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The Three States of Matter |The Particle Theory of Matter | Temperature | Diffusion


Matter is anything that has mass and takes up space.
Matter is the atoms that material things are made of.

The mass of an object is a measure of the quantity of matter it contains.
Mass Symbol: m
Unit of Mass: kilogram (kg)

The Three States of Matter:  solid, liquid and gas.

Comparing the Three States of Matter

 Solid   Liquid  Gas
 Definite shape  No definite shape  No definite shape
 Definite volume  Definite volume  No definite volume
 Almost incompressible  Almost incompressible  Compressible
 Does not flow   Flows  Diffuses filling the space
 Does not diffuse   Diffuses slowly  Diffuses rapidly
 Fixed particles  Rolling particles    Flying particles

The Particle Theory of Matter (The Kinetic Theory of Matter)

Matter is composed of tiny particles (atoms, ions or molecues)
The state of matter depends on the ‘movement’ of the particles.
In a solid the particles are very close but do not move out of position, they ‘vibrate’ or ‘jog on the spot’.
In a liquid the particles are very close but they are in constant ‘random rapid rolling’ over each other.
In a gas the particles are relatively very far apart but are in constant ‘haphazard haste flying and hitting’ off each other.


The temperature of a substance is a measure of the average movement energy of its particles.
The higher the temperature the greater the average movement energy of its particle i.e. the greater the vibration of the solid particles or the great the speed of movement of the particles of a liquid or gas.

Temperature and Change of State
Matter changes state because of a change in the movement of its particles.
(a) Solid » Liquid » Gas;
     Change in movement: fixed » rolling » flying
      heat gain, increasing temperature, increasing motion of the particles
(b) Gas » Liquid » Solid
      Change in movement: flying » rolling » fixed
      heat loss, decreasing temperature, decreasing motion of the particles.

Change of state, melting and boiling points, heat and temperature are dealt with in the chapter on Heat in Physics.


  • Diffusion is the movement of the free particles of a substance throughout a liquid or gas resulting in their equal distribution.
  • The direction of diffusion is from an area where the concentration of the particles is higher to where they are in lower concentration.
  • Diffusion is said to have stopped when the concentration of the particles of the substance is the same in all places in the liquid or gas.
  • Diffusion in gases is about 10,000 faster than in liquids.
  • The faster diffusion in gases is due to the much higher speed of the particles plus the much more open space for the particles to move.


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