FoxChild@Learn
KS3 Science Study Pack: Particles
Key Knowledge
The big idea: matter is made of particles
Everything that has mass and takes up space is matter. Air, water, metal, plastic, wood, glass, skin, food and smoke are all examples of matter. The particle model says that all matter is made from tiny particles.
Particles are much too small to see with a school microscope. A grain of salt looks tiny, but it is still made from a huge number of particles. A drop of water also contains a huge number of particles. We use the particle model because we cannot see individual particles directly in a normal school laboratory, but we can use the model to explain what substances do.
The particle model helps explain:
- why solids keep their shape
- why liquids flow
- why gases spread out to fill a container
- why gases are easier to compress than solids and liquids
- why diffusion happens in liquids and gases
- why heating and cooling can change the state of a substance
- why mass is conserved when a substance changes state in a closed system
In KS3, you should practise explaining observations using three important particle ideas:
- arrangement: how close together the particles are and whether they are in fixed positions
- movement: whether particles vibrate, slide past each other or move freely in all directions
- energy: whether particles have more or less energy, which affects how fast they move or vibrate
You should also use the idea of forces of attraction. Particles in a solid are strongly attracted to each other, so they stay close together in fixed positions. Particles in a liquid are still close together, but they can move around each other. Particles in a gas are much further apart and move freely.
Scientific models: what the particle model shows and what it leaves out
A scientific model is a simplified way of representing something complicated. Models help scientists explain observations, make predictions and communicate ideas.
The particle model is useful because it shows that substances are made from particles and that the particles behave differently in solids, liquids and gases. It can explain why a gas spreads out, why a liquid takes the shape of a container and why a solid is difficult to squash.
However, particle diagrams are not exact pictures of real particles. They are simplified.
A particle diagram usually shows:
- particles as circles
- whether particles are close together or far apart
- whether particles are in a regular pattern or a random arrangement
- whether particles can move freely or only vibrate
A particle diagram does not usually show:
- the true size of the particles
- the true distance between every particle
- the actual shape of different particles
- the forces between particles in detail
- all the particles in a real sample
For example, a gas particle diagram may show only ten circles in a box, but a real gas sample contains an enormous number of particles. The spaces between gas particles are also usually much larger than a simple diagram can show.
Good science explanations use the model carefully. You should write that the diagram is a model, not an exact image.
Comparing solids, liquids and gases
Most substances can exist as a solid, a liquid or a gas. These are called states of matter. The state depends on how the particles are arranged, how they move and how strongly they are attracted to each other.
| State | Particle arrangement | Particle movement | Shape | Volume | Compression |
|---|---|---|---|---|---|
| Solid | Close together in a fixed, regular or mostly regular arrangement | Vibrate around fixed positions | Fixed shape | Fixed volume | Very difficult to compress |
| Liquid | Close together in a random arrangement | Move and slide past each other | Takes the shape of its container | Fixed volume | Difficult to compress |
| Gas | Far apart in a random arrangement | Move quickly in all directions | Fills its container | No fixed volume | Easy to compress |
Solids, liquids and gases behave differently because their particles behave differently. You should avoid explanations that only describe what you can see. A stronger scientific explanation links the visible property to particles.
For example:
- weak explanation: "A liquid flows because it is runny."
- stronger explanation: "A liquid flows because its particles are close together but can move and slide past each other."
Solids
In a solid, the particles are close together. They are arranged in fixed positions. In many solid particle diagrams, the particles are shown in a regular pattern, but some real solids may have less regular arrangements. At KS3, the key idea is that solid particles stay in fixed positions and only vibrate.
Solid particles do not move from place to place. They vibrate around fixed positions. This means a solid keeps its shape. A block of ice stays as a block unless it melts, breaks or is cut. A metal spoon keeps its shape because its particles cannot slide around each other like liquid particles.
Solids also keep a fixed volume. If you put a wooden cube into a beaker, it does not spread out to fill the beaker. Its particles are already close together and strongly held in place.
Solids are difficult to compress because there is very little empty space between the particles. If you try to squash a solid, you cannot easily push its particles much closer together.
Worked example: explain why a solid keeps its shape
Question: Explain why a solid keeps its shape using the particle model.
Step-by-step answer:
- Solid particles are close together.
- They are held in fixed positions by forces of attraction.
- They can only vibrate around these fixed positions.
- Because the particles cannot move or slide past each other, the solid keeps a fixed shape.
Full model answer:
A solid keeps its shape because its particles are close together and held in fixed positions. The particles can only vibrate around these positions. They cannot move freely or slide past each other, so the solid has a fixed shape.
