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

11. Explaining melting and freezing using the kinetic particle model

State changes: solid <==> liquid (also called phase changes)

GCSE (~US grades 8-10) level multiple choice QUIZ on the states of matter: gases, liquids & solids

11a. Melting (solid to liquid)

• 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

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11b. Freezing (liquid to solid)

Explained using the kinetic particle theory of liquids and solids

• 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

KEY POINTS about melting and freezing:

Designed for syllabus-aligned revision notes on melting and freezing explained with the kinetic particle model, tailored for WJEC, CCEA, CIE IGCSE, AQA, Edexcel, OCR Gateway, and OCR 21st Century GCSE Chemistry.

Kinetic Particle Model: Melting and Freezing

Melting

• Definition: Solid → liquid change of state.
• Explanation (kinetic particle model theory):
  • In solids, particles are closely packed, vibrating in fixed positions.
  • Heating increases kinetic energy → vibrations become stronger.
  • At the melting point, vibrations overcome intermolecular forces holding particles in place.
  • Particles break free from fixed positions → liquid forms.
• Example: Ice melting at 0 °C into liquid water.

Freezing

• Definition: Liquid → solid change of state.
• Explanation (kinetic particle model theory):
  • In liquids, particles move freely but remain close together.
  • Cooling reduces kinetic energy → particles move more slowly.
  • At the freezing point, intermolecular forces become strong enough to lock particles into fixed positions.
  • Ordered solid lattice forms.
• Example: Water freezing at 0 °C into ice.

Key Differences between melting and freezing

Typical Exam Board Requirements about melting and freezing

• Describe in terms of particle motion and energy transfer during melting/freezing.
• Use particle diagrams showing ordered versus disordered arrangements.
• Using kinetic theory to describe energy changes at melting/freezing points.
• Links melting/freezing to heating/cooling curves (see Part 13).
• Includes practical contexts (melting ice, freezing water in calorimetry).
• Stresses energy transfer and particle motion.
• Applies melting/freezing to everyday contexts (food science, weather).

Student Exam Tips about melting and freezing

• Use particle diagrams: Show solid lattice breaking apart (melting) versus liquid particles locking into lattice (freezing).
• Mention energy transfer: Melting absorbs energy; freezing releases energy.
• State conditions: Both occur at fixed temperatures (melting/freezing points).
• Link to heating/cooling curves: Temperature stays constant during melting/freezing until the change is complete.
• Give everyday examples: Ice cubes melting, water freezing in winter.

Common Misconceptions about melting and freezing

•  “Melting point and freezing point are different” → For a pure substance, they are the same temperature.
•  “Particles gain energy during freezing” → They lose kinetic energy.
•  “Temperature changes during melting/freezing” → It stays constant until the state change finishes.
•  “Melting breaks chemical bonds” → Only intermolecular forces are overcome, not covalent/ionic bonds.
•  “Freezing is the same as condensation” → Freezing = liquid → solid; condensation = gas → liquid.

Quick Recap of melting and freezing

• Melting: Solid → liquid, particles absorb energy, overcome forces, move freely.
• Freezing: Liquid → solid, particles lose energy, forces fix them into lattice.
• Both explained by kinetic particle model: energy changes alter particle motion and intermolecular forces.

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Website content © Dr Phil Brown 2000+. All copyrights reserved on Doc Brown's Chemistry revision notes, images, quizzes, worksheets etc. Copying of website material is NOT permitted. GCSE level and advanced pre-university level revision notes. Detailed notes on the states of matter and their properties. Based on the syllabus-specifications for students taking the IGCSE/GCSE level physics examinations summary revision notes and key points about explaining melting and freezing using the kinetic particle model, for students taking the WJEC gcse chemistry/physics, CCEA gcse chemistry/physics, CIE igcse chemistry/physics, AQA igcse/gcse physics, Edexcel gcse chemistry/physics, OCR 21st century chemistry/physics, OCR gateway chemistry/physics or any other GCSE or IGCSE level chemistry/physics exams e.g. US grade 9-10 physics courses INDEX of all my notes on the states of matter GCSE (~US grades 8-10) level multiple choice QUIZ on the states of matter: gases, liquids & solids