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

11. Explaining melting and freezing using the kinetic particle model (solid <=> liquid)

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.


11a. Melting (solid to liquid)

Explained using the kinetic particle theory of liquids and solids

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  • When a solid is heated the particles vibrate more strongly as they gain kinetic energy and the particle attractive forces are weakened.
  • Eventually, at the melting point, the attractive forces are too weak to hold the particles in the structure together in an ordered way and so the solid melts.
    • Note that the intermolecular forces are still there to hold the bulk liquid together but the effect is not strong enough to form an ordered crystal lattice of a solid.
    • Metals, ionic compounds and giant covalent structures where the atoms are held together by strong chemical bonding, will tend to have high melting points.
  • The particles become free to move around and lose their ordered arrangement.
  • Energy is needed to overcome the attractive forces and give the particles increased kinetic energy of vibration.
  • So thermal energy (heat energy) is taken in from the surroundings and so melting is an endothermic process (ΔH +ve).
  • The energy required for the state change of solid to liquid is called the latent heat of fusion.
  • Energy changes for these physical changes of state for a range of substances are dealt with in a section of the Energetics Notes and also here in section 14.
  • 14. Comparison of latent heat changes in physical changes of state for different substances

11b. Freezing (liquid to solid)

Explained using the kinetic particle theory of liquids and solids

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  • The freezing point of a liquid substance is the same as the melting point of the solid substance, its just a question of which direction the state of the substance changes and the direction of thermal energy change (added for melting and removed for freezing).
  • On cooling, liquid particles lose kinetic energy and so can become more strongly attracted to each other.
  • When the temperature is low enough, the kinetic energy of the particles is insufficient to prevent the particle attractive forces causing a solid to form.
  • Eventually at the freezing point the forces of attraction are sufficient to remove any remaining freedom of movement (in terms of one place to another) and the particles come together to form the ordered solid arrangement (though the particles still have vibrational kinetic energy.
  • Since thermal energy (heat energy) must be removed to the surroundings, so strange as it may seem, freezing is an exothermic process (ΔH ve).
  • Energy changes for these physical changes of state for a range of substances are dealt with in a section of the Energetics Notes and also here in section 14.
  • 14. Comparison of latent heat changes in physical changes of state for different substances

Learning objectives for the state of matter changes of melting and freezing

Know that the freezing point of a liquid substance is the same as the melting point of the solid substance, its just a question of which direction the state of the substance changes.

Be able to draw particle pictures to illustrate and explain melting and freezing.

Know what we mean by melting and freezing and they are the opposite of each other.

Be able to use the kinetic particle theory of matter to explain the state of matter change of melting (solid to liquid)

Be able to use the kinetic particle theory of matter to explain the state of matter change of freezing (liquid to solid)

Know that particles in a solid vibrate more forcefully when heated to increase their temperature.

Know that thermal energy is released by a liquid or removed it to freeze it - endothermic processes.

Know that thermal energy must be absorbed by a solid before it can melt - endothermic processes.


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INDEX of all my notes on the states of matter

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Detailed notes on the states of matter and their properties

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