Liquids
In a liquid, the particles are close together but arranged randomly. They are not held in fixed positions. Liquid particles can move and slide past each other.
This explains why liquids flow. If you pour water from a jug into a beaker, the water changes shape to fit the beaker. The particles are still close together, so the liquid keeps the same volume, but the particles can move around each other, so the liquid takes the shape of its container.
Liquids are difficult to compress because the particles are already close together. There is not much empty space between them. This is why a sealed syringe full of water is difficult to push in.
Liquids have a fixed volume but not a fixed shape. If you pour 100 cm3 of water into a narrow measuring cylinder, it looks tall and thin. If you pour the same 100 cm3 into a wide dish, it looks shallow. The shape changes, but the volume stays the same.
Gases
In a gas, particles are far apart and arranged randomly. They move quickly in all directions. Gas particles are not fixed in place and are not close together like particles in solids or liquids.
Gases fill their container because their particles move freely in all directions until they spread through all the available space. If you spray perfume in one part of a room, particles of perfume vapour eventually spread out through the air.
Gases do not have a fixed shape or a fixed volume. A gas takes the shape and volume of its container. If a gas is in a small container, it fills the small container. If the same gas is released into a larger container, it spreads out to fill the larger container.
Gases are easy to compress because there is lots of empty space between the particles. When a gas is compressed, the particles are pushed closer together and the volume decreases.
Worked example: explain why a gas can be compressed
Question: Explain why a gas can be compressed.
Step-by-step answer:
- Identify the state as a gas.
- Gas particles are far apart.
- There is lots of empty space between the particles.
- When the gas is squashed, the particles can be pushed closer together, so the gas volume decreases.
Full model answer:
A gas can be compressed because its particles are far apart with lots of empty space between them. When a force squashes the gas, the particles can be pushed closer together. This reduces the volume of the gas.
Explaining properties using arrangement and movement
A good particle explanation links a property to arrangement and movement.
| Property | Particle explanation |
|---|---|
| A solid keeps its shape | Particles are close together and held in fixed positions. They only vibrate. |
| A liquid flows | Particles are close together but can move and slide past each other. |
| A gas fills a room | Particles are far apart and move quickly in all directions. |
| A solid is hard to compress | Particles are already close together, so there is very little empty space. |
| A gas is easy to compress | Particles are far apart, so there is lots of empty space. |
| A liquid keeps its volume | Particles are close together, so the volume is difficult to reduce. |
When answering questions, include the particles first, then link them to the property. For example:
"A liquid takes the shape of its container because its particles are close together but can move and slide past each other."
This is stronger than:
"A liquid takes the shape of its container because it is wet."
Density and the particle model
Density means how much mass is in a given volume. A dense material has a large amount of mass in a certain volume. A less dense material has a smaller amount of mass in the same volume.
The equation for density is:
density = mass ÷ volume
At KS3, you should understand what density means, even if you are not doing many calculations. If two blocks have the same volume but one has more mass, the one with more mass is denser.
Particle spacing helps explain density. If particles are packed closely together, the same volume can contain more particles and therefore more mass. If particles are far apart, the same volume contains fewer particles and therefore less mass.
This is why gases are usually much less dense than solids and liquids. Gas particles are far apart, so a given volume of gas has much less mass than the same volume of a solid or liquid.
However, you should be careful. Density depends on the substance, not only the state. A small piece of lead can be denser than a large piece of wood because lead has more mass for each unit of volume.
Compression and the particle model
Compression means squashing a substance into a smaller volume. The particle model explains why gases can be compressed much more than solids or liquids.
In solids and liquids, particles are already close together. There is very little empty space between them, so they cannot easily be pushed much closer together. This makes solids and liquids difficult to compress.
In gases, particles are far apart. There is lots of empty space between them. When a gas is compressed, the particles move closer together, but the particles themselves do not shrink. The volume of the gas decreases because the empty space between the particles decreases.
Example: A bicycle pump compresses air. When the handle is pushed down, air particles are forced into a smaller space. This increases the pressure and helps push air into the tyre.
Heating, cooling and particle energy
Heating transfers energy to particles. When particles gain energy, they move or vibrate more. Cooling removes energy from particles. When particles lose energy, they move or vibrate less.
In a solid, heating makes particles vibrate more strongly around their fixed positions. If enough energy is transferred, the particles can overcome some of the forces holding them in fixed positions and the solid melts.
In a liquid, heating makes particles move faster. If enough energy is transferred, particles can separate from the liquid and become a gas. This may happen by evaporation from the surface or by boiling throughout the liquid at its boiling point.
