# Is it a fluke that the triple point of water is almost exactly 0°C ?

• I
• peanutaxis
In summary, the triple point of water being slightly above 0°C is not a coincidence, but rather a result of the arbitrary definition of Celsius. The difference between the temperature of freezing water and the triple point is due to the slope of the phase diagram and the pressure of the system. This can be seen in the example of using pressure to shift the equilibrium and change the freezing temperature in the process of making ice cream.
peanutaxis
Seems a crazy coincidence that the tiple point of water is also virtually the same temperature at which water freezes/melts. Or is it that the triple point of water was always going to be at the temperature that water freezes/melts (so those two neccessarily co-exist) and then above water there is always going to be some vapour?
Why is the triple point slightly above 0°C? (0.0100°C)

Thanks
p

It's not a coincidence but it has been defined, arbitrarily, in this way by Celsius. For good reasons today the SI defines the units by fixing the values of fundamental constants, which provides much better accuracy and sustainability of the units, but of course in "redefining" the SI units one has been invested decades of painstaking effort to make them as accurately as possible to be consistent with the previous definition.

Look at the phase diagram of water (taken from here):

The difference between the temperature of freezing water and the triple point is only a function of the slope of the line separating solid from liquid and where it intersects with the line separating gas from liquid/solid. The fact that it is a small difference is an accident. Had we lived on Venus, where the atmospheric pressure is 75 times bigger, we would have found a greater difference between the two (since the freezing point is a function of pressure).

TSny, Lord Jestocost and vanhees71
vanhees71 said:
It's not a coincidence but it has been defined, arbitrarily, in this way by Celsius. For good reasons today the SI defines the units by fixing the values of fundamental constants, which provides much better accuracy and sustainability of the units, but of course in "redefining" the SI units one has been invested decades of painstaking effort to make them as accurately as possible to be consistent with the previous definition.
The difference in T between the freezing point of water at atmospheric pressure and the triple point is independent of the system of units, apart from the trivial point that had Celsius decided that the boiling point of water was at 10000 °C, then the difference would have been 1 °C instead of 0.01 °C.

TSny and vanhees71
peanutaxis said:
Why is the triple point slightly above 0°C? (0.0100°C)
This seems straightforward enough to reason out from simple principles. Let us see whether that is so.

Zero degrees is, at least traditionally, the freezing/melting point of pure water under ordinary atmospheric pressure -- dry air at 1 atm.

Under these conditions, the water/ice mixture will be out-gassing water vapor. We are not at the triple point yet.

So we allow the mixture to outgas until the ambient partial pressure of water vapor is high enough so that equilibrium is attained. And we remove the air. Now we have water, ice and water vapor. The pressure of the vapor on the water/ice mixture is far less than 1 atm.

Water expands when it freezes and contracts when it melts. One effect of that original 1 atm pressure was to force the equilibrium in the direction of melting more ice. Like the idea of ice skates melting a track in the ice. Now that we have removed the pressure, the equilibrium shifts in the direction of allowing more ice to freeze. The freezing temperature has been elevated.

If you get confused trying to figure how the freezing temperature shifts, think about the trick of using salt and ice to make ice cream in an old fashioned ice cream freezer. If you shift the equilibrium of an ice/water mixture in the direction of melting ice, you depress the freezing temperature -- so that you can make the ice cream. If you shift the equilibrium in the direction of freezing ice you do the opposite. You raise the freezing temperature.

QED.

Edit: Scooped by @DrClaude but I like to think in terms of ice skates and ice cream rather than phase diagrams.

Vanadium 50, TSny, Lord Jestocost and 1 other person

## What is the triple point of water?

The triple point of water is the unique combination of temperature and pressure at which water can coexist in three phases: solid (ice), liquid (water), and gas (vapor). This occurs at exactly 0.01°C (273.16 K) and a pressure of 611.657 pascals.

## Why is the triple point of water important in science?

The triple point of water is crucial because it provides a precise reference point for the calibration of thermometers and for defining temperature scales. It is used as a standard to define the Kelvin temperature scale, which is essential for scientific measurements.

## Is it purely coincidental that the triple point of water is close to 0°C?

Yes, it is largely coincidental. The Celsius scale was originally defined based on the freezing and boiling points of water, with 0°C set as the freezing point. The triple point of water, defined more precisely, happens to be very close to this original reference point but is not exactly 0°C.

## How does the triple point of water differ from the freezing point?

The freezing point of water is the temperature at which water transitions from liquid to solid at standard atmospheric pressure (1 atmosphere). The triple point, however, is a specific temperature and pressure where all three phases coexist. The freezing point is 0°C at 1 atmosphere, while the triple point is 0.01°C at a much lower pressure.

## Can the triple point of water be used to define other substances' phase transitions?

While the triple point of water is a well-known and standardized reference, each substance has its own unique triple point where three phases coexist. These points can also be used to study and define phase transitions for other materials, but they are specific to each substance and not as universally applicable as the triple point of water.

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