OCR · GCSE · Physics
Course version: J249 · Version from Sept 2018
Explain the differences in density between the different states of matter…
Full specification wording
The particle model: explain the differences in density between the different states of matter in terms of the arrangements of the atoms and molecules
Density differs between solids, liquids and gases because of how the particles are arranged and how closely they sit.
Checked against the OCR J249 specification (opens in a new tab) on .
- Point 6 of 7
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Density differs between solids, liquids and gases because of how the particles are arranged and how closely they sit. In a solid, particles are packed tightly in a regular arrangement and only vibrate about fixed positions, so a given volume holds many particles and the density is high. In a liquid, particles are still close together but are irregularly arranged and can slide past one another, so the density is usually slightly lower than the solid but still high. In a gas, particles are far apart and move randomly at high speed, so the same volume contains far fewer particles and the density is much lower. The key idea is that density depends on how much mass is squeezed into a volume, which follows from particle spacing. In a multiple-choice question, match each state to its spacing and arrangement.
What earns the marks
- States that solid particles are closely packed in a regular arrangement, giving high density.
- States that liquid particles are close together but irregularly arranged and able to move past each other.
- States that gas particles are far apart and move randomly, giving much lower density.
- Links the density of each state to the spacing and arrangement of its particles.
Common mistakes and how to put them right
Claims gas particles are heavier or lighter than solid particles.
Put it right by explaining that the difference in density comes from spacing, not particle mass.
Says liquid particles are far apart like a gas.
Put it right by stating that liquid particles remain close together but are irregularly arranged.
Describes solid particles as free to move around.
Put it right by stating that they vibrate about fixed positions in a regular arrangement.
Exam tips
- For each option, check both the spacing and the arrangement described before deciding.
- Remember the order of density: generally solid highest, liquid next, gas lowest.
- Link any statement about density back to how closely the particles are packed.
Try the lesson
The start of the The particle model: explain the differences in density between the different states of matter in terms of the arrangements of the atoms and molecules lesson, made from checked teaching notes for this specification point.
Have a go first
In which state of matter are the particles furthest apart on average?
Listen
Density differs between solids, liquids and gases because of how the particles are arranged and how closely they sit. In a solid, particles are packed tightly in a regular arrangement and only vibrate about fixed positions, so a given volume holds many particles and the density is high. In a liquid, particles are still close together but are irregularly arranged and can slide past one another, so the density is usually slightly lower than the solid but still high. In a gas, particles are far apart and move randomly at high speed, so the same volume contains far fewer particles and the density is much lower. The key idea is that density depends on how much mass is squeezed into a volume, which follows from particle spacing. In a multiple-choice question, match each state to its spacing and arrangement. A strong answer earns marks for these points. States that solid particles are closely packed in a regular arrangement, giving high density. States that liquid particles are close together but irregularly arranged and able to move past each other. States that gas particles are far apart and move randomly, giving much lower density. Links the density of each state to the spacing and arrangement of its particles. A common slip: Claims gas particles are heavier or lighter than solid particles. Instead, explaining that the difference in density comes from spacing, not particle mass. Exam tip: For each option, check both the spacing and the arrangement described before deciding.
See it
Key words
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Particle arrangement in solids, liquids and gases
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Particle spacing and its link to density
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Movement of particles in each state
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Explaining density differences using the particle model
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See every definition
- Particle arrangement in solids, liquids and gases
- Solid particles are closely packed in a regular arrangement, liquid particles are close together but irregularly arranged, and gas particles are far apart.
- Particle spacing and its link to density
- Density depends on how much mass is squeezed into a volume, which follows from particle spacing.
- Movement of particles in each state
- Solid particles vibrate about fixed positions, liquid particles can slide past one another, and gas particles move randomly at high speed.
- Explaining density differences using the particle model
- A given volume holds many particles when they are packed tightly, giving high density, and far fewer particles when they are far apart, giving much lower density.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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