Lesson Plan

Describe the four phases of matter at the particle level and the transitions between them

Explain why temperature stays constant during a phase change

Define and distinguish the latent heat of fusion and the latent heat of vaporization

Apply Q = mL_f and Q = mL_v to calculate the heat for a phase change

Solve multi-step problems that combine temperature changes (Q = mcΔT) and phase changes (Q = mL), including heat-balance problems

11.3

"Instead, the additional thermal energy acts to loosen bonds between molecules or atoms and causes a phase change. Because this energy enters or leaves a system during a phase change without causing a temperature change in the system, it is known as latent heat (latent means hidden)." "The heat, Q, required to change the phase of a sample of mass m is Q = mL_f (for melting/freezing), Q = mL_v (for vaporization/condensation), where L_f is the latent heat of fusion, and L_v is the latent heat of vaporization." "The ice starts out at −20 °C, and its temperature rises linearly, absorbing heat at a constant rate until it reaches 0°. Once at this temperature, the ice gradually melts, absorbing 334 kJ/kg. The temperature remains constant at 0 °C during this phase change. ... At 100 °C, the water begins to boil and the temperature again remains constant while the water absorbs 2256 kJ/kg during this phase change."