# Why 273.15 and 273.16

1. May 12, 2006

### al_201314

Hi guys,

I've got a question regarding temperature. Here goes, on the Kelvin scale, I understand that at 273.15K, it is equivalent to 0.00 degrees celsius. What is the purpose then for the choice of 273.16K at the triple point of water? My text states that this number is deliberate so as to keep the interval between ice point and steam point a total of 100. I couldn't really understand that. Isn't the triple point of water the same as 0 degrees celsius, at 273.15K and 373.15K for steam point which is 100 degrees celsius?

Where is the point coming from?

thanks guys

2. May 12, 2006

### Farsight

The freezing point of water is 273.15K. At this temperature you've got ice. The triple point, where ice and water and vapour can all co-exist, is just a little warmer at a notional 273.16K, and is a more measurable flag for setting international standards.

3. May 12, 2006

### vanesch

Staff Emeritus
Yes, this is correct. It is maybe useful to specify that this freezing point is the equilibrium between water and ice, with one atmosphere of air pressure. This is the defining point for 0 Celcius, and happens at about 273.15K.

The triple point of water, where water and ice are in equilibrium with WATER VAPOR only (no air) happens slightly higher, at about 0.01 Celcius, or 273.16K (exactly). As this is a more accurate standard, the 273.16 K has been set EXACTLY to this point, and 273.16 has been chosen so as to stick as closely as possible to 1 K = 1 degree Celcius step.

4. May 12, 2006

### Gokul43201

Staff Emeritus
The triple point is the point of intersection of the three phase boundaries in the P-T phase diagram. The point, hence, represents a temperature and a pressure. For water the temperature is 273.16K and the pressure is 612 Pa (or about 0.006 atm).

Also, unlike almost every other melting curve, the solid liquid phase boundary has a negative slope for the water system. So, as you increase the pressure (from the triple-point pressure), the freezing temperature decreases very slowly. At a pressure of 1 atm, the freezing point is 273.15K.

Note: The normal freezing/melting point is defined as the temperature at which the solid and liquid are in equilibrium at 1 atm pressure.

5. May 12, 2006

### al_201314

Thanks for the responses. One more thing, why define the Kelvin as 1/273.16 of the triple point of water and not 1/273.15 of the ice point or freezing point of water? Any particular reason? This part is just for my curiosity. Thanks again.

Last edited: May 12, 2006
6. May 12, 2006

### Gokul43201

Staff Emeritus