States of Matter (Cambridge International IGCSE Chemistry)

State changes

Spec point — 1.1.2 · Describe changes of state in terms of melting, boiling, evaporation, freezing & condensing

• State changes occur when:

  • Solids become liquids
  • Liquids become gases
  • Gases become liquids
  • Liquids become solids

• Each state change requires a change in the energy, arrangement and movement of the particles

The state changes

The inter-conversions / state changes are shown in relation to energy

Melting

• Melting is when a solid changes into a liquid
• Requires heat energy which transforms into kinetic energy, allowing the particles to move
• Occurs at a specific temperature known as the melting point (m.p.)

Freezing

• Freezing is when a liquid changes into a solid
• This is the reverse of melting and occurs at exactly the same temperature as melting, hence the melting point and freezing point of a pure substance are the same. Water, for example, freezes and melts at 0 ºC
• Requires a significant decrease in temperature (or loss of thermal energy) and occurs at a specific temperature

Boiling

• Boiling is when a liquid changes into a gas
• Requires heat which causes bubbles of gas to form below the surface of a liquid, allowing for liquid particles to escape from the surface and within the liquid
• Occurs at a specific temperature known as the boiling point (b.p.)

Evaporation

• Evaporation occurs when a liquid changes into a gas and occurs over a range of temperatures
• Evaporation occurs only at the surface of liquids where high energy particles can escape from the liquid's surface at low temperatures, below the b.p. of the liquid
• The larger the surface area and the warmer the liquid surface, the more quickly a liquid can evaporate

Condensation

• Condensation occurs when a gas changes into a liquid on cooling and it takes place over a range of temperatures
• When a gas is cooled its particles lose energy and when they bump into each other they lack the energy to bounce away again, instead they group together to form a liquid

Exam Hint Questions on the particle theory of matter show interconversion of states with a reversible arrow: ⇌, which means that the process can go forwards and backwards.

Read the question carefully and pick the direction of the change in state that the question refers to.

State Changes & Kinetic Theory

Spec point — 1.1.2 · Explain changes of state in terms of kinetic particle theory, including the interpretation of heating and cooling curves

Extended tier only

• When substances are heated, the particles absorb thermal energy which is converted into kinetic energy

  • This is the basis of the kinetic theory of matter

• Heating a solid causes its particles to vibrate more

  • As the temperature increases, the particles vibrate so much that the solid expands until the structure breaks
  • This is when the solid melts into a liquid

• Heating a liquid causes its particles to move more and spread out

  • Some particles at the surface gain sufficient energy to overcome the intermolecular forces
  • This is when a liquid starts to evaporate
  • When the boiling point is reached, all of the particles gain enough energy to escape and the liquids boils into a gas

• These changes in state can be shown on a graph called a heating curve:

A heating curve showing the states, state changes and temperature changes as time progresses

• Cooling down a gas has the reverse effect and this would be called a cooling curve:

A cooling curve is like a heating curve, but is the mirror image

• Heating and cooling curves are used to show how changes in temperature affect changes of state

• The horizontal sections occur when there is a change of state but there is no change in temperature

• During a change of state, the thermal energy supplied is used to overcome the attractive forces between particles rather than increasing their kinetic energy

  • Since temperature is a measure of average kinetic energy, it remains constant until the state change is complete