The Physical Forms of Matter

  • The three states of matter are solids, liquids and gases.
  • Individual atoms themselves do not share the same properties as bulk matter.
  • Particle theory uses models based on particles that are used to graphically illustrate the states of bulk matter.

The arrangement of particles in solids, liquids and gases, IGCSE & GCSE Chemistry revision notes

Summary of the properties of solids, liquids and gases

  • The forces acting between the particles of matter can be weak or very strong, depending on the state of matter.
  • The properties of each state of matter are as follows.


  • Strong forces of attraction between particles, particles are packed very closely together in a fixed and regular pattern.
  • Atoms vibrate in position but can’t change position or move.
  • Solids have a fixed volume and shape and have high density.
  • Solid particles have only a small amount of energy.


  • Weaker attractive forces in liquids than in solids, particles are close together in an irregular, unfixed pattern.
  • Particles can move and slide past each other which is why liquids adopt the shape of the container they’re in and also why they are able to flow.
  • Liquids have a fixed volume but not a fixed shape and have a moderate to high density.
  • Liquid particles have more energy than those in a solid but less than gaseous particles.


  • No intermolecular forces, particles are in random movement and so there is no defined pattern.
  • Particles are far apart and move quickly (around 500 m/s) in all directions, they collide with each other and with the sides of the container (this is how pressure is created inside a can of gas).
  • No fixed volume, since there is a lot of space between the particles, gases can be compressed into a much smaller volume. Gases have low density.
  • Gaseous particles have the highest amount of energy.

Phase Changes

  • When matter changes from one state to another due to changes in temperature or pressure, the change is called an interconversion of state.
  • It is a physical change involving changes in the forces between the particles of the substances, the particles themselves remain the same, as do the chemical properties of the substance.
  • Physical changes are relatively easy to reverse as no new substance is formed during interconversions of state.
  • The interconversions have specific terms to describe them.

Interconvention Table, IGCSE & GCSE Chemistry revision notes

  • Melting is when a solid changes into a liquid.
  • Requires heat energy which transforms into kinetic energy, allowing the particles to move.
  • Occurs at a specific temperature known as the melting point (m.p.) which is unique to each pure solid.


  • Boiling is when a liquid changes into a gas.
  • Requires heat which causes bubbles of gas to form below the surface of a liquid, allowing for liquid particles to escape from the surface and from within the liquid.
  • Occurs at a specific temperature known as the boiling point (b.p.) which is unique to each pure liquid.


  • Freezing is when a liquid changes into a solid.
  • This is the reverse of melting and occurs at exactly the same temperature as melting, hence the melting point and freezing point of a pure substance are the
  • same. Water for example freezes and melts at 0ºC.
  • Requires a significant decrease in temperature (or loss of thermal energy) and occurs at a specific temperature which is unique for each pure substance.


  • When a liquid changes into a gas.
  • Evaporation occurs only at the surface of liquids where high energy particles can escape from the liquids surface at low temperatures, below the b.p. of the liquid.
  • The larger the surface area and the warmer the liquid/surface, the more quickly a liquid can evaporate
  • No heat is required and evaporation occurs over a range of temperatures.


  • When a gas changes into a liquid, usually on cooling.
  • When a gas is cooled its particles lose energy and when they bump into each other, they lack energy to bounce away again, instead grouping together to form a liquid.
  • No energy is required for condensation to occur and it takes place over a range of temperatures.


  • When a solid changes directly into a gas.
  • This happens to only a few solids such as iodine or solid carbon dioxide.
  • The reverse reaction also happens and is also called sublimation (sometimes called deposition or desublimation).
  • Sublimation occurs at a specific temperature which is unique for a pure substance.

Changing state, IGCSE & GCSE Chemistry revision notes

Interconversion of solids, liquids and gases

  • The physical state of a substance under certain conditions can be predicted from a given set of data.
  • Normally you are given melting and boiling point data for a substance and asked to predict its physical state in specified conditions.
  • At temperatures below the melting point:
    • The substance will be in the solid state
  • At temperatures above the melting point:
    • The substance will be in the liquid state
  • At temperatures above the boiling point:
    • The substance will be in the gaseous state.


  • The table below indicates melting and boiling point data for four different substances named A, B, C and D. Predict the states of the following substances:
    • Substance A at -150ºC
    • Substance B at 50ºC
    • Substance C at 1400ºC
    • Substance D at 400ºC

melting and boiling point table, IGCSE & GCSE Chemistry revision notes

  • A boils at temperatures above -173ºC so at -150 ºC A is a gas.
  • B melts at 1736ºC so at 50ºC it is a solid.
  • C melts at 1105ºC and boils at 1450ºC so at 1400ºC it is a liquid
  • D melts at 650ºC so at 400ºC it is a solid.

Particle Theory & its Limitations

Particle Theory

  • Particle theory explains how matter changes state depending on the energy and forces present between the particles in the substance.
  • The amount of energy needed to change from a solid to a liquid and from a liquid to a gas depends on the relative strength of the forces acting between the particles.
  • There are many different types of substances which contain different amounts of elements and compounds.
  • Since each substance contains different particles, then the amount of energy needed to induce a change of state is different for each individual substance.
  • The stronger the forces between the particles then the higher the energy needed for melting and boiling to occur.
  • When substances are heated, the particles absorb thermal energy which is converted into kinetic energy.
  • Heating a solid causes its particles to vibrate more and as the temperature increases, they vibrate so much that the solid expands until the structure breaks and the solid melts.
  • On further heating, the now liquid substance expands more and some particles at the surface gain sufficient energy to overcome the intermolecular forces and evaporate.
  • When the b.p. temperature is reached, all the particles gain enough energy to escape and the liquid boils.
  • While changing state, the temperature of the substance remains the same as the heat energy is rapidly converted into kinetic energy. This is called latent heat.
  • The entire process can be summarized in a diagram called a heating and cooling curve.

Heating & cooling curve for pure substance, IGCSE & GCSE Chemistry revision notes

Heating and cooling curve for a pure substance

Limitations of the Particle Theory

  • Particle theory considers all particles, irrespective of their state or chemical identity, to be small, solid and inelastic spheres.
  • It doesn’t consider the difference caused by different particles such as atoms, ions or molecules or mixtures of all three.
  • The theory also fails to consider the intermolecular forces that exist between different particles in different substances.

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Morgan Curtin Chemistry

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.