Specific Heat and Phase Change

Objective: Calculate Q = mcΔT for a specified temperature range.

Specific heat capacity c is the energy transfer required per unit mass per kelvin of temperature change under stated conditions; Q = mcΔT is a useful model when no phase change occurs and c is approximately constant over the range. The SI unit is J/(kg·K). For 2 kg of material with c = 1000 J/(kg·K) warming by 5 K, Q = 10,000 J. A larger c means more energy is required for the same mass and temperature rise, all else equal. The equation describes energy transfer by heating; sign conventions distinguish warming from cooling. Real c can depend on temperature, pressure, and phase, and losses to surroundings complicate experiments.

Objective: Connect constant-temperature transitions to changed particle arrangements.

During a phase transition at a particular pressure, energy transfer can change the arrangement and interactions of particles while temperature remains approximately constant for a pure substance undergoing an equilibrium transition. For melting or vaporization, Q = mL under stated conditions, where L is specific latent energy in J/kg; condensation or freezing releases energy to surroundings. For 0.5 kg with L = 200,000 J/kg, a complete transition requires 100,000 J. Boiling occurs throughout a liquid at its boiling temperature for the given pressure, whereas evaporation can occur at the surface below that temperature. Pressure and composition affect transition temperatures. A heating curve must identify both sloped warming regions and flat transition regions.

Objective: Interpret slopes and plateaus without confusing energy and temperature.

A heating curve plots temperature against energy supplied or time under a specified heating rate. Sloped regions typically represent temperature change within a phase, modeled with Q = mcΔT. Plateaus in a suitable pure-substance model correspond to phase change with energy transfer at approximately constant temperature, modeled with Q = mL. The width of a plateau on a time graph depends on input power, mass, and losses as well as latent energy; it is not automatically a pure material constant. Cooling reverses transfer direction, with possible real-world supercooling or nonideal behavior. Data interpretation requires axis units, pressure, and whether the plotted sample is pure or mixed.