In a gas, heating makes particles move faster. Cooling a gas removes energy. If enough energy is removed, gas particles move more slowly and come closer together to form a liquid. This is condensation.
Important rule:
- heating gives particles more energy
- cooling removes energy from particles
- particles do not get bigger when heated
- particles do not disappear when cooled
Changes of state explained step by step
A change of state happens when a substance changes between solid, liquid and gas. Changes of state are physical changes. This means the substance is still the same substance, but the arrangement and movement of its particles have changed.
The main changes of state are:
| Change of state | Description |
|---|---|
| Melting | Solid to liquid |
| Freezing | Liquid to solid |
| Boiling | Liquid to gas throughout the liquid at its boiling point |
| Evaporation | Liquid to gas from the surface below boiling point |
| Condensation | Gas to liquid |
| Sublimation | Solid to gas without becoming liquid |
Melting
Melting is the change from solid to liquid. When a solid is heated, energy is transferred to its particles. The particles vibrate more. At the melting point, the particles have enough energy to overcome some of the forces holding them in fixed positions.
The particles are still close together after melting, but they are no longer fixed in place. They can move and slide past each other. This is why the substance becomes a liquid and can flow.
Example: Ice melts to form liquid water. The water particles are still close together, but they can move around each other.
Freezing
Freezing is the change from liquid to solid. When a liquid is cooled, energy is removed from its particles. The particles move more slowly. At the freezing point, the particles no longer have enough energy to move freely around each other. They become held in fixed positions.
The particles are close together in both the liquid and solid states, but in the solid they are fixed in position and only vibrate.
Example: Liquid water freezes to form ice.
Boiling
Boiling is the change from liquid to gas throughout the liquid. Boiling happens at a substance's boiling point. When a liquid reaches its boiling point, particles throughout the liquid have enough energy to separate and form gas bubbles.
For water at normal air pressure, boiling happens at 100 °C. Bubbles of water vapour form inside the liquid and rise to the surface.
Boiling is different from evaporation because boiling happens throughout the liquid and at a particular boiling point.
Evaporation
Evaporation is the change from liquid to gas from the surface of a liquid. It can happen below the boiling point.
In a liquid, particles have different amounts of energy. Some particles at the surface have enough energy to escape from the liquid and become gas particles. This is evaporation.
Evaporation happens faster when:
- the temperature is higher, because particles have more energy
- there is a larger surface area, because more particles are at the surface
- air moves over the surface, because gas particles are carried away
Example: A puddle dries up on a warm day. Water particles leave the surface of the puddle and spread into the air as water vapour.
Evaporation compared with boiling
| Feature | Evaporation | Boiling |
|---|---|---|
| Where it happens | At the surface only | Throughout the liquid |
| Temperature | Can happen below boiling point | Happens at boiling point |
| Bubbles | Does not produce bubbles throughout the liquid | Produces bubbles of gas in the liquid |
| Example | A puddle drying | Water bubbling in a kettle |
Both evaporation and boiling involve liquid particles becoming gas particles. In both cases, particles gain enough energy to separate from the liquid. The difference is where and when the change happens.
Condensation
Condensation is the change from gas to liquid. When a gas is cooled, energy is removed from its particles. The particles move more slowly. The forces of attraction between particles pull them closer together until they form a liquid.
Example: After a hot shower, water vapour in the air touches a cooler mirror. The particles lose energy and condense into tiny droplets of liquid water on the mirror.
Sublimation
Sublimation is the change from solid directly to gas, without becoming a liquid first. This is less common in everyday life than melting or boiling.
Dry ice is solid carbon dioxide. At normal room conditions, it sublimes to form carbon dioxide gas. The particles gain energy and separate from the solid arrangement into a gas.
At KS3, you do not need to explain sublimation in detail, but you should know that it is a change of state from solid to gas.
Conservation of mass during changes of state
During a change of state, particles are rearranged or move differently. The particles do not disappear, and new particles are not created. This means mass is conserved if the system is closed.
A closed system means matter cannot enter or leave. For example, if ice melts inside a sealed plastic bag, the mass before melting and after melting should be the same. The water particles are still there. They have changed arrangement and movement, but the number of particles has not changed.
In an open system, it may look as if mass has been lost. For example, if water evaporates from an open beaker, the mass of liquid water in the beaker decreases. This does not mean the water particles disappeared. They escaped into the air as water vapour.
Reading Particle Diagrams
Particle diagrams use circles to represent particles. The pattern and spacing of the circles help you identify the state.
Solid
● ● ● ●
● ● ● ●
● ● ● ●
● ● ● ●
Particles are close together in fixed positions.
They vibrate but do not move from place to place.
