Changes of State (DP IB Chemistry): Revision Note

The kinetic molecular theory

• The kinetic molecular theory explains the physical properties of matter (solids, liquids, and gases) by describing particles in terms of:

  • Energy

  • Arrangement

  • Motion

What are the three states of matter?

• The three standard states of matter are solid, liquid and gas

Solids

• Have a fixed shape and volume

• Particles are closely packed in a regular, ordered pattern

• Particles can only vibrate in place

  • They do not move around

• Have a high density due to tightly packed particles

• Particles have the lowest energy compared to liquids and gases

Liquids

• Have a fixed volume but take the shape of their container

• Particles are close together but randomly arranged

• Can slide past one another, which allows liquids to flow

• Have a medium density (less than solids, more than gases)

• Particles have more energy than in solids, but less than in gases

Gases

• Have no fixed shape or volume

  • They expand to fill the container

• Particles are widely spaced and randomly arranged

• Move quickly and randomly in all directions

• Can be compressed easily due to the large space between particles

• Have a very low density

• Particle motion causes collisions, which generate pressure

• Particles have the highest energy of all three states

Changes of state

• State changes are physical changes and are reversible

• They involve energy transfer but do not alter the chemical identity of the substance

Names of state changes

• Melting – solid to liquid

• Freezing – liquid to solid

• Vaporisation – liquid to gas, including:

  • Boiling – occurs throughout the liquid at a specific temperature and takes place when the vapour pressure reaches the external atmospheric pressure

  • Evaporation – occurs only at the surface and takes place at temperatures below the boiling point

• Condensation – gas to liquid

• Sublimation – solid to gas

• Deposition – gas to solid

Changes of state diagram

State symbols in chemical equations

• In chemical equations, the physical state of a substance is shown using the following state symbols:

  • (s) – solid

  • (l) – liquid

  • (g) – gas

  • (aq) – aqueous (dissolved in water)

• In exams, marks are often awarded for correct state symbols, especially in thermodynamic contexts such as:

  • Ionisation energy

  • Lattice enthalpy (HL)

  • Enthalpy of formation (HL)