How does the particle model explain the states of matter and how they change?
Describe the arrangement and motion of particles in solids, liquids and gases, and explain changes of state, diffusion and the kinetic particle theory
A focused answer to the O-Level Combined Science outcome on the particulate nature of matter. The kinetic particle theory, the three states and their arrangement, changes of state, and diffusion.
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What this dot point is asking
SEAB wants you to use the kinetic particle theory to describe how particles are arranged and how they move in solids, liquids and gases, and to explain changes of state and diffusion in terms of those particles. The marks come from clear, particle-level descriptions: arrangement, spacing and motion for each state, and what happens to the particles during melting, boiling and diffusion.
The answer
The kinetic particle theory
All matter is made of tiny particles that are in constant motion. The hotter the substance, the more energy the particles have and the faster they move. The way the particles are arranged and how freely they move decides whether a substance is a solid, a liquid or a gas.
The three states
- Solid. Particles are packed closely in a regular, fixed pattern. They vibrate about fixed positions but cannot move around, so a solid has a fixed shape and a fixed volume.
- Liquid. Particles are close together but arranged randomly. They can slide past one another, so a liquid has a fixed volume but takes the shape of its container.
- Gas. Particles are far apart and move quickly in random directions. A gas has no fixed shape or volume and fills any container; it can be compressed because of the large gaps.
Changes of state
Heating a solid makes the particles vibrate harder until, at the melting point, they break free of their fixed positions and the solid melts to a liquid. More heating lets the particles escape the liquid as a gas at the boiling point. Cooling reverses these: a gas condenses to a liquid and a liquid freezes to a solid. During a change of state the temperature stays constant while energy goes into changing the arrangement.
Diffusion
Diffusion is the spreading of particles from a region of high concentration to a region of low concentration, caused by their random motion. It happens faster in gases than liquids because gas particles move more quickly and are further apart. Lighter (less dense) particles diffuse faster than heavier ones at the same temperature.
Examples in context
Example 1. Smelling food from another room. The particles of the scent are gases in constant random motion. They diffuse through the air, spreading from the kitchen where they are concentrated until they reach your nose, which is why the smell takes a little while to arrive.
Example 2. Drying washing. Water particles at the surface of wet clothes gain enough energy to escape as a gas (evaporation), even below the boiling point. A warm, breezy day speeds this up because the particles have more energy and are carried away, so the washing dries faster.
Try this
Q1. Describe the arrangement of particles in a liquid. [2 marks]
- Cue. The particles are close together but arranged randomly, and they can slide past one another, giving a fixed volume but no fixed shape.
Q2. Name the change of state when a gas turns into a liquid. [1 mark]
- Cue. Condensation (the gas particles lose energy and come close together to form a liquid).
Q3. Explain why diffusion is faster in a gas than in a liquid. [2 marks]
- Cue. Gas particles move faster and are much further apart than liquid particles, so they spread out and mix more quickly.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original4 marksUsing the kinetic particle theory, describe the arrangement and movement of the particles in (a) a solid and (b) a gas.Show worked answer →
(a) In a solid the particles are packed closely in a regular, fixed arrangement. They vibrate about fixed positions but cannot move from place to place, so a solid has a fixed shape and volume.
(b) In a gas the particles are far apart and arranged randomly. They move quickly in all directions and collide with each other and the walls, so a gas has no fixed shape and fills its container.
Markers reward describing both the arrangement (close and fixed versus far apart and random) and the motion (vibration versus rapid random movement) for each state.
Original3 marksA few drops of brown bromine vapour are placed at the bottom of a gas jar. After some time the whole jar turns pale brown. (a) Name the process. (b) Explain it using the particle model.Show worked answer →
(a) The process is diffusion.
(b) The bromine particles are in constant random motion. They spread out from the region where they are concentrated into the empty space among the air particles until they are evenly mixed, colouring the whole jar.
Markers reward naming diffusion and explaining it as the random movement of particles spreading from high to low concentration until evenly spread.
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