Cambridge IGCSE Physics 0625 – 2.1.1 States of Matter
2.1.1 States of Matter
Learning Objective
Students should be able to name and describe the four principal changes of state that involve solids, liquids and gases (excluding direct solid‑gas transitions).
Key Terminology
Melting – solid → liquid
Freezing – liquid → solid
Evaporation – liquid → gas (occurs at temperatures below the boiling point)
Condensation – gas → liquid
Boiling – rapid evaporation when a liquid reaches its boiling point
Phase‑Change Summary Table
Change of State
Symbolic Form
Common Term
Energy Change
Typical Conditions
Solid to Liquid
\$s \rightarrow l\$
Melting
Absorbs latent heat of fusion
Temperature reaches the melting point of the substance
Liquid to Solid
\$l \rightarrow s\$
Freezing (Solidification)
Releases latent heat of fusion
Temperature falls to the freezing point
Liquid to Gas (below boiling point)
\$l \rightarrow g\$
Evaporation
Absorbs latent heat of vaporisation
Surface molecules gain enough kinetic energy to escape; occurs at any temperature
Liquid to Gas (at boiling point)
\$l \rightarrow g\$
Boiling
Absorbs latent heat of vaporisation
Temperature reaches the boiling point; bubbles form throughout the liquid
Gas to Liquid
\$g \rightarrow l\$
Condensation
Releases latent heat of vaporisation
Gas is cooled to or below its dew point
Energy Considerations
During any change of state, the temperature of the material remains constant while energy is either absorbed or released as latent heat. This can be expressed by the equation:
\$Q = mL\$
where \$Q\$ is the heat energy, \$m\$ is the mass of the substance, and \$L\$ is the appropriate latent heat (fusion or vaporisation).
Typical Examples
Ice melting at \$0^\circ\text{C}\$ – solid \$\rightarrow\$ liquid.
Water freezing at \$0^\circ\text{C}\$ – liquid \$\rightarrow\$ solid.
Water evaporating from a puddle on a warm day – liquid \$\rightarrow\$ gas.
Steam condensing on a cold surface – gas \$\rightarrow\$ liquid.
Water boiling at \$100^\circ\text{C}\$ (at 1 atm) – liquid \$\rightarrow\$ gas (rapid evaporation).
Suggested Diagram
Suggested diagram: A phase‑change diagram showing the four required transitions (melting, freezing, evaporation, condensation) with temperature on the vertical axis and arrows indicating the direction of change.
Common Misconceptions
Thinking that evaporation only occurs at the boiling point – it can happen at any temperature.
Confusing the terms “boiling” and “evaporation” – boiling is a rapid, bulk process at a specific temperature, while evaporation is a surface phenomenon.
Assuming that heat is “used up” during a phase change – the energy is stored as latent heat within the material.
Quick Revision Checklist
Can you write the symbolic form for each change of state?
Do you know whether heat is absorbed or released for each transition?
Can you give a real‑world example for melting, freezing, evaporation, and condensation?
Are you able to explain why temperature does not change during a phase change?