Particle Theory & its Limitations (AQA GCSE Chemistry)

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Particle Theory & its Limitations

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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, 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 bonds break 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 boiling point is reached, all the particles gain enough energy for the intermolecular forces to break and the molecules to escape as the liquid boils
  • While changing state, the temperature of the substance remains the same as the heat energy goes into breaking the bonds between the particles
    • This is called latent heat

  • The entire process can be summarized in a diagram called a heating and cooling curve

heating-and-cooling-curve-for-pure-substance-igcse-and-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
  • 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

Exam Tip

Remember that the horizontal sections of a heating / cooling curve represent phase changes during which the temperature doesn’t change as the heat energy is absorbed during the process of changing state.

Predicting Physical State

  • 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 between the melting point and the boiling point:
    • The substance will be in the liquid state

  • At temperatures above the boiling point:
    • The substance will be in the gas state

Worked example

The table below indicates melting and boiling point data for four different substances named A, B, C and D.

Melting Point Table, downloadable IGCSE & GCSE Chemistry revision notes

Predict the states of the following substances:

  • Substance A at -150 ºC
  • Substance B at 50 ºC
  • Substance C at 1400 ºC
  • Substance D at 400 ºC

Answer:

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

Exam Tip

Questions on the particle theory of matter show interconversion of states with a reversible arrow: ⇌

This means that the process can go forwards and backwards. Read the question carefully and pick the direction of the change in state that the question refers to.

Solids, Liquids & Gases - A Summary of Properties

Solids

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

Liquids

  • There are weaker attractive forces between the particles of a substance in a liquid than in its corresponding solid form
  • Particles are close together in an irregular, unfixed form
  • Particles can move and slide past each other which is why liquids adopt the shape of the container they are 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

Gases

  • 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

Exam Tip

Solids, liquids and gases have different physical properties. The difference in these properties comes from differences in how the particles are arranged in each state.

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Stewart

Author: Stewart

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.