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

2.2.3 Melting, Boiling and Evaporation

Objective

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

Key Concepts

• Melting (fusion): the change of a substance from solid to liquid when heat is added.
• Boiling (vaporisation): the change of a substance from liquid to gas throughout the bulk of the liquid when its vapour pressure equals the external pressure.
• Evaporation: the surface‑only change from liquid to gas that can occur at any temperature.

Standard Atmospheric Pressure

Standard atmospheric pressure (often denoted as \$P_{\text{atm}}\$) is defined as 101.3 kPa (1 atm). All temperature values given below refer to this pressure.

Temperatures for Water

Why These Temperatures Are Important

1. They provide reference points for the Celsius temperature scale.
2. In experiments, they allow students to verify the concept of latent heat:

   • During melting, heat added goes into breaking intermolecular bonds, not raising temperature.
   • During boiling, heat added also goes into overcoming intermolecular forces.

3. They are used in calculations involving the equation \$Q = mL\$, where \$L\$ is the latent heat of fusion or vaporisation.

Typical 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 amount of heat required to convert 250 g of ice at 0 °C to water at 0 °C, given \$L_{\text{fusion}} = 334 \text{J g}^{-1}\$.
• Describe how the boiling point of water would change if the external pressure were reduced to 80 kPa.

Suggested Diagram