1.1States of Matter
Solids, liquids and gases — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions
5 min read 11 parts Lesson 1 of 2 1 quiz
Lesson summary
1.1 Solids, liquids and gases — Notes — IGCSE Chemistry notes: Describe the particle model of solids, liquids and gases, explain changes of state and heating curves, and relate gas pressure and volume to particle motion.Model: matter is made of tiny particles (atoms, molecules or…
- Kinetic theory
- Evaporation
- Condensation
- Model: matter is made of tiny particles (atoms, molecules or ions) in constant motion
- The arrangement and movement of the particles differ in each state, which gives each state its physical properties
- Particles in a gas move faster and particles in a solid vibrate more vigorously
Part 1 of 9Kinetic particle theory
1
Model: matter is made of tiny particles (atoms, molecules or ions) in constant motion.
2
The arrangement and movement of the particles differ in each state, which gives each state its physical properties.
3
Temperature relates to the average kinetic energy of the particles, so raising the temperature makes them move faster.
- Heating converts absorbed thermal energy into kinetic energy.
- Particles in a gas move faster and particles in a solid vibrate more vigorously.
4
Particles do not get bigger when heated, and thermal expansion is a change in the average spacing between them.
5
Pure substances melt and boil at fixed, sharp temperatures.
6
Impurities lower the melting point, raise the boiling point and make melting happen over a range of temperatures.
Part 2 of 9Particle model of solids, liquids and gases
| Property | Solid | Liquid | Gas |
|---|---|---|---|
| Arrangement | Regular, closely packed lattice | Irregular, close together | Random, far apart |
| Movement | Vibrate about fixed positions | Move and slide past each other | Move quickly and randomly in all directions (about 500 m/s) |
| Shape | Fixed | Takes the shape of the container | Fills the whole container |
| Volume | Fixed | Fixed | Not fixed |
| Compressibility | Not compressible | Almost incompressible | Easily compressed |
| Density | High | Medium (water is the exception) | Very low |
| Energy of particles | Lowest | Higher | Highest |
1
Solids: strong forces hold the particles in fixed positions, so a solid has a fixed shape and volume.
2
Liquids: particles are close, so the volume is fixed and the liquid is hard to compress, but they move past each other, so a liquid flows.
- Liquids are generally less dense than solids (water is the exception) and much denser than gases.
3
Gases: forces between particles are negligible and there is a lot of space between them, so a gas spreads out and can be compressed.
4
Air in a sealed syringe can be compressed because the large spaces between gas particles get smaller.
5
Water is much harder to compress because its particles are already close together.
6
Compression does not squash the particles themselves.
Part 3 of 9Changes of state
| Change | Name | Energy change |
|---|---|---|
| Solid to liquid | Melting, at the melting point | Energy absorbed (endothermic) |
| Liquid to gas | Boiling, at the boiling point | Energy absorbed (endothermic) |
| Liquid to gas | Evaporation, below the boiling point | Energy absorbed (endothermic) |
| Gas to liquid | Condensation | Energy released (exothermic) |
| Liquid to solid | Freezing, at the freezing point | Energy released (exothermic) |
1
State changes are physical changes, so the chemical identity of the substance stays the same.
2
Melting: heated particles vibrate more until they have enough energy to break out of the fixed lattice.
3
Freezing is the reverse of melting and needs a loss of thermal energy.
4
The melting point and freezing point of a pure substance are the same (water: 0 °C).
5
Condensation: cooled gas particles lose energy, can no longer bounce away from each other when they collide and group together as a liquid.
6
Forces broken: changes of state overcome the attractive forces between molecules, and the covalent bonds inside the molecules do not break.
Part 4 of 9Boiling compared with evaporation
| Feature | Boiling | Evaporation |
|---|---|---|
| Location | Throughout the liquid | At the surface only |
| Temperature | At the boiling point | Any temperature below the boiling point |
| Bubbles | Yes, gas forms below the surface | Not required |
1
Boiling: particles gain enough energy to overcome the attractive forces completely and escape as a gas throughout the liquid.
2
Evaporation: the most energetic particles escape from the surface.
3
Evaporation is faster with a higher temperature, a larger surface area and moving air.
4
The remaining liquid cools during evaporation because the escaping particles carry away energy.
Part 5 of 9Energy and particles during changes of state (Supplement)
Energy and particles during changes of state (Supplement)
Heating a solid: particles absorb thermal energy as kinetic energy and vibrate more,
- and at the melting point the structure breaks and the solid melts.
