Changes of State

Each change of state has a name and a direction:

• Melting (solid → liquid) and freezing (liquid → solid) happen at the melting point.
• Evaporation / boiling (liquid → gas) and condensation (gas → liquid) happen at the boiling point.
• Sublimation (solid → gas directly) — e.g. dry ice, $\text{CO}_2$.

Melting and boiling absorb energy (particles need energy to break apart); freezing and condensation release energy. For pure water at sea level, the melting point is $0\,^\circ\text{C}$ and the boiling point is $100\,^\circ\text{C}$.