Particles and states of matter

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Big idea: matter is a system of moving particles

Big idea. Key concept: Systems. Everything around you is matter, and matter behaves like a system of tiny particles that are always interacting.
Related concept: Models. The particle model is a simple picture that scientists use to explain what we see, such as why ice melts and why smells spread.
Global context: Scientific and technical innovation. Engineers use the particle model to design everything from fridges and pressure cookers to the fuel tanks of rockets.
Matter is anything that has mass and takes up space. Rock, water, air and your own body are all matter.
Matter exists in three main states: solid, liquid and gas. The state depends on how the particles are arranged and how they move.
A model is never the full truth. It leaves things out so that we can explain and predict. Later in this lesson you will test the particle model against real observations.

Solids, liquids and gases

In a solid, the particles are close together in a regular pattern. They cannot move from place to place, but they vibrate on the spot. So a solid has a fixed shape and a fixed volume.
In a liquid, the particles are still touching, but they are arranged in an irregular way and can slide past one another. So a liquid flows and takes the shape of its container, but its volume stays fixed.
In a gas, the particles are far apart and move quickly in random directions. So a gas spreads out to fill its container and has no fixed shape or volume.
Gases are easy to compress (squash into a smaller volume) because there is lots of empty space between the particles. Solids and liquids are very hard to compress because their particles are already touching.
The particles themselves do not change size or change into something else when the state changes. Only their arrangement, spacing and movement change.
Remember that the particle drawings are a model. Real particles are far too small to see, and they do not look like coloured balls.

Particles in a solid, a liquid and a gas

Changes of state

A change of state happens when a substance turns from one state into another. It is a physical change, so the substance is still the same substance. Ice and steam are both water.
Melting is solid to liquid. Freezing is liquid to solid. Boiling and evaporation are liquid to gas. Condensation is gas to liquid.
Sublimation is solid straight to gas, with no liquid stage. Solid carbon dioxide (dry ice) does this. The reverse change, gas straight to solid, is called deposition.
To melt or boil a substance, energy must be transferred to it. The particles gain energy, vibrate or move faster, and overcome the forces holding them together. Cooling takes energy away and reverses the changes.
Boiling happens throughout the liquid at one particular temperature, the boiling point. Evaporation happens only at the surface and can happen at any temperature, so a puddle dries without ever reaching 100 °C.
Mass is conserved in a change of state. If 50 g of ice melts, you get 50 g of water, because no particles are lost.

Changes of state between solid, liquid and gas

Melting points, boiling points and heating curves

The temperature at which a solid turns into a liquid is its melting point. The temperature at which a liquid boils is its boiling point. A pure substance has one fixed value for each.
Water melts at 0 °C and boils at 100 °C. Ethanol melts at −114 °C and boils at 78 °C. Iron melts at about 1538 °C. Mercury melts at −39 °C, so it is a liquid at room temperature.
To find the state of a substance at a given temperature, compare the temperature with the two fixed points. Below the melting point it is a solid. Between the melting point and the boiling point it is a liquid. Above the boiling point it is a gas.
A heating curve is a graph of temperature against time as a substance is heated steadily. It slopes upwards while one state warms up, but it goes flat (a plateau) while the substance melts or boils.
During a plateau the temperature does not rise even though energy is still being transferred. The energy is being used to separate the particles and overcome the forces between them, not to make them move faster.
When the substance has finished melting or boiling, the temperature starts to rise again.

Diffusion and gas pressure

Diffusion is the net movement of particles from a region where they are at a higher concentration to a region where they are at a lower concentration. It happens because particles move randomly and collide.
Diffusion explains why the smell of cooking spreads across a house, and why a drop of food colouring slowly spreads through still water without anyone stirring it.
Diffusion is much faster in gases than in liquids, because gas particles are far apart and move quickly. It is faster when the substance is warmer, because the particles have more energy and move faster.
Gas particles move randomly and collide with the walls of their container. Each collision pushes on the wall. Millions of collisions every second add up to gas pressure.
If you squeeze the same gas into a smaller volume, the particles hit the walls more often, so the pressure goes up. If you heat a gas in a sealed container, the particles move faster and hit the walls harder and more often, so the pressure also goes up.
A gas does not stop at the surface of the liquid or solid below it. Its particles keep moving, which is why a bottle left open loses its fizz and a smell reaches you from across a room.

Bromine gas spreading out until it is evenly mixed with air

Think like a scientist: does temperature affect diffusion?

