THE KELVIN
 

The SI unit of temperature is the kelvin, symbolized K (uppercase and nonitalicized). It is a measure of how much heat exists relative to absolute zero, which represents the absence of all heat and which is therefore the coldest possible temperature. A temperature of 0 K represents absolute zero. Formally, the kelvin is defined as a temperature increment (an increase or decrease) of 0.003661 part of the thermodynamic temperature of the triple point of pure water. Pure water at sea level freezes (or melts) at +273.15 K and boils (or condenses) at +373.15 K. What, you might ask, is the meaning of triple point? In the case of water, it’s almost exactly the same as the freezing point. For water, it is the temperature and pressure at which it can exist as vapor, liquid, and ice in equilibrium. For practical purposes, you can think of it as freezing.