How does the particle model explain the three states of matter and the changes between them?
The particle model of solids, liquids and gases, the arrangement and movement of particles in each state, changes of state and their names, the energy changes involved, and the limitations of the simple particle model.
A focused answer to the OCR Gateway GCSE Combined Science A topic C1 on the particle model, covering the arrangement and movement of particles in solids, liquids and gases, the changes of state and their names, the energy changes, and the limitations of the simple particle model.
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What this topic is asking
OCR wants you to use the particle model to describe solids, liquids and gases, explain the changes of state and their names, describe the energy changes involved, and state the limitations of the simple particle model.
The three states of matter
- In a solid, the particles are packed closely together in a regular arrangement, touching, held by strong forces of attraction. They cannot move from place to place; they only vibrate about fixed positions. So a solid has a fixed shape and a fixed volume and cannot be compressed.
- In a liquid, the particles are still close together but are randomly arranged and can move around each other. So a liquid has a fixed volume but flows and takes the shape of its container, and can only be compressed very slightly.
- In a gas, the particles are far apart with large spaces between them and move randomly at high speed. So a gas has no fixed shape or volume, fills its container, and can be compressed easily because of the empty space.
Changes of state
When a substance is heated or cooled, energy is transferred and it can change state. These changes are physical, not chemical, because no new substance forms and they can be reversed.
When a solid melts, the particles gain energy, vibrate more, and the forces holding them in fixed positions are partly overcome so they can move around each other. When a liquid boils, the particles gain enough energy to fully overcome the forces and move far apart as a gas. During a change of state the temperature stays constant while the energy goes into overcoming the forces rather than raising the temperature.
Limitations of the simple particle model
The simple model is useful but has limitations OCR expects you to know. It treats particles as solid inelastic spheres, which ignores that real particles are not solid balls, that there are forces between them (the model does not show these forces or how strong they are), and that particles are mostly empty space inside. These simplifications mean the model cannot fully explain every property, but it is good enough to explain states of matter and changes of state.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20184 marksUse the particle model to explain why a gas can be compressed easily but a solid cannot, and why a solid has a fixed shape.Show worked answer →
A Chemistry Paper 3 structured question on the particle model. Reward: in a gas the particles are far apart with large spaces between them and move randomly at high speed, so they can be pushed closer together (compressed) into the empty space. In a solid the particles are packed closely together in a regular arrangement, touching, with almost no space between them, so they cannot be pushed closer (a solid cannot be compressed). The particles in a solid only vibrate about fixed positions and are held by strong forces of attraction, so the solid keeps a fixed shape. Markers credit the spacing (far apart versus close), the movement, and the link from fixed positions to fixed shape.
OCR 20203 marksName the change of state when a solid turns directly into a gas, and describe what happens to the energy and arrangement of the particles when a solid melts.Show worked answer →
A C1 recall question on changes of state. Reward: a solid turning directly into a gas is sublimation. When a solid melts, energy is transferred to the particles (the solid is heated), the particles gain energy and vibrate more until the forces holding them in fixed positions are partly overcome; the regular arrangement breaks down and the particles can move around each other, forming a liquid. Markers credit naming sublimation and describing energy being taken in, the forces being overcome, and the particles becoming able to move (the arrangement becoming less ordered).
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