Liquid
● ● ● ●
● ● ●
● ● ●
● ●
Particles are close together but randomly arranged.
They can move and slide past each other.
Gas
● ●
●
●
●
Particles are far apart and randomly arranged.
They move quickly in all directions.
When interpreting a particle diagram, ask:
- Are the particles close together or far apart?
- Are they in fixed positions or randomly arranged?
- Can they only vibrate, slide past each other or move freely?
- Which state of matter does this represent?
- What property can be explained by this arrangement and movement?
Common diagram mistake: students sometimes say there is air between gas particles. In a pure gas, the gaps are empty space, not air. Air itself is a mixture of gas particles.
Reading Heating and Cooling Curves
A heating curve shows how temperature changes as a substance is heated. A cooling curve shows how temperature changes as a substance is cooled.
On a heating curve:
- sloping sections show temperature increasing
- flat sections show a change of state
- the melting point can be read from the flat section where solid changes to liquid
- the boiling point can be read from the flat section where liquid changes to gas
During a flat section, energy is still being transferred to the substance. However, the temperature does not rise because the energy is used to overcome forces of attraction between particles during the change of state.
Example heating curve:
Temperature (°C)
120 | gas heating
100 |---------------------------- boiling
80 | /
60 | /
40 | / liquid heating
20 |------------------- melting
0 | /
-20 | / solid heating
+--------------------------------
0 2 4 6 8 Time
In this example:
- the melting point is 20 °C
- the boiling point is 100 °C
- the flat sections show changes of state
- the sloping sections show the substance warming up in one state
On a cooling curve:
- sloping sections show temperature decreasing
- flat sections show a change of state
- condensation happens when gas changes to liquid
- freezing happens when liquid changes to solid
During freezing or condensation, energy is transferred from the substance to the surroundings. The temperature remains constant during the change of state because the particles are changing arrangement.
Diffusion in Liquids and Gases
Diffusion is the spreading out of particles from an area of higher concentration to an area of lower concentration.
Concentration means how much of a substance there is in a certain volume. A high concentration means many particles in a certain volume. A low concentration means fewer particles in that volume.
Diffusion happens because particles move. In gases, particles move quickly in all directions. In liquids, particles move and slide past each other. This random movement causes particles to spread out.
Diffusion does not need stirring. Stirring can make spreading happen faster, but diffusion is caused by particle movement.
Examples of diffusion:
- perfume particles spread through the air in a room
- food colouring spreads through water
- oxygen diffuses from the air into water in a pond
- carbon dioxide diffuses from a fizzy drink into the air
Diffusion is faster at higher temperatures because particles have more energy and move faster. If food colouring is added to hot water and cold water, it usually spreads faster in the hot water.
Diffusion happens in liquids and gases because their particles can move from place to place. Diffusion is much slower in solids because solid particles are fixed in position and only vibrate.
Density, Compression and Diffusion Together
The same particle ideas can explain many observations.
If a balloon is squeezed, the gas inside can be compressed because gas particles are far apart. The particles are pushed closer together. The particles themselves do not become smaller.
If a metal block feels heavy for its size, it may have a high density. This means it has a large mass in a small volume. Its particles are closely packed and the substance has a lot of mass for its volume.
If the smell of food spreads through a kitchen, particles from the food have diffused through the air. They moved from a region of higher concentration near the food to lower concentration further away.
In all three examples, the particle model connects what we observe to what particles are doing.
Working Scientifically: Investigating Diffusion
Investigation question
How does temperature affect the rate of diffusion of food colouring in water?
Prediction
Food colouring will diffuse faster in warmer water because the particles have more energy and move faster.
Variables
| Variable type | Example in this investigation |
|---|---|
| Independent variable | Temperature of the water |
| Dependent variable | Time taken for the colour to spread through the water |
| Control variables | Volume of water, size of beaker, amount of food colouring, colour used, starting position of the drop, whether the water is stirred |
The independent variable is the variable you change. The dependent variable is the variable you measure. Control variables are kept the same to make the test fair.
Method
- Measure 100 cm3 of cold water into a beaker.
- Record the temperature of the water using a thermometer.
- Add one drop of food colouring to the centre of the water surface.
- Start a timer immediately.
- Do not stir the water.
- Stop the timer when the colour has spread evenly through the water.
- Repeat the test with warm water and hot water.
- Repeat each temperature at least three times and calculate a mean time.
