Chapter 7: Model of Matter – The Particulate Nature of Matter (Summary Sheet)

Part A · Matter as a Model

1 Matter and the Particle Model

Matter is anything that has mass and takes up space — which is why light and sound are not matter.
The kinetic particle theory explains the properties of matter with three ideas, all of which you can write as a mark each:
All matter is made up of particles too small to be seen directly.
The particles in matter attract one another — it is these forces of attraction that hold a substance together.
The particles have kinetic energy and are in constant, random motion.
Since the particles themselves cannot be seen, scientists use a model — the particulate nature of matter — to represent matter as small discrete particles, drawn differently for solids, liquids and gases.

2 Constant Random Motion — and How We Know

Brownian motion is the never-ending random jiggle of tiny specks floating in a fluid — a liquid or a gas — and it happens because the fluid's own particles keep colliding with them.
Shine a strong light on smoke under a microscope and the smoke specks jiggle about in no particular direction — they are being knocked about by air particles they cannot see.
That jiggling is the evidence for the motion of the invisible particles: something must be hitting the smoke, and the only thing there is air.
It is also why the motion is called random: nothing steers the particles, they simply collide and change direction.

3 Why Models — and the Attitudes Behind Them

A model is a simplified picture of something we cannot see: it explains what has been observed and predicts what will happen next.
The particle model asks you to accept something odd — that a solid block is mostly empty space — so it took creativity and open-mindedness to propose it in the first place.
Models are kept only while they explain the evidence: when Brownian motion was observed, the particle picture was re-examined and strengthened rather than abandoned.
That is the scientific attitude the syllabus asks for: be willing to change the model when new evidence appears, instead of defending the old one.

Part B · The Three States of Matter

4 Arrangement and Movement: Solid, Liquid, Gas

State	Arrangement	Space between	Forces of attraction	Movement of particles
Solid	Packed very closely, in an orderly arrangement	Very little	Very strong	Vibrate and rotate, staying in fixed positions
Liquid	Packed closely but in a disorderly arrangement	Little	Strong — but weaker than in a solid	Slide past one another within the liquid
Gas	Very far apart, in a disorderly arrangement	A lot	Very weak	Move randomly in any direction, at high speed

Energy runs the same way as the spacing: as temperature rises, the average kinetic energy of the particles rises, so they move more quickly — solid → liquid → gas.

5 Explaining the Properties of Each State

Property	Solid	Liquid	Gas
Particle diagram
Volume	Fixed	Fixed	Not fixed — fills the container
Shape	Fixed	Takes the shape of the container	Takes the shape of the whole container
Can it be compressed?	No	No	Yes

Never answer with the property alone — link it back to the particles: a solid keeps its shape because its particles are packed closely with very strong forces, holding them in place.
A gas is compressible because of the large space between its particles: pushing them closer takes very little effort.

Part C · Heating, Cooling and the Conservation of Mass

6 Expansion and Contraction

Expansion is the increase in volume of a substance when it is heated.
Contraction is the decrease in volume of a substance when it is cooled.
The explanation is always the same: heating gives the particles more kinetic energy, so they vibrate and move more quickly and end up further apart; cooling removes energy, so they slow down and sit closer together.
The particles themselves do not change in size — only the space between them changes. Writing that sentence is usually where the mark is.
Familiar example: run a metal lid under hot water and it expands, so it unscrews easily — cool it again and it is hard to unscrew.

7 Conservation of Mass

Changes when the substance is heated or cooled	Stays the same
The space between the particles	The number of particles
The arrangement of the particles	The size of each particle
How quickly the particles move	The total mass of the substance

Mass is conserved during expansion and contraction: nothing has been added or removed, so the mass does not change.
The same is true across a change of state — melt an ice cube and the water weighs exactly what the ice did, because every particle is still there.

8 Melting

Melting is the change from solid to liquid, and it absorbs thermal energy from the surroundings.
It happens at the melting point of the substance — a fixed temperature for a pure substance.
What the model says: the particles vibrate so vigorously that they break free from their fixed positions and take on a disorderly arrangement, moving further apart.
During melting the temperature stops rising. The energy being absorbed is used to overcome the forces of attraction between the particles, so their average kinetic energy does not change.
While it is melting, the substance is a mixture of solid and liquid.

Part of the heating curve	Temperature	What the particles are doing
Before melting	Rises	Solid: they vibrate and rotate more quickly about fixed positions
While melting	Constant — a flat plateau	Taking in energy to break free of the forces holding them in place; arrangement becomes disorderly
After melting	Rises again	Liquid: they slide past one another more quickly

9 Boiling

Boiling is the change from liquid to gas, and it absorbs thermal energy as well.
It happens at the boiling point, another fixed temperature for a pure substance.
What the model says: the particles gain enough kinetic energy to overcome the forces of attraction completely, then move randomly in any direction at high speed, far apart.
The temperature is constant while boiling, exactly as in melting — the energy goes into pulling the particles apart, not into speeding them up.
While it is boiling, the substance is a mixture of liquid and gas.
So the three states convert in a chain: solid → liquid → gas as energy is absorbed, and back again as it is released.

Part D · Diffusion

10 Diffusion

Diffusion: the net movement of particles from a region of higher concentration to a region of lower concentration.
Concentration here means how many particles there are in a given volume — more particles in the same volume is a higher concentration, fewer is a lower concentration.
Net movement is the key word: particles move in every direction, but more of them move from the crowded region to the emptier one, so overall the crowd spreads out.
It stops when the particles are evenly mixed — the concentrations are then the same everywhere.
It needs no pushing and no stirring, because the particles are already in constant random motion — diffusion is the same motion, simply spreading out.
Diffusion happens in gases and liquids, because their particles are free to move past one another.

11 Diffusion in Evidence

Purple crystals dropped into a beaker of water colour the water at the bottom first; leave it a few hours and the whole beaker is evenly coloured.
A few drops of bromine in one gas jar, with a jar of air above it, end up as one jar of evenly pale brown gas — the bromine and air particles have spread through both jars.
Everyday versions: a perfume smells across the room without anyone fanning it, and a drop of colouring spreads through a glass of water on its own.
Answer questions the same way every time: name the particles that moved, say they went from higher to lower concentration, and finish with evenly mixed.

Part E · Exam Tips

12 Exam Tips: What Each Syllabus Pointer Asks For

The pointer	What earns the mark
Show an awareness about the particulate nature of matter being a model — matter made up of small discrete particles in constant and random motion	Say it is a model, that the particles are far too small to see, and that they are in constant random motion — offer Brownian motion as the evidence
Describe the arrangement and movement of the particles in the solid, liquid and gaseous states	Give both halves for each state: how they are arranged (spacing, order) and how they move (vibrate about fixed positions · slide past one another · move randomly at high speed)
Show an understanding that diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration	Use the words net movement and higher to lower concentration, and say it ends when the particles are evenly mixed
Explain expansion and contraction, and the conservation of mass during these processes, using models	Say the space between the particles changes while the number and size of the particles stay the same — so the mass is conserved
Explain melting and boiling in terms of the conversion of three states of matter, using models	Name the change and the energy taken in, then explain the flat part of the graph: the energy overcomes the forces of attraction instead of raising the temperature
Show an appreciation of scientific attitudes such as creativity and open-mindedness in creating models, and the willingness to re-examine existing models (Brownian Motion)	Use Brownian motion as the example: a model must explain the evidence, and scientists changed their thinking when new observations appeared

The mark most often lost in this topic: saying the particles "expand" or "get hotter" during melting and boiling. The particles never change size, and during the change their average kinetic energy stays the same — it is the spacing and the arrangement that change.