Question. Does the temperature of the water change how fast a drop of food colouring spreads? A class could test this with beakers of water at different temperatures. The water must be warm, not hot enough to burn, and your teacher sets up the warm water.
The independent variable is the one you change on purpose: the temperature of the water, for example 10 °C, 20 °C, 30 °C and 40 °C.
The dependent variable is the one you measure: the time, in seconds, for the colour to spread evenly through the water.
The control variables must stay the same to keep the test fair: the volume of water, the same type of beaker, the same number of drops of colouring, and no stirring.
Repeat each temperature at least three times and calculate a mean. Repeating shows whether your results are reliable and helps you spot an anomaly.
Inquiry task. Predict what the results will be, using the particle model to explain your prediction. Then plan a table for the results. After the investigation, evaluate it: how could the method be improved to measure “evenly spread” more accurately?

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1.Which state of matter has a fixed shape and a fixed volume?
Easy
- ASolid
- BLiquid
- CGas
- DLiquid and gas
2.Which sentence describes the particles in a gas?
Easy
- AThey are close together in a regular pattern and only vibrate.
- BThey are far apart and move quickly in random directions.
- CThey touch one another and slide past each other.
- DThey are close together and cannot move at all.
3.Which state of matter takes the shape of its container but keeps a fixed volume?
Easy
- ASolid
- BGas
- CLiquid
- DNone of them
4.What is the name for a liquid turning into a gas from its surface at any temperature?
Easy
- ACondensation
- BMelting
- CSublimation
- DEvaporation
5.What is the name for a gas turning into a liquid?
Easy
- ACondensation
- BFreezing
- CEvaporation
- DMelting
6.What happens to the particles in a solid when it is heated, before it melts?
Easy
- AThey get bigger.
- BThey vibrate more strongly.
- CThey stop moving.
- DThey turn into a different substance.
7.Why can a gas be compressed much more easily than a liquid?
Easy
- AThe particles in a gas are bigger than those in a liquid, so they squash more easily.
- BGas particles are lighter than liquid particles, so they are easier to push together.
- CThe particles in a gas are far apart, so there is empty space to squash them into.
- DGas particles do not move, so nothing resists the squeezing.
8.Why does a solid keep its shape?
Easy
- AIts particles are not moving at all, so nothing can change their positions.
- BIts particles are larger than liquid particles and cannot fit past each other.
- CIts particles are far apart and are held in place by the surrounding air.
- DStrong forces hold its particles in fixed positions, so they can only vibrate.