Results table
| Water temperature (°C) | Time 1 (s) | Time 2 (s) | Time 3 (s) | Mean time (s) |
|---|---|---|---|---|
| 10 | 210 | 205 | 215 | 210 |
| 30 | 125 | 120 | 130 | 125 |
| 50 | 68 | 72 | 70 | 70 |
Conclusion
As the temperature of the water increased, the time taken for the food colouring to spread decreased. This means diffusion was faster at higher temperatures. The particle model explains this because particles in warmer water have more energy and move faster.
Reliability, accuracy and precision
Reliability is improved by repeating measurements and calculating a mean. If repeated results are similar, the results are more reliable.
Accuracy means how close a measurement is to the true value. Using a thermometer correctly and measuring the water volume carefully improves accuracy.
Precision means how detailed or exact the measurements are. A digital thermometer that measures to 0.1 °C is more precise than one that measures only to the nearest degree.
Evaluation
Possible improvements:
- use the same volume of water each time
- use the same number of drops of food colouring
- use a white background so the spreading colour is easier to see
- repeat each test more times
- use a clear rule for deciding when the colour has fully spread
- keep the beakers away from draughts or vibration
Possible limitation:
It can be difficult to judge exactly when the food colouring has spread evenly. Different students might stop the timer at slightly different times.
Working Scientifically: Investigating a Change of State
Investigation question
What happens to the temperature of ice as it is heated until it becomes liquid water?
Prediction
The temperature will increase while the ice warms up, stay the same while the ice melts, and then increase again after all the ice has melted.
Variables
| Variable type | Example |
|---|---|
| Independent variable | Time heated |
| Dependent variable | Temperature of the ice or water |
| Control variables | Mass of ice, heating power, beaker size, starting temperature, how often the mixture is stirred |
Method
- Place crushed ice in a beaker.
- Put a thermometer into the ice.
- Record the starting temperature.
- Heat the beaker gently.
- Record the temperature every minute.
- Stir carefully to help keep the temperature even.
- Continue until all the ice has melted and the water has warmed.
- Plot a graph of temperature against time.
Expected graph interpretation
The temperature rises as the solid ice warms. During melting, the graph has a flat section because energy is being used to overcome forces of attraction between particles instead of increasing temperature. After all the ice has melted, the temperature of the liquid water rises.
Safety
- Wear eye protection.
- Take care with hot equipment.
- Do not touch the hot beaker or tripod.
- Keep bags and paper away from flames if using a Bunsen burner.
Real-World Examples
Puddles drying
A puddle dries because water particles evaporate from the surface. The particles gain enough energy to leave the liquid and become water vapour in the air. This can happen below the boiling point.
Steam on a mirror
After a shower, warm water vapour touches a cooler mirror. The particles lose energy, move more slowly and come closer together. They condense into liquid water droplets.
A balloon being squeezed
A balloon contains gas particles. The particles are far apart, so the gas can be compressed. When the balloon is squeezed, the gas particles are pushed closer together and the volume decreases.
Smell spreading across a room
Perfume or cooking smells spread because particles diffuse through the air. They move from a region of higher concentration near the source to lower concentration further away.
Syringes with air and water
A sealed syringe filled with air can be pushed in because air is a gas and its particles are far apart. A sealed syringe filled with water is difficult to push in because liquid water particles are close together.
Ice cream melting
Ice cream melts when it gains energy from warmer surroundings. Its particles vibrate more and then become able to move around each other, changing the solid into a liquid mixture.
