Triple Point
Definition and meaning of Triple Point in chemistry.
A triple point is a very specific combination of temperature and pressure. At this exact point, a substance exists as a solid, liquid, and gas. The three different physical phases rest together in a perfect balance.
In more detail
Most of the time, changing the temperature causes a substance to change its state. Solid ice melts into liquid water when it gets warm. Liquid water boils into a gas when it gets very hot.
But at the triple point, a very strange physical balance happens. The solid, liquid, and gas forms all exist at the exact same time. Six different phase changes happen simultaneously inside a closed container.
Solid turns to gas, and gas turns back to solid. Solid melts to liquid, and liquid freezes back to solid. Liquid boils to gas, and gas condenses back to liquid.
All these phase changes happen at the exact same speed. Because the speeds match perfectly, the total amount of each phase stays constant. We say the system has reached a state of perfect thermodynamic equilibrium.
Every pure chemical substance has its own unique triple point. You can find this point on a graph called a phase diagram. It looks like a single dot where three boundary lines meet.
Scientists use these reliable points to calibrate extremely precise lab thermometers. They know the triple point of a pure substance will never change.
Key facts
| Field | Physical Chemistry |
|---|---|
| Key Feature | Three phases coexist at once |
| Equilibrium | All phase changes happen at equal speeds |
| Phase Diagram | Appears as a single intersection point |
| Water Triple Point Temperature | 273.16 K (0.01 °C) |
| Water Triple Point Pressure | 611.657 Pa |
The triple point of pure water is a famous example in chemistry. This specific state occurs at a temperature of exactly 0.01 degrees Celsius. It also requires a very low pressure of exactly 611.657 pascals. This pressure is less than one percent of normal atmospheric pressure. If you have water at these exact conditions, you will see a fascinating mix. You will see solid ice cubes floating in boiling liquid water. This exact condition was historically used to define the Kelvin temperature scale.
Frequently asked questions
Can a substance have more than one triple point?
Yes, a substance can have multiple solid phases or distinct crystal structures. This means it can have several triple points for different phase combinations.
What happens if you change the temperature at the triple point?
The perfect balance will be destroyed. One or more of the phases will completely disappear as the substance changes state.
Is a triple point the same thing as a freezing point?
A freezing point only involves a balance between a solid and a liquid. A triple point requires a solid, a liquid, and a gas to all exist together.