MCAT content library / Chemistry & physics / Phase changes and phase diagrams
Separate energy that changes temperature from energy that changes phase, then follow pressure–temperature paths across phase boundaries.
About 17 minutes, including a heating-curve calculation and hypothetical phase diagram.
Useful foundations
On this page
- Temperature change and phase change use different energy models
- Concept: heating-curve plateaus mark phase coexistence
- Worked application: heat a hypothetical substance
- Pattern: phase diagrams map the stable phase
- Rule: triple and critical points mean different things
- Check your reasoning
- Continue learning
- Sources
Temperature change and phase change use different energy models
Within one phase, sensible heating is modeled by
q = mcΔT.
During a phase change at the equilibrium transition temperature and fixed pressure, added energy can change intermolecular organization without raising temperature. That energy is modeled with a latent heat or molar enthalpy such as
q = mL or q = nΔHphase.
A multi-segment heating problem therefore must be broken into separate temperature-change and phase-change steps.
Concept: heating-curve plateaus mark phase coexistence
On an idealized constant-pressure heating curve, sloped segments correspond to warming within one phase. A flat segment occurs while two phases coexist at the transition temperature. Energy added during melting increases the fraction liquid rather than raising temperature; energy added during boiling increases the fraction vapor.
The slope of a temperature-versus-heat curve depends on heat capacity. For the same mass, a phase with larger specific heat requires more energy for the same temperature increase and therefore shows a shallower temperature rise per unit heat added.
Worked application: heat a hypothetical substance
A 50-g hypothetical substance begins as a solid at 0°C. It melts at 20°C. The solid specific heat is 2.0 J/(g·°C), and the latent heat of fusion is 100 J/g. How much heat is required to reach fully liquid material at 20°C?
First warm the solid:
q₁ = (50 g)(2.0 J/g·°C)(20°C) = 2000 J.
Then melt it:
q₂ = (50 g)(100 J/g) = 5000 J.
Total:
q = 7000 J.
| Step | Temperature behavior | Heat |
|---|---|---|
| Warm solid 0→20°C | Temperature rises | 2000 J |
| Melt at 20°C | Temperature constant | 5000 J |
| Total | Final material just fully liquid | 7000 J |
Using q = mcΔT for the melting plateau would be incorrect because ΔT = 0 while substantial energy is still absorbed.
Pattern: phase diagrams map the stable phase
A pressure–temperature phase diagram shows which phase is thermodynamically stable at each combination of pressure and temperature. Lines separating regions represent equilibrium coexistence of two phases.
Imagine a hypothetical substance at a temperature where low pressure lies in the gas region and higher pressure lies in the liquid region. Increasing pressure at fixed temperature crosses the liquid–gas coexistence line and condenses the gas. At a different temperature below the triple point, the liquid region may not be accessible at equilibrium, so increasing pressure can instead cross directly from gas to solid.
The direction of a phase-boundary slope is substance dependent. Water’s solid–liquid line is an important exception to the common positive-slope pattern, so use the actual diagram provided rather than assuming every substance behaves like water.
Rule: triple and critical points mean different things
The triple point is the unique pressure–temperature condition at which solid, liquid, and gas coexist in equilibrium. The critical point is the endpoint of the liquid–gas coexistence curve. Above the critical temperature and pressure region, liquid and gas are no longer separated by an ordinary first-order phase boundary and are described as a supercritical fluid.
These points should not be confused with melting or boiling points at one chosen pressure. Normal boiling point, for example, refers specifically to the temperature where vapor pressure equals 1 atm.
ACTIVE PRACTICE · THERMAL PHYSICS
Separate Heating From Melting
The lesson’s hypothetical 50-g solid starts at 0°C, melts at 20°C, has c = 2.0 J/(g·°C), and requires 100 J/g for fusion. Predict the total energy to obtain fully liquid material at 20°C.
Temperature: °C. State:. Fraction melted:.
Heat for warming: 2,000 J; heat to melt completely at 20°C: 5,000 J. The plateau absorbs latent heat with no temperature rise.
Ideal constant-pressure illustrative model without supercooling, overheating, or heat loss. The slider ends once the sample has fully melted; it does not model later liquid warming.
Check your reasoning
Choose an answer before opening its feedback.
Check 1
During an ideal constant-pressure melting plateau, heat continues to enter but temperature stays constant. Where does the energy go?
A. Into the phase change and intermolecular organization
Correct. The liquid fraction increases while temperature remains at the melting point.
B. Nowhere; no energy can enter at constant temperature
Review this choice. Latent heat is absorbed during the phase change.
C. Only into increasing translational temperature
Review this choice. Temperature is constant during the ideal plateau.
D. Into converting mass into charge
Review this choice. The process is an ordinary phase change.
Check 2
For 20 g of a substance with latent heat of fusion 150 J/g, how much energy is required to melt it at its melting temperature?
A. 3000 J
Correct. q = mL = 20×150.
B. 170 J
Review this choice. Mass and latent heat are multiplied.
C. 7.5 J
Review this choice. This reverses the ratio.
D. 0 J
Review this choice. Constant temperature does not mean zero phase-change energy.
Check 3
What is special about the triple point on a phase diagram?
A. Solid, liquid, and gas coexist in equilibrium.
Correct. Three phase boundaries meet there.
B. Liquid and gas become indistinguishable.
Review this choice. That describes the critical point.
C. Temperature must be absolute zero.
Review this choice. Triple points occur at substance-specific nonzero temperatures.
D. No phase is stable.
Review this choice. Three phases are simultaneously stable in equilibrium.