Key Vocabulary
| Term | Definition |
|---|---|
| Particle | A tiny piece of matter, too small to see with a school microscope. |
| Particle model | A scientific model that represents matter as tiny particles. |
| Matter | Anything that has mass and takes up space. |
| Solid | A state of matter with particles close together in fixed positions. |
| Liquid | A state of matter with particles close together that can move and slide past each other. |
| Gas | A state of matter with particles far apart moving quickly in all directions. |
| Volume | The amount of space something takes up. |
| Mass | The amount of matter in an object or substance. |
| Density | How much mass is in a given volume. |
| Compress | To squash something into a smaller volume. |
| Diffusion | The spreading of particles from higher concentration to lower concentration. |
| Concentration | How much of a substance there is in a certain volume. |
| Melting | The change of state from solid to liquid. |
| Freezing | The change of state from liquid to solid. |
| Boiling | The change of state from liquid to gas throughout a liquid at its boiling point. |
| Evaporation | The change of state from liquid to gas from the surface, below boiling point. |
| Condensation | The change of state from gas to liquid. |
| Sublimation | The change of state from solid directly to gas. |
| Melting point | The temperature at which a solid changes to a liquid. |
| Boiling point | The temperature at which a liquid boils. |
| Heating curve | A graph showing how temperature changes as a substance is heated. |
| Cooling curve | A graph showing how temperature changes as a substance is cooled. |
| Independent variable | The variable changed in an investigation. |
| Dependent variable | The variable measured in an investigation. |
| Control variable | A variable kept the same to make a test fair. |
| Reliability | How consistent repeated results are. |
| Accuracy | How close a measurement is to the true value. |
| Precision | How exact or detailed a measurement is. |
Common Misconceptions
| Misconception | Correct explanation |
|---|---|
| Particles in a solid do not move at all. | Solid particles vibrate around fixed positions. |
| Particles get bigger when heated. | Particles gain energy and move or vibrate more; they do not get bigger. |
| There is air between gas particles. | In a gas, there is empty space between particles. Air itself is made of gas particles. |
| Liquids are easy to compress because they flow. | Liquids flow, but their particles are close together, so liquids are difficult to compress. |
| Boiling and evaporation are the same. | Both form gas, but boiling happens throughout the liquid at boiling point, while evaporation happens from the surface below boiling point. |
| When water evaporates, it disappears. | Water particles spread into the air as water vapour. They have not disappeared. |
| A particle diagram is an exact picture of matter. | It is a simplified model that helps explain behaviour but is not drawn to scale. |
| During melting, particles are destroyed. | Particles remain the same substance; their arrangement and movement change. |
| Diffusion only happens if something is stirred. | Stirring speeds up mixing, but diffusion happens because particles move randomly. |
| The bubbles in boiling water are air. | The bubbles are mostly water vapour forming throughout the liquid. |
Diagram Interpretation Practice
Diagram 1: identifying states
A B C
● ● ● ● ● ● ● ● ●
● ● ● ● ● ● ●
● ● ● ● ● ● ● ● ●
● ● ● ● ● ● ● ●
Questions:
- Which diagram shows a solid?
- Which diagram shows a gas?
- Which diagram shows a liquid?
- Explain your answer for diagram B using particle spacing and movement.
Model answers:
- Diagram A shows a solid.
- Diagram B shows a gas.
- Diagram C shows a liquid.
- Diagram B shows a gas because the particles are far apart and randomly arranged. Gas particles move quickly in all directions and fill their container.
Diagram 2: compression in a syringe
Before compression
|-----------------------|
| ● ● ● |
| ● ● |
| ● ● ● |
|-----------------------|
After compression
|------------|
| ● ● ● |
| ● ● ● |
| ● ● ● |
|------------|
Questions:
- What state of matter is shown in the syringe?
- What happens to the distance between particles after compression?
- Do the particles themselves get smaller?
- Explain why the volume decreases.
Model answers:
- The syringe contains a gas.
- The distance between particles decreases.
- No. The particles themselves do not get smaller.
- The volume decreases because gas particles are far apart with empty space between them, so they can be pushed closer together.
Diagram 3: diffusion
Start
High concentration Low concentration
● ● ● ● ● ● ● .
● ● ● ● ● ● .
● ● ● ● ● ● .
Later
● ● ● ● ● ●
● ● ● ●
● ● ● ● ●
Questions:
- Define diffusion.
- In which direction do the particles spread overall?
- Why does diffusion happen faster at higher temperature?
Model answers:
- Diffusion is the spreading of particles from an area of higher concentration to an area of lower concentration.
- The particles spread from the high concentration region to the low concentration region.
- At higher temperature, particles have more energy and move faster, so they spread out more quickly.
Diagram 4: heating curve
Temperature (°C)
100 | D
80 | /
60 |------------- C ------/
40 | /
20 | B
0 |------/
-20 | A
+----------------------------
0 2 4 6 Time
Questions:
- Which labelled section shows melting?
- Which labelled section shows a liquid warming up?
- What is the melting point in this graph?
- Why is the graph flat during section B?
Model answers:
- Section B shows melting.
- Section C shows the liquid warming up.
- The melting point is 0 °C.
- The graph is flat because energy is being used to overcome forces of attraction between particles during melting rather than increasing the temperature.
Exam-Style Questions
Multiple choice
- Which statement best describes particles in a solid?
A. They are far apart and move quickly in all directions.
B. They are close together and vibrate around fixed positions.
C. They are close together and move freely to fill any container.
D. They are far apart and arranged in a regular pattern.
- Why are gases easy to compress?
A. Gas particles are soft.
B. Gas particles disappear when pushed.
C. There is lots of empty space between gas particles.
D. Gas particles are arranged in a fixed pattern.
- Which change of state is liquid to gas from the surface below boiling point?
A. Condensation
B. Freezing
C. Evaporation
D. Melting
- What does a flat section on a heating curve show?
