States of Matter Notes

Cambridge IGCSE Physics: Revision notes

Key facts

  • The three states are solid, liquid and gas; they differ in how close the particles are and how they move.
  • A solid has a fixed shape and fixed volume; a liquid has a fixed volume but takes the shape of its container; a gas has neither and fills its container.
  • Gases are easily compressed; solids and liquids are not.
  • The six changes of state: melting, freezing, evaporation, condensation, sublimation and deposition.
  • During melting and boiling the temperature stays constant while energy breaks the forces between particles.

Three states

Solids, liquids and gases differ in how the particles are arranged and move.

Matter exists as a solid, a liquid or a gas. In a solid the particles are tightly packed and have the least energy. In a liquid they are close but free to move. In a gas they are far apart and have the most energy. The state of a substance depends on its temperature and pressure.

Solid

Liquid

Gas

Particle diagrams for the three states

Which state has the particles with the highest energy?

Properties

Compare shape, volume, compressibility and flow.

A solid keeps its shape and volume because its particles are held in fixed positions by strong forces. A liquid has a fixed volume but takes the shape of its container, because its particles can move past each other. A gas has no fixed shape or volume and fills its container. Because gas particles are far apart, a gas can be compressed; a solid or liquid cannot (easily).

Solid

Fixed shape:
Yes
Fixed volume:
Yes
Easily compressed:
No
Can flow:
No

Liquid

Fixed shape:
No
Fixed volume:
Yes
Easily compressed:
No
Can flow:
Yes

Gas

Fixed shape:
No
Fixed volume:
No
Easily compressed:
Yes
Can flow:
Yes

Water is poured from a bottle into a bowl. What changes and what stays the same?

Particles in each state

Arrangement, motion and forces explain the properties.

In a solid the particles are in fixed, regular positions and only vibrate. In a liquid they are close together in a random arrangement and move around each other. In a gas they are widely spaced and move rapidly and randomly in all directions, and the forces between them are negligible. So solids and liquids are dense and gases have a very low density.

Solid

Arrangement:
Fixed, regular
Motion:
Vibrate about fixed positions
Density:
High
Forces between particles:
Strong

Liquid

Arrangement:
Close, random
Motion:
Move around each other
Density:
High
Forces between particles:
Weaker

Gas

Arrangement:
Far apart, random
Motion:
Fast, in all directions
Density:
Very low
Forces between particles:
Negligible

Why can a gas be compressed but a liquid cannot (easily)?

Changes of state

There are six changes: three on heating and three on cooling.

Heating gives melting (solid to liquid), evaporation (liquid to gas) and sublimation (solid directly to gas). Cooling gives freezing (liquid to solid), condensation (gas to liquid) and deposition (gas directly to solid). All are reversible. Melting and freezing happen at the same temperature, the melting point.

Heating

  • Melting: solid to liquid
  • Evaporation: liquid to gas
  • Sublimation: solid to gas

Cooling

  • Freezing: liquid to solid
  • Condensation: gas to liquid
  • Deposition: gas to solid
  1. 1

    Solid

    Particles vibrate in fixed positions

  2. 2

    Melting

    Forces break, particles move freely

  3. 3

    Liquid

    Particles close and random

  4. 4

    Evaporation

    Particles escape from the surface

  5. 5

    Gas

    Far apart and moving fast

Heating a solid

Solid iodine turns directly into purple vapour when warmed. What is this change called?

Constant temperature

The energy supplied breaks forces between particles instead of raising their speed.

Ice at 0 °C melts to water at 0 °C when heat is supplied, and the temperature does not rise during melting. The energy goes into overcoming the forces between the particles, not into increasing their kinetic energy. That is why a pure substance has a single melting point, not a range.

  1. 1

    Heat supplied

    Energy transferred to the ice

  2. 2

    Forces overcome

    Particles break free of fixed positions

  3. 3

    Temperature constant

    Kinetic energy does not rise

  4. 4

    All melted

    Temperature can rise again

Melting ice at 0 °C

A pure solid is melting. What happens to its temperature while it melts?

Try an exam question

(a) Describe the arrangement and motion of the particles in a gas. (b) Explain why a gas can be compressed. (c) Name the change of state when a solid turns directly into a gas.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.