Mehr Fragen

9.Which sentence correctly compares boiling and evaporation?
Medium
- ABoiling happens throughout the liquid at its boiling point; evaporation happens only at the surface.
- BBoiling happens only at the surface, while evaporation happens throughout the liquid.
- CBoth can only happen when the liquid reaches exactly 100 °C.
- DBoiling needs no energy to be supplied, while evaporation needs a lot of energy.
10.The smell of freshly baked bread reaches the next room. Which process explains this?
Medium
- AMelting of the bread
- BDiffusion of gas particles
- CCondensation of water vapour
- DCompression of the air
11.A drop of food colouring spreads faster in warm water than in cold water. What is the best explanation?
Medium
- AWarm water has bigger particles, which push the colouring outwards.
- BThe food colouring melts when the water is warm and then spreads.
- CThe particles in warm water have more energy and move faster.
- DThe particles in cold water have stopped moving completely.
12.A sealed syringe of air is pushed in so the air takes up less volume. What happens to the pressure, and why?
Medium
- AIt decreases, because the particles slow down.
- BIt stays the same, because there are the same number of particles.
- CIt decreases, because the particles are closer together.
- DIt increases, because the particles hit the walls more often.
13.Mercury melts at −39 °C and boils at 357 °C. What state is mercury at 20 °C?
Medium
- ALiquid
- BSolid
- CGas
- DA mixture of solid and gas
14.Oxygen melts at −218 °C and boils at −183 °C. What state is oxygen at −200 °C?
Medium
- ASolid
- BLiquid
- CGas
- DSolid and liquid at the same time
15.What are the bubbles inside a pan of boiling water made of?
Easy
- AAir
- BHydrogen
- CWater vapour
- DOxygen from the water
16.On a heating curve for ice, the temperature stays at 0 °C for a while even though the ice is still being heated. Why?
Hard
- AThe ice has stopped absorbing energy, so the temperature cannot change.
- BThe thermometer cannot measure temperatures below 1 °C accurately during melting.
- CAll the particles stop moving until every piece of ice has melted.
- DThe energy is being used to separate the particles as the ice melts, not to raise the temperature.
17.In diagrams of the particle model, every particle is drawn as a small identical sphere. Which is a limitation of this model?
Hard
- AReal particles are not simple solid spheres, and they come in different sizes and types.
- BIt cannot show that a solid has a fixed shape, because the spheres are all the same.
- CIt cannot explain why gases can be compressed, because no space is drawn.
- DIt suggests that particles are bigger than the object they make up.
18.A class investigates whether the temperature of water affects how fast food colouring diffuses. What is the independent variable?
Medium
- AThe time for the colour to spread
- BThe temperature of the water
- CThe volume of water in each beaker
- DThe number of drops of food colouring
19.In the same investigation, which of these is a control variable?
Medium
- AThe temperature of the water
- BThe time for the colour to spread
- CThe number of drops of food colouring
- DWhether the colour spreads fastest in the warmest beaker
20.In the same investigation, what is the best dependent variable to measure?
Medium
- AThe colour of the food colouring used in each beaker
- BThe temperature of the room where the test is done
- CThe size and shape of the beaker used each time
- DThe time for the colour to spread evenly through the water
21.Which plan is the fairest way to test whether warmer water makes colouring diffuse faster?
Hard
- AUse four identical beakers with the same volume of water at 10, 20, 30 and 40 °C, the same drops of colouring, and time each without stirring.
- BUse one beaker, add the colouring, then warm the water as it spreads and time how long the colour takes to spread evenly.
- CUse a small beaker of cold water and a large beaker of hot water, add colouring to both and see which spreads first.
- DStir the warm beaker and leave the cold beaker still, then compare how long each takes to look evenly coloured.
22.In one experiment, a student times the colouring in a 20 °C beaker (90 s) and a 60 °C beaker (35 s) once each. She concludes that particles always move faster at higher temperatures. What is the best evaluation of her conclusion?
Hard
- AHer conclusion is proven, because the result matches what the particle model predicts.
- BIt supports the idea, but one trial at two temperatures cannot show “always”.
- CHer result is useless, because she should have used the same beaker for both.
- DHer conclusion is wrong, because 90 s is a longer time than 35 s.
23.Which of these are properties of a liquid? (select all that apply)
Easy
- AIt has a fixed volume
- BIt takes the shape of its container
- CIts particles are in a regular pattern
- DIt is easy to compress
- EIts particles are touching
24.Which of these changes need energy to be transferred to the substance? (select all that apply)
Medium
- AMelting
- BCondensation
- CBoiling
- DFreezing
- EEvaporation
25.Which of these are examples of diffusion? (select all that apply)
Medium
- APerfume spreading across a room
- BColour spreading out from a tea bag in still water
- CAn ice cube melting in a drink
- DA ball rolling across a table
26.In a solid, the particles can vibrate but they cannot move from place to place.
Easy
True or false?
27.The particles in a gas are arranged in a regular pattern.
Easy
True or false?
28.The mass of a substance stays the same when it melts.
Easy
True or false?
29.When a solid is heated and expands, its particles become bigger.
Medium
True or false?
30.A pure substance melts at one fixed temperature.
Easy
True or false?
31.Water can only evaporate when it is at 100 °C.
Medium
True or false?
32.Diffusion is usually faster in a gas than in a liquid at the same temperature.
Hard
True or false?
33.Match each change to its name.
Easy
- Solid to liquid
- Liquid to gas by boiling
- Gas to liquid
- Liquid to solid
- Melting
- Boiling
- Condensation
- Freezing
34.Match each state to its particle description.
Medium
- Solid
- Liquid
- Gas
- Far apart, moving quickly and randomly
- Regular pattern, vibrating on the spot
- Touching, irregular, sliding past one another
35.Match each substance to its melting point.
Hard
- Water
- Ethanol
- Mercury
- Iron
- −114 °C
- 0 °C
- −39 °C
- About 1538 °C
36.Put these in order as an ice cube is heated, starting with the coldest.
Easy
- Liquid water
- Steam (gas)
- Ice (solid)
37.Put these in order to show what happens to the particles as a solid is heated until it becomes a gas.
Medium
- Particles move far apart, quickly and randomly
- Particles vibrate in fixed positions in a pattern
- Particles gain energy and begin to slide past one another
38.Put the stages of a fair-test investigation in a sensible order.
Medium
- Record the results in a table
- Decide what to change and what to measure
- Write a conclusion and evaluate the method
- Carry out the method and repeat the readings
- Decide which variables to keep the same
39.Complete the sentence about melting.
Easy
The change of state from solid to liquid is called ____.
40.Complete the sentence about gases.
Easy
In a ____, the particles are far apart and move randomly.
41.Complete the sentence about diffusion.
Medium
Diffusion is the net movement of particles from a region of ____ concentration to a region of ____ concentration.
42.Complete the sentence about gas pressure.
Medium
Gas pressure is caused by particles ____ with the walls of the container.
43.Complete the sentence about sublimation.
Hard
A change directly from solid to gas, with no liquid stage, is called ____.
44.Explain why a gas can be compressed but a solid cannot.
Medium
45.Use the particle model to explain why the temperature stays constant while ice is melting, even though it is still being heated.
Hard
46.A student says, “The particles in a solid do not move at all.” Explain what is wrong with this idea.
Hard
47.Describe a safe investigation, run by your teacher, to show that diffusion happens in a liquid. Say what you would see.
Hard
48.Water boils at 100 °C. A pan of water starts at 20 °C. By how many degrees Celsius must the temperature rise before it boils?
Easy
49.
In one experiment, a student heated a solid and recorded its temperature every minute.
Time (min) Temperature (°C)
0 -10
1 -5
2 0
3 0
4 0
5 0
6 5
7 10
For how many minutes did the temperature stay constant while the substance was melting?
Medium
50.Ethanol melts at −114 °C and boils at 78 °C. Over how many degrees Celsius is ethanol a liquid?
Hard

