2.1.1Thermal Physics
States OF Matter — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions
5 min read 10 parts Lesson 2 of 45 1 quiz
Lesson summary
2.1.1 States of Matter — IGCSE PHYSICS notes: Describe the properties, particle arrangement and changes of state of solids, liquids and gases.PropertySolidLiquidGasShapeFixedTakes the shape of the containerTakes the shape of the containerVolumeFixedFixedNot fixed; fills the conta…
- Energy of the particles
- Freezing (solidification)
- Boiling (vaporising)
- Evaporation
- Condensing
- Does not change
Part 1 of 8Properties of solids, liquids and gases
| Property | Solid | Liquid | Gas |
|---|---|---|---|
| Shape | Fixed | Takes the shape of the container | Takes the shape of the container |
| Volume | Fixed | Fixed | Not fixed; fills the container |
| Flow | Cannot flow | Flows | Flows freely |
| Compressible | No | Almost none | Yes, easily |
| Density | High | Medium | Low |
Part 2 of 8Arrangement and motion of particles
Arrangement and motion of particles
All particles are in constant motion at any temperature.
Solid
- Solid particles are very close together in a regular pattern and vibrate about fixed positions.
Liquid
- Liquid particles are close together with no gaps but not in a regular pattern, and they slide past each other.
Gas
- Gas particles are widely separated, about 10 times further apart in each direction, and move randomly at high speeds.
Energy of the particles
- Energy of the particles lowest in a solid, higher in a liquid and highest in a gas.
Part 3 of 8Forces and distances between particles (Extended)
Forces and distances between particles (Extended)
Solid
- Solid strong intermolecular forces hold particles in place, so the solid has a rigid shape and fixed volume.
Liquid
- Liquid particles have enough energy to slide past one another but not to escape the forces holding them close.
- Liquids have a fixed volume, flow and are hard to compress because repulsive forces are large when particles are close.
Gas
- Gas particles have overcome the forces holding them together, so large spaces allow easy compression, expansion and free flow.
Attractive forces are strongest in a solid and weakest in a gas.
Part 4 of 8Changes of state
Changes of state
Melting
- Melting a solid turns into a liquid.
Freezing (solidification)
- Freezing (solidification) a liquid turns into a solid.
Boiling (vaporising)
- Boiling (vaporising) a liquid turns into a gas at its boiling point, with bubbles forming throughout the liquid.
Evaporation
- Evaporation a liquid turns into a gas from its surface at any temperature.
Condensing
- Condensing a gas turns into a liquid.
The number of molecules and the mass do not change; only the energy changes.
Changes of state are physical changes and are reversible.
Part 5 of 8Energy and temperature during a change of state
1
Melting and boiling need energy input; freezing and condensing release energy.
2
For a pure substance, melting and boiling happen at fixed temperatures.
- Pure water melts at 0 °C and boils at 100 °C at standard atmospheric pressure.
3
During a change of state the temperature stays constant, shown as a flat section on a heating or cooling curve.
4
State at a given temperature:
- Below the melting point — solid.
- Between the melting point and boiling point — liquid.
- Above the boiling point — gas.
Part 6 of 8What changes and what does not
Changes
- Changes the spacing, arrangement and speed of the particles.
Does not change
- Does not change the particles themselves, which never grow, shrink or turn into something else.
Does not change
- Does not change the total mass, because no particles are gained or lost.
Heating makes particles move further apart and the particles themselves do not expand.
Ice is less dense than liquid water, so ice floats.
Part 7 of 8Cooling curve of stearic acid
1
Melt stearic acid in a boiling tube in a hot water bath.
2
Remove the tube and place a thermometer in the liquid.
3
Record the temperature every 30 s as it cools.
4
Plot temperature against time.
5
Identify the flat section, which is the freezing (melting) point.
Part 8 of 8More from the specification
More from the specification
Kinetic model
- Matter is made of particles in constant motion;
- solids vibrate, liquids move past each other, gases move randomly with large spaces between them.
Gas pressure
- Pressure arises from particles that collide with the walls of a container, each collision exerting a small force;
- more frequent, faster collisions increase pressure.
Brownian motion
- The random jerky movement of smoke or pollen particles is caused by unequal collisions with fast invisible air or water molecules,
- giving evidence for particles.
Absolute zero and kelvin
- The kelvin scale starts at absolute zero (-273 °C),
- where particles have minimum kinetic energy; temperature in K is proportional to average kinetic energy.
Balloon example
- Heating air in a balloon raises particle speed, so pressure increases and the balloon expands.
Kinetic model
- Matter is made of particles in constant motion;
- solids vibrate, liquids move past each other, gases move randomly with large spaces between them.
Gas pressure
- Pressure arises from particles that collide with the walls of a container, each collision exerting a small force;
- more frequent, faster collisions increase pressure.
Brownian motion
- The random jerky movement of smoke or pollen particles is caused by unequal collisions with fast invisible air or water molecules,
- giving evidence for particles.
Absolute zero and kelvin
- The kelvin scale starts at absolute zero (-273 °C),
- where particles have minimum kinetic energy; temperature in K is proportional to average kinetic energy.
Balloon example
- Heating air in a balloon raises particle speed, so pressure increases and the balloon expands.
Kinetic energy
- K = ½mv2, energy of motion.
Inversely proportional
- One quantity falls as the other rises in the same ratio.
Average speed
- Total distance divided by total time.
Kelvin
- SI unit of temperature on the absolute scale.
Collision
- Short interaction with large internal forces.
Brownian motion
- Random jiggling of small visible particles hit by fluid molecules.
Particle model
- Representing an object as a single point.
p1V1
p1V1 p2V2
What the symbols mean: p1, V1 = pressure and volume before · p2, V2 = after; only for a fixed mass of gas at constant temperature
Key terms
| Term | Meaning |
|---|---|
| Solid | State with a fixed shape and a fixed volume. |
| Liquid | State with a fixed volume that takes the shape of its container. |
| Gas | State with no fixed shape and no fixed volume. |
| Melting | Change from solid to liquid. |
| Condensing | Change from gas to liquid. |
| Compressible | Able to be squeezed into a smaller volume because of gaps between particles. |
| Attractive force | Pull between particles that holds them together. |
Avoid these mistakes
1
Saying particles expand when heated — they move further apart, but the particles themselves do not change size.
2
Saying a gas has a fixed volume — a gas fills its container.
3
Saying the temperature rises during melting — it stays constant while energy changes the arrangement.
4
Confusing evaporation with boiling — evaporation happens at the surface at any temperature.
Marked practiceQuizzes for this lesson
Last updated:
\n