Heating a liquid: particles move more and spread out.
- Some surface particles gain enough energy to overcome the intermolecular forces, and the liquid starts to evaporate.
- At the boiling point all particles gain enough energy to escape and the liquid boils.
Cooling reverses these changes.
Part 6 of 9State at a given temperature
1
Solid: a substance is solid at 25 °C if its melting point is above 25 °C.
2
Liquid: it is liquid if its melting point is below 25 °C and its boiling point is above 25 °C.
3
Gas: it is a gas if its boiling point is below 25 °C.
4
Water has a melting point of 0 °C and a boiling point of 100 °C at standard atmospheric pressure.
Part 7 of 9Heating and cooling curves
Heating and cooling curves
Heating curve: a graph of temperature (y-axis) against time (x-axis) showing the states, state changes and temperature changes.
Method
- Place a solid such as stearic acid in a boiling tube with a thermometer.
- Heat it gently in a water bath.
- Record the temperature every 30 seconds while stirring gently.
- Read the melting point from the horizontal section of the curve.
Cooling curve: the mirror image of the heating curve, and the flat section gives the freezing point.
| Stage on a heating curve | Explanation |
|---|---|
| Solid heated (slope) | Temperature rises as particles vibrate more |
| Melting point (flat) | Solid and liquid coexist; temperature constant; energy overcomes forces between particles |
| Liquid heated (slope) | Temperature rises as particles move faster |
| Boiling point (flat) | Liquid and gas coexist; temperature constant; energy lets particles escape as a gas |
| Gas heated (slope) | Temperature rises as particles gain kinetic energy |
Heating and cooling curves
On flat sections energy is still being supplied, and it is used to overcome the forces between particles, not to raise their kinetic energy.
Temperature measures average kinetic energy, so it stays constant until the change of state is complete.
Reading a curve: name the change of state, identify the energy transfer, then explain the particle behaviour.
Part 8 of 9Gas pressure and temperature (Supplement)
1
Gas pressure is the force per unit area caused by gas particles colliding with the walls of the container.
2
Gas particles collide with each other and with the walls, and only the wall collisions create pressure.
3
Temperature at constant volume: pressure increases because particles move faster and hit the walls more often and with more force.
4
Temperature at constant pressure: volume increases because faster particles hit the walls harder and more often and push them outwards.
- A hot air balloon expands, its density falls and it rises.
Part 9 of 9Gas pressure and volume (Supplement)
1
Pressure at constant temperature: increasing the pressure decreases the volume because particles are pushed closer together.
2
The same number of particles in a smaller volume hit the walls more frequently, as in a bicycle pump.
3
At constant temperature, doubling the pressure halves the volume.
4
Too high a pressure can make a container leak or burst.
5
Before predicting a change, identify what stays constant.
| Situation | Constants | Change and explanation |
|---|---|---|
| Heat gas in a rigid sealed container | Amount and volume | Pressure rises because particles collide with the walls more often and with more force |
| Heat gas under a movable piston | Amount and pressure | Volume rises as the gas pushes the piston out until the pressures balance |
| Compress gas slowly | Amount and temperature | Pressure rises because wall collisions become more frequent |
- Wordingparticles spread out or move faster, and only the material expands, as particles never expand.
Key terms
| Term | Meaning |
|---|---|
| Kinetic theory | the model that matter consists of particles in constant motion. |
| Melting | solid to liquid at the melting point, absorbing energy. |
| Boiling | liquid to gas throughout the liquid at the boiling point. |
| Evaporation | liquid to gas at the surface only, below the boiling point. |
| Condensation | gas to liquid on cooling, releasing energy. |
| Freezing | liquid to solid at the freezing point, releasing energy. |
| Plateau | a flat section of a curve where a change of state occurs at constant temperature. |
| Gas pressure | force per unit area from gas particles colliding with the container walls. |
Avoid these mistakes
1
Particles expand when heatedthey move faster and spread out, but do not change size.
2
Particles in a solid do not move: they vibrate about fixed positions.
3
Bonds break when water boilsthe attractive forces between molecules are overcome, not the covalent bonds inside them.
4
Flat parts of a heating curve mean heating has stopped: energy is still supplied and is used to separate particles.
5
Higher gas pressure means faster particlesit can mean the same particles in a smaller volume.
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