Describe melting and boiling in terms of energy input without a change in temperature

2.2.3 Melting, Boiling and Evaporation

Objective: Describe melting and boiling in terms of energy input without a change in temperature. 🔥🌡️

Melting (Fusion)

When a solid absorbs heat, its molecules vibrate faster. At a specific temperature (the melting point), the absorbed energy is used to break the bonds that keep the molecules in a fixed arrangement. The temperature stays constant while the solid turns into a liquid. This extra energy is called the latent heat of fusion.

• Think of a crowded dance floor: everyone is moving, but the room’s temperature stays the same because the energy goes into the dancing, not into heating the air.
• Example: An ice cube in a glass of water at 0 °C. Adding heat melts the ice, but the water stays at 0 °C until all ice is gone.
• Equation: $Q = mL_f$, where $L_f$ is the latent heat of fusion.

Boiling (Vaporisation)

Boiling occurs when a liquid is heated to its boiling point. The absorbed energy breaks intermolecular bonds, turning the liquid into gas. The temperature remains constant during the phase change. The energy required is the latent heat of vaporisation.

1. Imagine a pot of soup: the water stays at 100 °C while bubbles rise, until all the water has turned into steam.
2. Example: Water boiling in a kettle at 100 °C. Heat keeps the temperature at 100 °C until every drop has vaporised.
3. Equation: $Q = mL_v$, where $L_v$ is the latent heat of vaporisation.

Evaporation

Evaporation is the surface version of boiling. It can occur at any temperature below the boiling point. The molecules at the surface gain enough energy to escape into the gas phase. The surrounding temperature remains unchanged during the process.

Example: A puddle drying on a sunny day. The water molecules at the surface evaporate, but the air temperature stays the same.

Key Equations

Latent heat is the energy per unit mass needed for a phase change:

$Q = mL$

Where:

• $Q$ – heat added (J)
• $m$ – mass of the substance (kg)
• $L$ – latent heat (kJ kg⁻¹)

Latent Heat Table

Key Takeaways

• During melting or boiling, added heat is used to change the state, not to raise temperature.
• The temperature remains constant until the phase change is complete.
• Latent heat values differ between substances and between fusion and vaporisation.
• Evaporation is a surface phenomenon that can happen below the boiling point.
• Latent heat is crucial for everyday processes: cooking, weather, refrigeration, and energy storage.