How does the particle model explain the three states of matter and the changes between them?
Properties of solids, liquids and gases in terms of particle arrangement and motion, changes of state, and interpreting heating curves.
A CCEA GCSE Physics answer on the arrangement and motion of particles in solids, liquids and gases, the changes of state between them, and how to interpret a heating curve including the flat sections.
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What this dot point is asking
CCEA wants you to describe solids, liquids and gases using the particle model, name the changes of state between them, and interpret a heating curve, including why the temperature stays constant during a change of state. This is core particle-model knowledge.
The answer
The three states of matter
Because solid and liquid particles are close together, they are hard to compress; gas particles are far apart, so gases are easily compressed.
Changes of state
A change of state is a physical change in which the particles are rearranged but the substance itself stays the same:
- Melting (solid to liquid) and freezing (liquid to solid).
- Boiling or evaporating (liquid to gas) and condensing (gas to liquid).
- Sublimation (solid straight to gas).
Heating curves
A heating curve plots temperature against time as a substance is heated steadily.
Worked example: reading a heating curve
Examples in context
Example 1. A puddle drying. Water evaporates from a puddle even below its boiling point: the fastest surface particles escape into the air, slowly turning the liquid into water vapour.
Example 2. Frost forming. Water vapour in cold air condenses and then freezes directly onto a surface, an everyday change from gas to solid that releases energy to the surroundings.
Try this
Q1. Describe the arrangement of particles in a liquid. [2 marks]
- Cue. Close together but irregular; they can move past one another.
Q2. Name the change of state from gas to liquid. [1 mark]
- Cue. Condensing (condensation).
Q3. Why does the temperature stay constant while a solid melts? [2 marks]
- Cue. The energy supplied breaks the forces between particles rather than raising their kinetic energy.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA style4 marksDescribe the arrangement and motion of the particles in a solid and in a gas, and use this to explain why a solid keeps its shape but a gas fills its container.Show worked answer →
In a solid the particles are packed closely in a regular, fixed arrangement and vibrate about fixed positions. Strong forces hold them in place, so a solid keeps a fixed shape and volume.
In a gas the particles are far apart and move quickly in all directions, with very weak forces between them. They are free to move anywhere, so a gas spreads out and fills its container.
Markers reward: solid particles close, regular, vibrating about fixed positions; gas particles far apart, fast, random; and linking these to fixed shape versus filling the container.
CCEA style3 marksA heating curve for a substance has a flat (horizontal) section even though heat is still being supplied. Explain what is happening to the substance and its energy during this flat section.Show worked answer →
The flat section is a change of state, for example melting or boiling. The temperature stays constant while the change happens.
The energy supplied is used to break the forces (bonds) between particles rather than to raise the temperature, so the kinetic energy of the particles does not increase during this section.
Markers reward: flat section is a change of state at constant temperature; and energy goes into breaking bonds / separating particles, not raising temperature.
Related dot points
- Explaining gas pressure in terms of particle collisions, and how gas pressure changes with temperature and volume at the particle level.
A CCEA GCSE Physics answer on how gas pressure arises from particle collisions with container walls, and how the pressure changes when the temperature or volume of a fixed mass of gas is changed.
- Heat transfer by conduction, convection and infrared radiation, the role of surface colour, and methods of reducing unwanted energy transfers.
A CCEA GCSE Physics answer on heat transfer by conduction, convection and infrared radiation, how surface colour affects emission and absorption, and how insulation reduces unwanted energy transfers from buildings.
- Density as mass per unit volume, the equation rho = m / V, and experiments to measure the density of regular solids, irregular solids and liquids.
A CCEA GCSE Physics answer on density as mass per unit volume, the equation rho = m / V, and the methods used to measure the density of regular solids, irregular solids and liquids.
- Pressure as force per unit area, P = F / A, pressure in liquids increasing with depth and density, and the equation P = rho g h.
A CCEA GCSE Physics answer on pressure as force per unit area, the equation P = F / A, how pressure in a liquid increases with depth and density, and using P = rho g h.
Sources & how we know this
- CCEA GCSE Physics specification — CCEA (2017)