Kinetic Molecular Theory of Matter

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Kinetic Molecular Theory of Matter

Matter is made up of particles which are in constant, random motion. It can be defined as anything which has mass or occupies space. Matter is classified by its state and type, of which there are three main types – solid, liquid and gas. These lead us to the Kinetic Molecular Theory of Matter.

Main States of Matter, Solid, Liquid, Gas


Particle Arrangement in Matter

  • Particles in the solid state are closely packed, in a regular arrangement, known sometimes as a lattice.
  • Particles in a liquid state are not as closely packed and are irregular in their arrangement.
  • In a gas, particles are separated.

Kinetic Molecular Theory of Matter, Structure and Properties of Matter


Forces Between Particles

  • In the solid state, the forces are strong enough to keep the particles in a fixed position. Particles do, however, vibrate and rotate in their positions.
  • In the liquid state, there are weak forces which hold it together. The greater energy of the particles and the weaker forces allows for the disruption of the lattice and particles are, therefore able to slide past one another.
  • In the gaseous state, particles possess even higher energy levels and the forces which hold the gas together are negligible. This explains why gases are able to isolate themselves completely from one another and have no fixed size or shape.


Changes in State

Changes in state (also called phase transitions) involve heat energy being supplied to or removed from the substance.  Increasing the amount of heat energy in a substance increases its kinetic energy since temperature is a measure of the amount of kinetic energy possessed by a substance.  In a solid, heat energy causes the particles to vibrate at a greater rate until they possess sufficient energy to break away from their fixed position and become a liquid (known as melting). The temperature at which this occurs is called a substances melting point. As heat is removed from the liquid, the particles return to their closely-packed, fixed positions, this process is called freezing.

Kinetic Molecular Theory of Matter

When heat energy is supplied to a liquid, the particles also take on more heat energy which causes them to move around at a greater speed. Fast-moving particles at the surface of the liquid eventually have sufficient energy to escape from the liquid and move into the gaseous state. Here, these particles move rapidly, at a large distance from the other particles. This process is known as vaporisation. The point at which a substance moves from the liquid to the gas state is known as its boiling point. As heat is removed from the substance the particles move closer together once more, this process is called condensation.


Some substances e.g. iodine crystals are able to change from a solid directly into a gas, without moving through the liquid state. This process is called sublimation.  Deposition is the reverse of sublimation and occurs when a substance moves directly from a gas to a solid, omitting the liquid state.  An example of this can be seen in sub-zero temperatures, where water vapor in the air changes directly into ice, without first becoming a liquid.



Select the best answer for each of the following questions.

  1. Which of the following is an example of a molecule, but not a compound?
  • SO2
  • S8
  • H2SO4
  • S-2
  1. Which of the following is the correct definition for an atom?
  • The smallest building block of matter
  • The smallest building block of matter that retains the chemical properties of the element.
  • The smallest, indivisible building block of matter.
  • The purest type of matter.


Decide whether the following statements are true or false.

  1. All molecules are compounds, but not all compounds are molecules.
  2. Compounds are pure substances.

Theobromine, is a bitter alkaloid of the cacao plant found in chocolate. It has the chemical formula C7H8N4O2.

  1. Calculate how many atoms are in two molecules of C7H8N4O2.


Plasma – the 4th State of Matter

Plasmas, like gases, have no fixed shape or volume. A gas can reach the plasma state when its atoms become ionized. This occurs when the atom loses some or all of the electrons leaving a positively charged nucleus.  This process is known as ionization and explains why plasmas are able to conduct electricity since the electrons are free to move around.  Recombination occurs when plasmas return to the gaseous state.


Neon signs are an example of plasma. The electricity flows through the glass tube containing the gas, stripping the atoms of their electrons. The electricity promotes the electrons to a higher energy level. As the electron returned to its former energy level the excess energy is carried away as a photon, which we see as coloured light.


Video Lesson States of Matter



  1. Decide if the following are true or false.
  • Solids possess a greater level of kinetic energy than gases.
  • The forces which hold a liquid together are weaker than those which hold a solid together.
  • Some solids exist in a regular structure known as a lattice.
  • Gases have a fixed shape when in a container.


  1. Decide whether energy needs to added or removed for each of the following changes of state.
  • Condensation
  • Melting
  • Vaporisation
  • Deposition


  1. Decide which of the properties described below can be attributed to solid, liquid or gas. (Note some properties will be relevant for more than one state.)
  • Can be compressed
  • Requires large amounts of energy input in order to change state.
  • Particles are isolated
  • Fits the shape of the container



Summarize the differences between solids, liquids and gases for the following:

  • Arrangement of particles
  • Shape (draw a diagram)
  • Shape of substance
  • Level of energy possessed by particles
  • Strength of forces involved
  • Processes involved to change state
  • Examples


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Unit 1 – Structure and Properties of Matter

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