5.3 Phase Changes and Thermal Energy

Why Phase Changes Matter on Praxis 5442

Physical science items ask teachers to name phase changes, decide whether energy is absorbed or released, read heating and cooling curves, and apply the specific-heat relationship q = mcΔT to simple temperature-change problems. Middle-school classrooms use ice melting, water boiling, and weather condensation as everyday hooks—your job is the particle-and-energy explanation behind those demos.

Phase Changes: Names, Directions, and Energy

A phase change (change of state) is a physical change: the substance’s identity stays the same (H₂O remains H₂O), but particle arrangement and motion change.

Which phase changes need energy input? Melting, evaporation/vaporization (including boiling), and sublimation. These are endothermic from the system’s point of view: the substance absorbs thermal energy from the surroundings (your hand feels cold holding melting ice because energy leaves your hand).

Which release energy? Freezing, condensation, and deposition are exothermic for the system: thermal energy flows out to the surroundings (steam can cause severe burns partly because condensation on skin releases large amounts of energy).

Evaporation vs boiling (teaching nuance):

• Evaporation can occur at the surface below the boiling point.
• Boiling occurs when vapor pressure equals external pressure; bubbles form throughout the liquid at the boiling temperature.

Both are liquid → gas and require energy input; Praxis stems may use either word.

Heating and Cooling Curves

A heating curve plots temperature of a sample versus time (or versus heat added) while energy is supplied at a steady rate. A cooling curve is the reverse while energy is removed.

How to read the graph

On a cooling curve, plateaus appear at condensation and freezing. Same temperatures as boiling and melting for a pure substance at fixed pressure (for water at 1 atm: 100°C and 0°C).

Exam-ready rule: sloped regions = temperature change (kinetic energy changing); flat regions = phase change (energy changing attractions/arrangement while temperature holds steady). Students often think “adding heat always raises temperature”—the plateau is the counterexample you must teach.

Water reference points (standard pressure)

• Melting / freezing: 0°C
• Boiling / condensation: 100°C

Ice water at 0°C can be all ice, all liquid, or a mixture during the phase change—temperature alone does not tell you the phase when you are exactly at the plateau.

Thermal Energy, Temperature, and q = mcΔT

Temperature measures average kinetic energy of particles. Thermal energy depends on temperature and how much matter is present (and the substance). Heating a thimble of water and a bathtub by 10°C are not the same energy cost.

When a single phase warms or cools without a phase change, use:

q = mcΔT

Water’s relatively high specific heat means it takes substantial energy to change water’s temperature—key for climate and organism temperature regulation discussions in crosscutting contexts.

Worked example: heating water from 20°C to 30°C

Problem: How much heat is needed to raise the temperature of 100 g of liquid water from 20°C to 30°C? Use c = 4.18 J/g·°C.

Step 1: Confirm no phase change—both temperatures are between 0°C and 100°C, so water stays liquid. q = mcΔT applies.

Step 2: Find ΔT:

ΔT = 30°C − 20°C = 10°C

Step 3: Substitute:

q = (100 g)(4.18 J/g·°C)(10°C) = 4,180 J

So about 4,180 J (or 4.18 kJ) must be absorbed by the water. On a heating curve, this process sits on a sloped liquid segment, not on a flat boiling or melting plateau.

Checks students should run:

• If mass doubles, q doubles (same ΔT).
• If ΔT is 5°C instead of 10°C, q halves.
• If the sample were ice warming below 0°C, you would still use q = mcΔT but with ice’s specific heat—not water’s—until melting begins.

Phase-change energy (beyond mcΔT) uses heats of fusion/vaporization (q = mH_f or q = mH_v). Praxis 5442 emphasizes recognizing that plateaus need energy without temperature change; quantitative H_f/H_v may appear in richer items, but mcΔT on a liquid segment is the calculation pattern highlighted here.

Instructional Scenario Hooks

• Sweat cooling the body: evaporation requires energy input, so energy leaves the skin.
• Lid on a pot: condensation returns water to liquid, releasing energy to the lid/surroundings.
• Graph item: “During which interval is potential energy of particles increasing while temperature stays constant?” → the melting or boiling plateau.

Always tie the macroscopic story (ice cube disappears, mirror fogs) to energy direction and particle spacing so answers stay consistent across multiple-choice and teaching-task formats.