States of Matter and Changes of State

OCR

GCSE

The kinetic particle model describes matter in terms of the arrangement, movement, and energy of particles in solids, liquids, and gases. Changes of state are physical transitions requiring energy transfer to overcome intermolecular forces or the release of energy when these forces form, distinct from chemical changes. Candidates must interpret heating and cooling curves to determine melting and boiling points, explain the energy constancy during state changes, and critically evaluate the limitations of representing particles as solid, inelastic spheres without forces.

Objectives

Exam Tips

Pitfalls

Key Terms

Mark Points

What You Need to Demonstrate

Key skills and knowledge for this topic

Award 1 mark for stating particles in a liquid are arranged randomly and are in contact with each other (touching)
Credit responses that link the flat section (plateau) of a heating curve to energy being used to overcome forces of attraction rather than raising temperature
Award 1 mark for identifying that the particle model does not represent the forces between particles or that particles are not solid spheres
Candidates must predict the physical state at a given temperature by correctly comparing it to provided melting and boiling point data
Award 1 mark for stating that gas particles move randomly at high speeds in all directions

Example Examiner Feedback

Real feedback patterns examiners use when marking

"You correctly identified the state, but you must explain *why* by explicitly comparing the temperature to the melting/boiling points"
"Good description of particle arrangement. To improve, explicitly mention the relative energy of the particles in your comparison"
"You mentioned bonds breaking. Remember, for simple molecules, it is the *forces between* molecules that are overcome, not the covalent bonds inside them"
"Excellent evaluation of the model. Ensure you also mention that particles are not actually solid spheres to secure the AO3 mark"

Marking Points

Key points examiners look for in your answers

Award 1 mark for stating particles in a liquid are arranged randomly and are in contact with each other (touching)
Credit responses that link the flat section (plateau) of a heating curve to energy being used to overcome forces of attraction rather than raising temperature
Award 1 mark for identifying that the particle model does not represent the forces between particles or that particles are not solid spheres
Candidates must predict the physical state at a given temperature by correctly comparing it to provided melting and boiling point data
Award 1 mark for stating that gas particles move randomly at high speeds in all directions

Examiner Tips

Expert advice for maximising your marks

💡When asked to predict a state at a specific temperature (e.g., -10°C), draw a number line with the melting and boiling points to visualize where the value falls
💡In 6-mark 'Level of Response' questions comparing states, ensure you cover three distinct aspects: arrangement, movement, and relative energy of particles
💡Always refer to 'intermolecular forces' when discussing simple covalent substances changing state; never say 'bonds break' unless referring to giant covalent structures

Common Mistakes

Pitfalls to avoid in your exam answers

Drawing particles in a liquid with significant gaps between them; examiners expect particles to be touching but irregularly arranged
Stating that 'covalent bonds' break when a simple molecular substance melts, rather than 'intermolecular forces' or 'forces between molecules'
Believing that temperature rises during a change of state; candidates often fail to recognize that temperature remains constant during melting and boiling
Confusing the terms 'evaporation' and 'boiling'; failing to note that boiling occurs at a specific temperature throughout the liquid

Study Guide Available

Comprehensive revision notes & examples

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Key Terminology

Essential terms to know

Particle arrangement and movement in three states

Interconversions and energy transfers (melting, freezing, boiling, condensing, subliming)

Interpretation of heating and cooling curves

Limitations of the simple particle model

State symbols in chemical equations

Likely Command Words

How questions on this topic are typically asked

State

Describe

Explain

Predict

Compare

Evaluate

Practical Links

Related required practicals

•{"code":"PAG C2","title":"Investigation of cooling curves (e.g., stearic acid or salol)","relevance":"Demonstrates temperature plateaus during changes of state"}

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