be compressed and retain their shape The particles of a solid have less intermolecular space, hence they cannot be compressed and retain their shape.
Most solids expand when they melt As the particles are slightly apart in a liquid

Properties of liquids and the kinetic model of matter

Property Reasoning
Liquids take up the shape of their container. As, their particles are free to move about within the bulk of the liquid.
Dissolved substances diffuse throughout a liquid As the mobile particles of the liquid can move, they carry the dissolved substances. The hotter the liquid, the faster the rate of diffusion.
Liquids expand a lot when they boil As the intermolecular space between the particles of a gas is larger than a liquid.

Properties of gasses and the kinetic model of matter

Property Reasoning
Gases take the shape of their container As their particles can move freely about
Gasses diffuse from one place to another As the particles of a gas are freely mobile
Gasses contract a lot when they condense As the intermolecular space between the particles of a liquid is lesser than a gas.



Evaporation is the change in state from a liquid to a gas at a temperature below its boiling point.

Evaporation occurs only when the particles of a liquid with the greatest energy escape from the surface of the liquid. This is the reason why the evaporation of sweat from the surface of your skin results in a decrease in your internal body temperature.

Factors affecting evaporation

Factor Explanation
A liquid evaporates more rapidly when it is hotter. High temperatures give the particles of the liquid energy to escape off its surface.
A liquid evaporates more rapidly when it has a greater surface area. An increased surface area means, more particles of the liquid are near the surface, and so they can escape easily.
A liquid evaporates more rapidly when a draught blows across its surface. Particles of the liquid nearer to its surface are blown away when a draught flows.

Common misconceptions: Often students mistake between boiling and evaporation; please be clear:

  • Evaporation occurs at any temperature – not just the boiling point
  • Evaporation only happens at the surface of the liquid – not throughout the liquid like boiling
  • Boiling requires a steady energy source; evaporation doesn’t require a steady energy source.

Brownian motion

Brownian motion is the motion of small particles suspended in a liquid or a gas, caused by molecular bombardment.

Brownian motion is named in the honour of its first investigator: Robert Brown. Brown was using a microscope to study pollen grains when he noticed tiny particles moving haphazardly.

This motion is called as Brownian motion.

Brownian motion occurs because the pollen grains are bombarded constantly by the smaller, lighter particles in which they are suspended (liquid or gas). This gives us the evidence to the properties of liquids and gasses.


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