Heat Transfer & Changes in State

Specific Latent Heat ($L$): The amount of energy required to change the state of $1$ kg of a substance while maintaining a constant temperature.

Latent Heat of Fusion ($L_f$): The energy involved in the transition between solid and liquid states; it is absorbed during melting and released during freezing.

Latent Heat of Vaporization ($L_v$): The energy involved in the transition between liquid and gaseous states; it is absorbed during boiling and released during condensation.

Molecular Mechanism: During a phase change, energy is used to overcome the attractive forces (bonds) between molecules rather than increasing their vibrational or translational speed.

Phase Change Formula: To calculate the energy required for a change of state, use the formula $Q = mL$, where $Q$ is energy in Joules, $m$ is mass in kg, and $L$ is the specific latent heat.

Temperature Change Formula: When a substance is within a single phase and its temperature is changing, use $Q = mc\Delta\theta$, where $c$ is the specific heat capacity and $\Delta\theta$ is the change in temperature.

Multi-Step Problems: For complex transitions (e.g., ice at $-10$°C to water at $20$°C), calculate the energy for each stage separately: heating the solid, melting the solid, and heating the resulting liquid.

Conservation of Energy: In closed systems, the energy lost by one substance (cooling/condensing) must equal the energy gained by another (heating/melting).