K223 / K224 / 2027
Kinetic particle model of matter overview

Lesson 1 of 3

States of matter

  • I can compare physical properties of solids, liquids and gases.Syllabus

    Syllabus K223 / K224, 6(a). Compare physical properties of solids, liquids and gases.

  • I can explain the properties of all three states using particle arrangement, motion, forces and distances.Syllabus

    Syllabus K223 / K224, 6(b). Explain the properties of all three states using particle arrangement, motion, forces and distances.

Make a guess. You are not marked.

A gas in a syringe is easy to squash, but the same syringe full of water is not. Why?

  1. Gas particles are smaller and softer than the particles in a liquid.
  2. Gas particles are far apart, while liquid particles are already touching.
  3. Liquid particles do not move, so they cannot be pushed together.
Show the answer

Gas particles are far apart, while liquid particles are already touching.

Squashing a gas pushes its particles into the large empty spaces between them. In a liquid the particles are already close together, so there is almost no space to remove.

In short

  • Solid: particles close together in fixed positions, vibrating. Fixed shape and volume.
  • Liquid: particles close together but able to slide past each other. It flows and keeps its volume.
  • Gas: particles far apart, moving fast in all directions. It fills its container and is easy to squash.

Use the arrangement, motion, forces and spacing of particles to explain the properties of solids, liquids and gases.

Particles may be atoms or molecules, depending on the substance. The kinetic particle model describes matter using these particles and their interactions. It links what we observe, such as a liquid flowing, to an explanation at a much smaller scale.

More detail (optional)

A drawing of circles is a model, not a literal microscope image. Its symbol sizes, gaps and particle counts help show differences; they do not give the actual sizes or numbers in a sample.

Use arrangement and motion to explain the properties

Circles represent particles such as atoms or molecules. The particle symbols stay the same size. These are schematic models, with selected motion indicators shown in blue.

Solid: a crystalline model

Solid: a crystalline model: Close particles vibrate about fixed mean positions.Fixed shape and approximately fixed volume. Close particles vibrate about fixed mean positions. Twelve particles occupy a regular arrangement. The double-ended blue arrows indicate vibration about fixed mean positions, not flow through the solid. This is a crystalline-solid model. All three models use the same particle-symbol size. Sizes, distances and particle counts are schematic, not measurements of the samples.Back-and-forth vibration

Fixed shape and approximately fixed volume. Close particles vibrate about fixed mean positions.

Liquid

Liquid: Close particles move past one another and change neighbours.Takes the container's shape; approximately fixed volume. Close particles move past one another and change neighbours. Twelve particles form a close but irregular arrangement in the lower part of a container. Selected blue arrows indicate motion past other particles; the remaining particles also move. All three models use the same particle-symbol size. Sizes, distances and particle counts are schematic, not measurements of the samples.

Takes the container's shape; approximately fixed volume. Close particles move past one another and change neighbours.

Gas

Gas: Widely separated particles move randomly through the space.Fills the available container and is easily compressed. Widely separated particles move randomly through the space. Twelve particles are spread throughout a much larger container. Blue motion arrows have varied lengths and directions. All three models use the same particle-symbol size. Sizes, distances and particle counts are schematic, not measurements of the samples.

Fills the available container and is easily compressed. Widely separated particles move randomly through the space.

Compare all four features: particle arrangement, motion, forces and spacing. The solid panel represents a crystalline arrangement. The symbols are the same size across the panels; a change of state does not mean that the atoms or molecules become bigger.

Solid

Observable properties: Usually keeps its own shape and volume under ordinary conditions. It does not flow like a liquid and is difficult to compress.

Arrangement and spacing
Particles are close together and remain near fixed positions. The regular pattern drawn here represents a crystalline solid; not every solid has that regular arrangement.
Motion
Particles vibrate about their positions. They are not motionless, even though they do not move freely through the solid.
Forces
Forces between particles keep them near their positions and resist changes of shape. Their close spacing makes a large reduction in volume difficult.

Liquid

Observable properties: Has a nearly fixed volume but takes the shape of the part of the container it occupies. It flows and is difficult to compress.

Arrangement and spacing
Particles are close together in an irregular arrangement. There is much less space between them than in a gas.
Motion
Particles move randomly and can move past their neighbours. This allows the liquid to flow and change shape.
Forces
Attractions keep the particles close while allowing them to rearrange. Liquid particles do not behave as if there are no forces between them.

Gas

Observable properties: Has no fixed shape or volume. It flows, spreads through the available container and can be compressed much more easily than a solid or liquid.

Arrangement and spacing
Particles are far apart compared with their own sizes. Much of the container is space between the particles.
Motion
Particles move rapidly in random directions through the available volume, changing direction when they collide.
Forces
In the ordinary gas model, attractions are negligible for most of the time between collisions. Particles are not held close together as in a liquid or solid.

Explain two properties of the same liquid

Water can be poured because its particles can move past one another. At the same time, it is difficult to compress because its particles are already close together. Being able to flow does not mean that there must be large empty spaces between the particles.

Worked explanation

Air and water in sealed syringes

Compare two otherwise identical sealed syringes, one containing air and the other filled with water and no trapped air. Apply comparable modest forces to their plungers.

  1. Observation: the air volume decreases much more than the water volume.
  2. Gas explanation: its particles are initially widely separated, so the spaces between them can become much smaller.
  3. Liquid explanation: its particles are already close together, so there is little space to remove without strong resistance.

The air molecules themselves are not being squeezed into much smaller molecules. The large change is in their spacing. Trapped air in the water syringe would make the comparison less useful because that air can compress.

A gas can fill a larger volume without larger particles

If a fixed amount of gas is allowed into a larger container, its random motion spreads the particles through the newly available space. The number and sizes of the particles do not need to increase. Their average spacing becomes greater.

More detail (optional)

Its mass stays the same while its occupied volume increases, so its density, mass divided by volume, decreases. A particle explanation and a bulk quantity can describe the same change from different viewpoints.

Use the feature that explains the property. Motion past neighbours explains flow. Close spacing explains the difficulty of compression. Forces help explain why particles remain close or near fixed positions. Listing "particles move" alone does not explain all three states.

Check your understanding

Water can be poured from one container into another, but a water-filled sealed syringe is difficult to compress. Which particle explanation accounts for both observations?

  1. The particles shrink during pouring but cannot shrink inside a syringe.
  2. Liquid particles have no forces between them and remain far apart.
  3. The particles can move past one another, while remaining close together.
Show the answer

The particles can move past one another, while remaining close together.

Movement past neighbours allows the liquid to flow and change shape. There is little space to remove between the already close particles, so the volume changes very little under a modest compressing force.

Practise an exam question

Write your answer first, then tick the mark points it makes.

Exam-style question

Describe the arrangement and motion of the particles in a solid and in a gas. Use your answer to explain why a gas can be compressed easily but a solid cannot. [3]

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