A phase diagram is one of the most useful visual tools in IB Chemistry Topic 1 (Stoichiometry) and Topic 5 (Energetics). It shows how a substance’s state—solid, liquid, or gas—changes depending on temperature and pressure. Phase diagrams help predict behavior under different conditions, explain unusual phase transitions, and provide insight into processes like boiling, melting, sublimation, and deposition. Understanding phase diagrams is essential for interpreting the relationships between pressure, temperature, and phase stability.
What Is a Phase Diagram?
A phase diagram is a graph showing the state of a substance at different combinations of temperature and pressure.
It contains three major regions representing:
- Solid
- Liquid
- Gas
And several important lines and points that indicate the boundaries between phases.
The axes are:
- x-axis: Temperature
- y-axis: Pressure
By locating a point on the graph, you can determine the phase the substance is in.
Key Parts of a Phase Diagram
A typical phase diagram includes the following features:
1. Fusion (melting) curve
The boundary between solid and liquid.
- Moving across this line indicates melting or freezing.
- Usually slopes slightly upward.
2. Vaporization (boiling) curve
The boundary between liquid and gas.
- Moving across this line indicates boiling or condensation.
- Ends at the critical point.
3. Sublimation curve
The boundary between solid and gas.
- Crossing this line represents sublimation or deposition.
These lines show where two phases coexist in equilibrium.
The Triple Point
The triple point is one of the most important features of a phase diagram.
Definition:
A temperature and pressure at which all three phases—solid, liquid, and gas—exist in equilibrium.
At this exact point:
- Sublimation, deposition, melting, freezing, boiling, and condensation can all occur simultaneously.
For water:
- Triple point occurs at 0.01°C and 0.006 atm.
Below the triple point, a substance cannot exist as a liquid, regardless of temperature.
The Critical Point
The critical point marks the end of the liquid–gas boundary.
At this point:
- The substance becomes a supercritical fluid
- Density of liquid and gas become equal
- The distinction between liquid and gas disappears
Beyond the critical point, a substance cannot be liquefied just by applying pressure.
For water:
- Around 374°C and 218 atm.
Supercritical fluids have unique properties useful in industrial extraction and purification.
How to Read a Phase Diagram
To determine the phase:
Locate a point on the graph and check which region it lies in.
To predict changes:
Move horizontally or vertically:
- Horizontal movement → change in temperature
- Vertical movement → change in pressure
This shows whether the substance will melt, boil, freeze, condense, sublime, or deposit.
Why Phase Diagrams Matter in IB Chemistry
Phase diagrams help you understand:
- Why water boils at lower temperatures at high altitude
- Why solid CO₂ (dry ice) sublimates at 1 atm
- Why pressure cookers raise boiling points
- Why frost forms on cold mornings
- How industrial processes adjust pressure to change phase behavior
They appear frequently in Paper 2 calculations and conceptual questions.
Unusual Phase Diagrams: Water vs Carbon Dioxide
Water has a negative slope on the solid–liquid boundary.
This means:
- Ice melts under increased pressure
- Ice is less dense than liquid water
- Water expands when frozen
Most substances have the opposite behavior, including CO₂.
Carbon dioxide:
- Cannot exist as a liquid at 1 atm
- Sublimes directly from solid to gas
- Has a triple point above atmospheric pressure
This explains why dry ice does not melt into a liquid.
Common IB Misunderstandings
“The triple point is where all three phases exist everywhere.”
No—it is a single, precise temperature and pressure.
“Critical point means boiling stops.”
Instead, above the critical point, the liquid–gas distinction disappears.
“Phase diagrams show chemical changes.”
They only show physical phase changes.
“Lines represent phase changes happening continuously.”
Lines represent equilibrium, not ongoing transitions.
FAQs
Why do some substances not have a liquid phase at 1 atm?
Because their triple point occurs above atmospheric pressure.
What happens at the critical point?
The substance becomes a supercritical fluid with combined liquid–gas properties.
How do phase diagrams relate to vapor pressure?
The vaporization curve is the vapor pressure curve.
Conclusion
A phase diagram is a map of the phases of a substance based on temperature and pressure. It reveals key concepts such as the triple point, critical point, sublimation line, and boiling point behavior. Mastering phase diagrams helps IB Chemistry students understand real-world phase changes, predict substance behavior, and interpret energetic transitions with confidence.
