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States of matter

Learning outcomes

After studying this page, you should be able to:

  • recall the names of the three states of matter and their interconversions
  • describe the arrangement, movement and the relative energy of particles in the three states of matter
  • explain what happens to particles during state changes
  • predict the physical state of a substance under given conditions

The particle model

The three states of matter are solid, liquid and gas. The particle model describes how the particles of a substance are arranged and move in each state.

 
Solid
Liquid
Gas
Relative distance apart
very close
close
far apart
Arrangement regular random random
Movement
vibrate about fixed positions
move around each other
rapid in all directions
2D diagram
solid.png
liquid.png
gas.png

The word 'matter' describes all the 'stuff' that is made of particles and which has mass. This includes you and all the substances and objects around you.

Depending on the substance, the particles can be atoms, molecules or ions.

The diagrams do not show the movement of the particles, the forces between them or their stored energy. Remember that there is empty space between particles, and that these are just 2D diagrams of 3D situations.

State changes

The diagram summarises the four main changes of state.

State changes.jpg

Evaporation only happens at the surface of a liquid. Boiling happens throughout the liquid, and occurs at or above its boiling point. Two changes of state (not shown in the diagram) do not involve the liquid state:

  • sublimation: solid → gas
  • deposition: gas → solid

The video shows 'dry ice' (solid carbon dioxide) subliming at room temperature. It plays at 10× normal speed so you can see more clearly how the solid disappears without becoming a liquid.

Attractive forces can exist between particles. These can be overcome or broken by heating.

When energy is transferred from the surroundings to the substance:

  • some of the forces are overcome during melting
  • all the remaining forces are overcome during evaporation or boiling.

When energy is transferred from the substance to the surroundings:

  • some forces form during condensing
  • many forces form during freezing.

The temperature of the substance stays the same during a state change.

Different attractive forces can exist between different types of particle. These include:

  • covalent bonds between atoms
  • intermolecular forces between simple molecules
  • ionic bonds between oppositely charged ions.

Predicting physical states at different temperatures

The melting point of a substance is the temperature at which it melts or freezes. The boiling point of a substance is the temperature at which it boils or condenses. You can predict the state of a substance at a given temperature if you know its melting point and boiling point. The diagram shows how this works.

Predicting state at different temperatures

Worked example

The melting point of substance X is –7.2 °C and its boiling point is 58.8 °C. Predict its state at:

(a) 25 °C
(b) –25 °C
(c) 75 °C

Click to see the answers

(a) liquid

(b) solid

(c) gas 

Steel in its liquid state

Fruit frozen in liquid nitrogen

How much do you know?

Click on the image to start the quiz. Good luck!

Worksheets

PDF code Title of worksheet Description
G-01-01-01 Changes of state Structured worksheet