Lernnotizen

## Particles and states of matter - All matter is made of tiny particles. The **particle model** explains solids, liquids and gases. - **Solid:** regular pattern, vibrating. **Liquid:** touching, sliding past. **Gas:** far apart, fast and random. - **Changes of state** are physical: melting, freezing, boiling, evaporation, condensation, sublimation. Mass is conserved. - A pure substance has a fixed **melting point** and **boiling point**. Compare the temperature with them to find the state. - On a **heating curve** the temperature is constant while the state changes, because the energy separates the particles. - **Diffusion** is movement from high to low concentration. It is faster in gases and when warm. - **Gas pressure** comes from particles colliding with the walls. Smaller volume or higher temperature means higher pressure. - In a fair test, change one variable, measure one, and keep the others the same.

Matter: Anything that has mass and takes up space.
Particle: A tiny piece of matter, too small to see, that the particle model uses to explain behaviour.
Particle model: A simple picture in which all matter is made of tiny moving particles.
Solid: A state of matter with a fixed shape and volume, whose particles vibrate in fixed positions.
Liquid: A state of matter with a fixed volume that flows, whose particles touch and slide past one another.
Gas: A state of matter with no fixed shape or volume, whose particles are far apart and move randomly.
Melting point: The temperature at which a solid turns into a liquid.
Boiling point: The temperature at which a liquid boils and turns into a gas throughout the liquid.
Evaporation: A liquid changing to a gas at its surface, at any temperature.
Condensation: A gas changing into a liquid.
Sublimation: A solid changing straight into a gas without becoming a liquid.
Diffusion: The net movement of particles from a region of higher concentration to a region of lower concentration.
Gas pressure: The force of gas particles colliding with the walls of their container.
Heating curve: A graph of temperature against time while a substance is heated.

Karteikarten

What is matter?

Anything that has mass and takes up space.

How are particles arranged in a solid?

Close together in a regular pattern, vibrating in fixed positions.

How are particles arranged in a liquid?

Touching but irregular, able to slide past one another.

How are particles arranged in a gas?

Far apart and moving quickly in random directions.

Why can gases be compressed?

There is a lot of empty space between the particles.

What is melting?

A solid changing into a liquid.

What is condensation?

A gas changing into a liquid.

What is sublimation?

A solid changing straight into a gas, with no liquid stage.

What is the difference between boiling and evaporation?

Boiling happens throughout the liquid at the boiling point. Evaporation happens only at the surface, at any temperature.

What happens to mass in a change of state?

It stays the same, because no particles are lost.

What is the melting point of water?

0 °C.

What is the boiling point of water?

100 °C.

Why is there a plateau on a heating curve?

The energy is used to separate the particles during the change of state, so the temperature does not rise.

What is diffusion?

The net movement of particles from a region of higher concentration to a region of lower concentration.

What speeds up diffusion?

Being in a gas rather than a liquid, and a higher temperature.

What causes gas pressure?

Gas particles colliding with the walls of their container.

What is an independent variable?

The variable you change on purpose in an investigation.

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Time (min)	Temperature (°C)
0	-10
1	-5
2	0
3	0
4	0
5	0
6	5
7	10