Kelvin
Definition and meaning of Kelvin in chemistry.
The kelvin is the SI base unit of thermodynamic temperature, defined such that absolute zero (0 K) represents the theoretical lower limit of the temperature scale, at which the thermal motion of particles approaches a minimum (though quantum mechanical zero-point energy prevents it from fully ceasing). The Kelvin scale has the same size increment as the Celsius scale, but begins at absolute zero rather than the freezing point of water.
In more detail
Because Kelvin is an absolute temperature scale, it is the standard unit in chemistry, physics, and all quantitative sciences where temperature must be accurately related to other properties. The relationships between temperature and physical quantities such as gas laws, equilibrium constants, and reaction rates are mathematically correct only when expressed in Kelvin. One kelvin equals one degree Celsius in magnitude; conversions follow K = °C + 273.15. Absolute zero (0 K, or -273.15°C) is unattainable in practice but serves as the essential reference point for all thermodynamic calculations and equations.
Key facts
| Field | Physical Chemistry |
|---|---|
| Absolute zero | 0 K = -273.15°C |
| Unit symbol | K |
| Conversion formula | K = °C + 273.15 |
At standard conditions, room temperature is approximately 298 K, the freezing point of water is 273.15 K, and the boiling point of water at atmospheric pressure is 373.15 K.
Frequently asked questions
Why must chemists use Kelvin instead of Celsius?
Kelvin is an absolute temperature scale beginning at absolute zero, which is necessary for the correct application of gas laws, equilibrium expressions, and thermodynamic equations. Using Celsius can produce mathematically incorrect results in these relationships.
What is the practical difference between a kelvin and a degree Celsius?
The Kelvin and Celsius scales have identical increments (one kelvin = one degree Celsius in size), but they differ in their zero points: Kelvin starts at absolute zero (-273.15°C), while Celsius starts at the freezing point of water.