A. The substance is cooling.
B. The temperature is increasing quickly.
C. A change of state is happening.
D. The particles have stopped moving.
- Which statement about diffusion is correct?
A. Diffusion only happens in solids.
B. Diffusion is faster at lower temperatures.
C. Diffusion is spreading from higher concentration to lower concentration.
D. Diffusion happens because particles are fixed in position.
- A liquid keeps its volume because:
A. its particles are far apart.
B. its particles are close together.
C. its particles cannot move at all.
D. its particles are arranged in a perfect regular pattern.
- What happens to particles when a substance is heated?
A. They always get larger.
B. They gain energy and move or vibrate more.
C. They lose energy and move more slowly.
D. They turn into air.
- Which statement correctly compares boiling and evaporation?
A. Boiling happens at the surface only, but evaporation happens throughout the liquid.
B. Evaporation can happen below boiling point, but boiling happens at boiling point.
C. Evaporation only happens in solids.
D. Boiling does not involve particles.
Short-answer questions
Describe the arrangement and movement of particles in a liquid.
Explain why a solid is difficult to compress.
A student sprays perfume at the front of a classroom. After a few minutes, students at the back can smell it. Explain this using diffusion.
Explain what happens to the particles when water vapour condenses on a cold window.
A sealed bag contains 50 g of ice. The ice melts completely. What should the mass of the liquid water be? Explain your answer.
A student says, "When a puddle dries, the water has vanished." Explain why this is not correct.
Why is the particle model described as a model rather than an exact picture?
Data and graph questions
- A student investigates diffusion using food colouring in water.
| Temperature of water (°C) | Time for colour to spread evenly (s) |
|---|---|
| 15 | 180 |
| 25 | 140 |
| 35 | 95 |
| 45 | 65 |
a. What is the independent variable?
b. What is the dependent variable?
c. Describe the trend in the results.
d. Explain the trend using the particle model.
e. Suggest one control variable.
- A substance is heated and its temperature is recorded.
| Time (min) | Temperature (°C) |
|---|---|
| 0 | -10 |
| 1 | 0 |
| 2 | 0 |
| 3 | 0 |
| 4 | 20 |
| 5 | 40 |
| 6 | 60 |
| 7 | 60 |
| 8 | 60 |
| 9 | 80 |
a. What is the melting point?
b. What is the boiling point?
c. During which times is the substance melting?
d. Why does the temperature stay the same during melting?
e. What state is the substance likely to be in at 5 minutes?
Longer-answer questions
Compare the particles in solids, liquids and gases. Include arrangement, movement, shape, volume and compression in your answer.
Explain how heating ice can eventually produce water vapour. Use particle ideas and name the changes of state.
Plan an investigation to find out whether food colouring diffuses faster in hot or cold water. Include variables, a brief method and one way to improve reliability.
Model Answers
Multiple choice answers
- B. Solid particles are close together and vibrate around fixed positions.
- C. Gases are easy to compress because there is lots of empty space between the particles.
- C. Evaporation is liquid to gas from the surface below boiling point.
- C. A flat section on a heating curve shows a change of state.
- C. Diffusion is spreading from higher concentration to lower concentration.
- B. A liquid keeps its volume because its particles are close together.
- B. Heating transfers energy to particles, so they move or vibrate more.
- B. Evaporation can happen below boiling point, but boiling happens at boiling point.
Short-answer model answers
In a liquid, particles are close together but randomly arranged. They can move and slide past each other, so the liquid flows and takes the shape of its container.
A solid is difficult to compress because its particles are already close together. There is very little empty space between the particles, so they cannot easily be pushed closer together.
Perfume particles diffuse through the air. They move from an area of higher concentration near the spray to areas of lower concentration around the room. Gas particles move quickly in all directions, so the smell spreads.
When water vapour condenses, the gas particles lose energy. They move more slowly and come closer together. Forces of attraction pull them into liquid droplets on the cold window.
The mass should be 50 g because the bag is sealed. The particles do not disappear when the ice melts. They change arrangement and movement, but the same particles remain in the closed system.
The water has not vanished. Water particles have evaporated from the surface of the puddle and spread into the air as water vapour.
The particle model is a model because it is a simplified representation. It shows useful ideas such as spacing and arrangement, but it is not drawn to scale and does not show the true size, shape or number of particles.
