The triple point is one of the most important features on a phase diagram in IB Chemistry. It represents a unique combination of temperature and pressure where all three states of matter—solid, liquid, and gas—exist in equilibrium. The triple point helps explain unusual phase behaviors, the design of scientific instruments, and why some substances cannot exist as liquids at normal pressures.
What Is the Triple Point?
The triple point is the specific temperature and pressure at which solid, liquid, and gas phases of a substance coexist in equilibrium.
At this exact point:
- A substance can melt, freeze, boil, condense, sublime, and deposit
- All phase changes occur simultaneously
- The substance is in complete thermodynamic balance
The triple point is a single point, not a range. Each substance has its own unique triple point.
Conditions at the Triple Point
For a substance to reach its triple point, the following must be true:
- Temperature must be exactly at the triple-point temperature
- Pressure must be exactly at the triple-point pressure
- The system must be closed and stable
Under these conditions, the substance can switch freely among all three states without any change in temperature or pressure.
Examples of Triple Points
Water
- Triple point temperature: 0.01°C
- Triple point pressure: 0.006 atm
Below this pressure, water cannot exist as a liquid.
This is why water on Mars often sublimates instead of melting.
Carbon dioxide
- Triple point temperature: –56.6°C
- Triple point pressure: 5.11 atm
At 1 atm (Earth’s atmosphere), CO₂ cannot form a liquid, which explains why dry ice sublimates directly into gas.
Iodine
- Triple point: around 113°C and 0.12 atm
Iodine can exist as solid, liquid, and gas at this point.
Triple Point on a Phase Diagram
On a phase diagram, the triple point appears where all three phase boundaries intersect:
- The solid–liquid line
- The liquid–gas line
- The solid–gas line
At this intersection:
- All three phases present
- System is in equilibrium
- Any movement away from this point leads to only one or two phases
Understanding how to read this point is crucial for IB exams.
Why the Triple Point Matters
The triple point is important because it shows:
1. Whether a substance can exist as a liquid
If the pressure is below the triple point pressure, the liquid phase cannot form.
This explains:
- Why dry ice never melts at normal pressure
- Why water behaves unusually in outer space
2. Phase change behavior
The triple point helps predict when substances will sublime, melt, or boil under unusual conditions.
3. Calibration of thermometers
The triple point of water is used to define temperature scales precisely.
4. Thermodynamic stability
It shows where all phase changes are in perfect balance.
Real-World Applications of the Triple Point
1. Scientific calibration
The triple point of water is used in standardized temperature measurements.
2. Space science
Triple points explain how substances behave on other planets with low pressures.
3. Industrial freeze-drying
Freeze-drying operates below the triple point so ice can sublime.
4. Meteorology
Triple points help describe the formation of frost, ice, and snow under different atmospheric pressures.
Common IB Misunderstandings
“The triple point is where most phase changes happen in nature.”
No—it is a rare, exact condition.
“A triple point is a line on the diagram.”
It is a single, precise point.
“All substances have the same triple point.”
Each substance has its own unique value.
“The triple point means the substance is unstable.”
Actually, it is at equilibrium among all three phases.
FAQs
Can you see all three phases at the triple point?
Yes—with the right equipment, solid, liquid, and gas coexist.
Why can’t CO₂ be a liquid at 1 atm?
Because atmospheric pressure is below the CO₂ triple point.
What happens if you heat or cool the triple point?
The system leaves equilibrium and shifts into one or two phases.
Conclusion
The triple point is the exact temperature and pressure at which a substance exists as a solid, liquid, and gas simultaneously. It is a key concept for understanding phase diagrams, freeze-drying, space chemistry, and the behavior of substances under extreme conditions. Mastering the triple point helps IB Chemistry students interpret phase transitions and predict how substances behave outside normal atmospheric conditions.
