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STATES OF MATTER - properties of gases and liquids (fluids) and solids

Doc Brown's chemistry revision notes: basic school chemistry science GCSE chemistry, IGCSE  chemistry, O level and ~US grades 8, 9 and 10 school science courses or equivalent for ~14-16 year old science students for national examinations in chemistry and also helpful for UK advanced level chemistry students aged ~16-18 and US grades 11-12 K12 honors.

2. What can we expect from gas–liquid–solid particle theory models?

The three states of matter are solid, liquid and gas. Either melting and freezing can take place at the melting point, whereas boiling and condensing take place at the boiling point. Evaporation can take place at any temperature from a liquid surface. You can represent the three states of matter with a simple particle model. In this model–diagrams, the particles are represented by small solid inelastic spheres (electron structure is ignored).

Kinetic particle theory can help to explain changes of state like melting, boiling, freezing and condensing. The amount of energy needed to change state from solid to liquid or from liquid to gas depends on the strength of the forces between the particles of the substance.

These inter-particle forces may be relatively weak intermolecular forces (intermolecular bonding) or strong chemical bonds (ionic, covalent or metallic). The nature of the particles involved depends on the type of chemical bonding and the structure of the substance. The stronger the attractive forces between the particles the higher the melting point and boiling point of the substance


    • Most materials can be simply described as a gas, a liquid or a solid.


    • Just knowing isn't enough, we need a comprehensive theory of gases, that can explain their behaviour and make predictions about what happens e.g. if we change temperature or pressure.


    • We need a theoretical model  e.g. 'particle theory' that is supported by experimental evidence.


    • In a word, YES!


    • It is important in the chemical industry to know about the behaviour of gases, liquids and solids in chemical processes e.g. what happens to the different states with changes in temperature and pressure.

  • What is the KINETIC PARTICLE THEORY of gases, liquids and solids?

    • The kinetic particle theory of the states of matter is based on the idea of all materials existing as very tiny particles which may be individual atoms or molecules and the their interaction with each other either by collision in gases or liquids or by vibration and chemical bonding in solids.


    • This page introduces general physical descriptions of substances in the simplest physical (non–chemical) classification level i.e. is it a gas, liquid or a solid.

    • BUT, this web page also introduces 'particle models' in which a small circle represents an atom or a molecule i.e. a particular particle or simplest unit of a substance.

    • This section is quite abstract in a way because you are talking about particles you can't see individually, you just the 'bulk' material and its physical character and properties.

  • Are there LIMITATIONS to the particle model?

    • Well, yes! e.g.

    • The particles are treated as simple minute inelastic spheres and just behave like minute snooker balls flying around, not quite true, but they do fly around non-stop at random!

    • Although the particles are assumed to be hard spheres and inelastic, in reality they are atoms, ions or molecules.

    • Apart from lone atoms, they can be all sorts of shapes and twist and bend on collision with other particles and when they react they split into fragments when bonds break.

    • The simple model assumes no forces between the particles, but this is untrue, the model takes little account of the forces between the particles, even in gases you get very weak intermolecular bonding forces.

    • The particle model takes no account of the actual size of the particles e.g. ions/molecules can be widely different in size e.g. compare an ethene molecule with a poly(ethene) molecule!

    • Neither does it take account of any space that may exist between the particles.

  • A note on 'forces'

    • Forces between particles are mentioned on this page and some ideas will seem more abstract than others – but think about it ...

    • A gas spreads everywhere in a given space, so there can't be much attraction between the molecules/particles.

    • Something must hold liquid molecules together or how can a liquid form from a gas?

    • In fact between liquid molecules there are actually weak electrical forces of attraction called intermolecular forces, but they can't be strong enough to create a rigid solid structure.

    • However, in solids, these forces must be stronger to create the rigid structure.

    • Intermolecular forces are also called 'intermolecular bonds' BUT these are not the same as covalent, ionic or metallic bonds and are much weaker than these true chemical bonds.

Learning objectives for the kinetic particle model of gases, liquids and solids

Know what do we want or expect from a particle model of gases, liquids and solids.

Know that any particle theory must be supported from experimental observations and experiments to match the kinetic particle theory.

Know why it is so important to understand the particle nature of gases, liquids and solids.

Know, and be able to explain and describe how the kinetic particle theory of the states of matter is based on the idea of all materials existing as extremely tiny particles which may be individual atoms or molecules and the their interaction with each other either by collision in gases or liquids or by vibration in solids.

Be able to describe and explain some limitations to the particle model e.g. they are not simple tiny inelastic spheres of zero volume with no attractive forces between them e.g. in a gas or liquid.

BUT ...

(i) The particles in gases have a real volume, which is assumed to be zero when doing gas law calculations.

(ii) There are always weak intermolecular forces attractive forces between ALL particles in gases, liquids or solids in the order solids > liquids >> gases (put in main text)

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INDEX of all my notes on the states of matter

(GCSE level and advanced pre-university level revision notes)

Detailed notes on the states of matter and their properties

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