Data and graph model answers
a. The independent variable is the temperature of the water.
b. The dependent variable is the time for the colour to spread evenly.
c. As the temperature increases, the time taken decreases. This means diffusion is faster at higher temperatures.
d. At higher temperatures, particles have more energy and move faster. This makes the food colouring particles spread through the water more quickly.
e. One control variable is the volume of water. Other valid answers include the amount of food colouring, the size of beaker or whether the water is stirred.
a. The melting point is 0 °C.
b. The boiling point is 60 °C.
c. The substance is melting from 1 minute to 3 minutes.
d. The temperature stays the same because energy is used to overcome forces of attraction between particles during melting, rather than increasing the temperature.
e. At 5 minutes, the substance is likely to be a liquid because it is between the melting point and boiling point.
Longer-answer model answers
- In a solid, particles are close together in fixed positions. They vibrate but do not move from place to place. This means a solid keeps a fixed shape and fixed volume. Solids are difficult to compress because there is very little empty space between the particles.
In a liquid, particles are close together but randomly arranged. They can move and slide past each other. This means a liquid takes the shape of its container but keeps a fixed volume. Liquids are difficult to compress because their particles are close together.
In a gas, particles are far apart and randomly arranged. They move quickly in all directions. This means a gas fills its container and has no fixed shape or volume. Gases are easy to compress because there is lots of empty space between the particles.
- When ice is heated, energy is transferred to its particles. The particles vibrate more strongly. At the melting point, the particles have enough energy to overcome some forces of attraction holding them in fixed positions, so the ice melts into liquid water. In the liquid, particles are still close together but can move and slide past each other.
If heating continues, the liquid particles gain more energy and move faster. Some particles may evaporate from the surface. At the boiling point, particles throughout the liquid have enough energy to separate and form gas bubbles. The liquid boils and becomes water vapour. The particles have not disappeared; their arrangement and movement have changed.
- To investigate whether food colouring diffuses faster in hot or cold water, change the temperature of the water. This is the independent variable. Measure the time taken for one drop of food colouring to spread evenly through the water. This is the dependent variable.
Use the same volume of water, the same size beaker, the same colour and amount of food colouring, and do not stir the water. Add one drop of food colouring to cold water and time how long it takes to spread evenly. Repeat with hot water. Repeat each test at least three times and calculate a mean to improve reliability.
Revision Checklist
Use this checklist to check your understanding.
Particle model
- I can state that all matter is made from tiny particles.
- I can explain why particles cannot be seen with a school microscope.
- I can describe the particle model as a simplified scientific model.
- I can explain what particle diagrams show and what they leave out.
Solids, liquids and gases
- I can describe the arrangement of particles in a solid.
- I can describe the movement of particles in a solid.
- I can explain why a solid keeps its shape and volume.
- I can describe the arrangement of particles in a liquid.
- I can explain why a liquid flows and takes the shape of its container.
- I can describe the arrangement and movement of particles in a gas.
- I can explain why a gas fills its container.
- I can explain why gases are easier to compress than solids and liquids.
Density and compression
- I can define density as mass in a given volume.
- I can compare the density of substances using mass and volume.
- I can use particle spacing to explain why gases are usually less dense than solids and liquids.
- I can define compression as squashing into a smaller volume.
- I can explain compression using empty space between particles.
Heating, cooling and changes of state
- I can explain that heating transfers energy to particles.
- I can explain that cooling removes energy from particles.
- I can define melting, freezing, boiling, evaporation, condensation and sublimation.
- I can compare evaporation and boiling.
- I can explain changes of state using particle arrangement, movement and energy.
- I can explain conservation of mass during a change of state in a closed system.
Diffusion
- I can define diffusion as spreading from higher concentration to lower concentration.
- I can explain why diffusion happens in liquids and gases.
- I can explain why diffusion is faster at higher temperatures.
- I can give examples of diffusion in everyday life.
Graphs and investigations
- I can identify sloping and flat sections on heating and cooling curves.
- I can read melting points and boiling points from a graph.
- I can explain why temperature stays constant during a change of state.
- I can identify independent, dependent and control variables.
- I can describe how to make an investigation fair.
- I can explain reliability, accuracy and precision in simple terms.
- I can evaluate a diffusion or change-of-state investigation.
Final Quick Review
The particle model explains matter using tiny particles. In solids, particles are close together in fixed positions and vibrate. In liquids, particles are close together but can move and slide past each other. In gases, particles are far apart and move quickly in all directions.
Heating gives particles more energy, so they move or vibrate more. Cooling removes energy, so particles move or vibrate less. Changes of state happen when particles gain or lose enough energy for their arrangement and movement to change.
Diffusion happens when particles spread from higher concentration to lower concentration. It happens faster at higher temperatures because particles have more energy and move faster.
Particle diagrams, heating curves and experiments all help us connect observations to particle behaviour. The strongest KS3 answers use precise particle language: arrangement, movement, energy, forces of attraction, density, compression and diffusion.