Kelvin
Definition and meaning of Kelvin in chemistry.
The kelvin is the standard scientific unit of thermodynamic temperature. It belongs to an absolute scale that starts exactly at absolute zero. Absolute zero is the coldest possible temperature in the universe where thermal motion practically stops.
In more detail
The Kelvin scale uses the exact same size steps as the Celsius scale. A single kelvin is equal in size to exactly one degree Celsius. The only real difference is where the two temperature scales officially begin.
The Celsius scale sets zero at the freezing point of water. The Kelvin scale instead sets its zero point at absolute zero. Because Kelvin is an absolute scale, it is required for chemistry math.
You must use it for gas laws, equilibrium constants, and reaction rates. Using Celsius in these equations will give you the completely wrong answer. This happens simply because the Celsius scale includes negative number values.
You cannot have a negative volume or a negative pressure in a gas law. Absolute zero is exactly equal to a freezing -273.15 degrees Celsius. Scientists have never actually reached absolute zero inside a physical laboratory.
However, it serves as the essential starting point for all thermodynamic rules. A very common student mistake is saying the phrase "degrees Kelvin." The correct scientific term is just "kelvins" when speaking the unit out loud. You also never use the degree symbol when writing the unit on paper.
Key facts
| Field | Physical Chemistry |
|---|---|
| Unit symbol | K (no degree symbol) |
| Absolute zero | 0 K or -273.15 degrees Celsius |
| Conversion formula | K = degrees Celsius + 273.15 |
| Water freezing point | 273.15 K |
| Water boiling point | 373.15 K |
Let us look at the common temperature phases of pure water. Water normally freezes at exactly zero degrees on the Celsius scale. To convert this to Kelvin, you just add 273.15 to the Celsius number. This makes the freezing point of water exactly 273.15 K. Water boils at 100 degrees Celsius under normal air pressure. You add 100 to 273.15 to find the boiling point of 373.15 K. A comfortable room temperature is usually about 25 degrees Celsius. This translates to a standard room temperature of roughly 298 K. This simple addition math works perfectly every time you need to switch scales.
Frequently asked questions
Why must chemists use Kelvin instead of Celsius?
Kelvin is an absolute temperature scale with no negative numbers. Using Celsius in gas laws or thermodynamic equations can produce mathematically impossible negative results.
What is the practical difference between a kelvin and a degree Celsius?
A single kelvin and a degree Celsius are the exact same size. They only differ in their starting zero points.
Is it possible to reach absolute zero?
It is practically impossible to cool a substance all the way down to absolute zero. Even at the lowest temperatures, quantum mechanics says a tiny bit of motion must remain.