What Is Triple point of water?
The triple point of water is one very exact temperature and pressure where water can be solid ice, liquid, and gas vapor at the same time. At this point all three states are in balance and none of them wins over the others. Scientists use this special point as a very precise mark on the temperature scale.
Definition
The triple point of water is defined as the unique condition where:
- Temperature is exactly 273.16 kelvin, which is 0.01 degree Celsius
- Pressure is about 611 pascals, which is much lower than normal air pressure at sea level
- Ice, liquid water, and water vapor are all present and stay in balance over time
At this point water can change between solid, liquid, and gas very easily, but the temperature stays the same as long as the pressure also stays at the right value. This makes it a perfect reference point for thermometers.
History / Origin
Scientists in the 19th century learned that many pure substances have a triple point. They saw that if you control temperature and pressure very carefully, three states of matter can exist together.
Water is the most important example, because water is common, cheap, and easy to clean. For many years, temperature scales were linked to two simple points:
- The melting point of ice
- The boiling point of water
Later, scientists found that the triple point of water is more stable and easier to repeat in different labs. In 1954 the international science community chose the triple point of water as the main fixed point to define the kelvin temperature scale. From that time, 273.16 kelvin at the triple point of water became the exact reference. In 2019 the kelvin was redefined using a constant from physics, but the triple point of water kept the same recommended value and is still used to check thermometers.
Symbol & Abbreviation
Several symbols are used when talking about the triple point of water.
- Ttpw or Ttr(H2O) is often used for the triple point temperature of water
- The value of this temperature is exactly 273.16 K
- In degree Celsius this is 0.01 °C
- The unit of temperature is the kelvin with the symbol K
So when you see Ttpw in a physics or chemistry text, it usually means the temperature at the triple point of water.
Current Use Around the World
The triple point of water is used worldwide as a standard reference in temperature measurement and calibration.
- National labs use triple point of water cells to check and set very accurate thermometers.
- High quality thermometers for science and industry are calibrated so that they read 273.16 K or 0.01 °C in a correct triple point of water cell.
- International temperature scales such as ITS 90 use the triple point of water as a main fixed point that all countries agree on.
Even though the official definition of the kelvin is now based on the Boltzmann constant, the triple point of water is still extremely important in practice. It connects real instruments to the abstract definition in a clear and repeatable way.
Example Conversions
Here are some useful numbers related to the triple point of water.
- Triple point of water in kelvin: 273.16 K
- In degree Celsius: 0.01 °C
- In degree Fahrenheit: about 32.018 °F
You can see how these values relate using simple formulas.
- From kelvin to Celsius: T(°C) = T(K) minus 273.15
- From Celsius to kelvin: T(K) = T(°C) plus 273.15
Examples:
- 273.16 K to °C
T(°C) = 273.16 minus 273.15 = 0.01 °C - 0.01 °C to K
T(K) = 0.01 plus 273.15 = 273.16 K - 273.16 K to °F
First to Celsius: 273.16 minus 273.15 = 0.01 °C
Then to Fahrenheit: 0.01 × 9 ÷ 5 plus 32 ≈ 32.018 °F
Related Units
Several other units and points are closely related to the triple point of water.
- Kelvin (K) the base unit of thermodynamic temperature in the International System of Units SI
- Degree Celsius (°C) a common everyday temperature scale, where 0 °C is close to the melting point of ice at normal pressure
- Degree Fahrenheit (°F) a temperature scale used mainly in the United States
- Absolute zero 0 K, the lowest possible temperature, used as the zero point for kelvin
- Boiling point of water 100 °C at standard pressure, another important reference point
- Other triple points triple points of other pure substances such as carbon dioxide
FAQs
What is the temperature at the triple point of water?
The temperature at the triple point of water is exactly 273.16 kelvin. This is the same as 0.01 degree Celsius and about 32.018 degree Fahrenheit.
Why is the triple point of water important?
It is important because it is very stable and easy to reproduce in different labs. This makes it an excellent reference for creating and checking accurate thermometers all over the world.
Is the triple point of water the same as the freezing point?
No. The freezing point of water at normal air pressure is 0 °C. The triple point of water is a little warmer at 0.01 °C and happens at much lower pressure. At the triple point all three states exist together, not just ice and liquid.
Can we see the triple point of water in daily life?
Not really. The triple point of water needs a special low pressure and a carefully controlled temperature. It is usually created in a sealed glass cell with pure water in a laboratory, not in normal outdoor conditions.
How do scientists create the triple point of water?
They use a triple point cell, which is a glass container with very pure water and water vapor. The pressure inside the cell is set by the water itself. By cooling and warming the cell in a controlled way, they create a stable mix of ice, water, and vapor all at the same time.
Did the triple point of water define the kelvin?
Yes, for many years the kelvin was defined so that the triple point of water was exactly 273.16 K. Since 2019 the kelvin is defined using a physics constant, but the triple point of water is still used as a key practical reference for temperature.
