Know the melting and boiling temperatures for water at standard atmospheric pressure

2.2.3 Melting, Boiling, Evaporation, Condensation & Solidification

Learning objective

State the melting and boiling points of water at 1 atm and describe the four phase‑change processes in terms of energy flow and particle behaviour.

Standard atmospheric pressure

1 atm = 101.3 kPa. All temperature values given below refer to this pressure unless otherwise noted.

Phase‑change processes

• Melting (fusion) – solid → liquid.
  Energy absorbed: latent heat of fusion.
  Temperature remains constant because the absorbed energy is used to break the ordered intermolecular bonds of the solid.
  Particle view: the rigid lattice of ice becomes a less‑ordered arrangement of water molecules that can move past one another.
• Boiling (vaporisation) – liquid → gas throughout the bulk.
  Energy absorbed: latent heat of vaporisation.
  Temperature stays at the boiling point while the supplied energy overcomes the attractive forces between molecules and creates vapour bubbles that rise to the surface.
  Particle view: molecules gain enough kinetic energy to escape the liquid phase.
• Evaporation – liquid → gas at the surface only.
  Can occur at any temperature; the rate increases with temperature, surface area and wind.
  The most energetic surface molecules leave the liquid, leaving the remaining molecules with a lower average kinetic energy – a cooling effect.
• Condensation – gas → liquid.
  Energy released: latent heat of vaporisation.
  Fast‑moving gas particles lose kinetic energy, collide, and form intermolecular bonds, producing a liquid. The surrounding environment gains the released heat.
• Solidification (freezing) – liquid → solid.
  Energy released: latent heat of fusion.
  As the temperature falls to the freezing point, liquid molecules lose kinetic energy, arrange into a regular lattice and release heat to the surroundings.

Melting and boiling temperatures of water (1 atm)

These two fixed points define the Celsius scale: 0 °C is the melting point of ice and 100 °C is the boiling point of water at 1 atm.

Boiling vs. evaporation – quick comparison

Why the reference temperatures matter

1. They provide the two fixed points that define the Celsius temperature scale.
2. In laboratory work they allow students to observe latent heat: temperature remains constant while the phase changes.
3. They are required when using the equation Q = m L (with L = latent heat of fusion or vaporisation) for energy calculations.

Typical IGCSE / A‑Level exam questions

• State the temperature at which water melts and boils at standard atmospheric pressure.
• Explain why the temperature of water does not rise while it is melting.
• Calculate the heat required to convert 250 g of ice at 0 °C to water at 0 °C, given L_fusion = 334 J g⁻¹.
• Describe how the boiling point of water would change if the external pressure were reduced to 80 kPa.
• Explain why sweating cools the body (evaporation).
• What happens to the temperature of steam when it condenses on a cold surface?

Suggested diagram