Temperature: The Kelvin

Introduction

On a cold winter night the temperature might drop to −10 °C. But what does a negative temperature really mean? The Celsius scale puts zero at the freezing point of water, which is convenient for everyday life but has nothing special to do with particles. Scientists wanted a scale that starts at the true bottom — the point where particles have the least possible energy. That scale is the kelvin scale, and its unit, the kelvin, is the SI base unit of temperature.

Core explanation

What temperature measures. From the particle model, you know that particles in any substance are always moving — vibrating in solids, and moving around in liquids and gases. Temperature is a measure of the average kinetic energy of these particles. The hotter the substance, the faster its particles move on average.

Absolute zero. If you cool a substance, its particles move more and more slowly. There is a limit: the temperature at which particles have the minimum possible energy. This is absolute zero , equal to −273.15 °C. Nothing can be colder, because particles cannot have less than the minimum energy. In practice absolute zero can be approached extremely closely but never quite reached.

The kelvin scale. The kelvin scale starts at absolute zero, so 0 K is absolute zero. Its steps are the same size as Celsius degrees: a rise of 1 K is exactly the same as a rise of 1 °C. This means:

Point Celsius Kelvin --- --- --- Absolute zero −273.15 °C 0 K Water freezes (1 atm) 0 °C 273.15 K Room temperature about 20–25 °C about 293–298 K Human body about 37 °C about 310 K Water boils (1 atm) 100 °C 373.15 K

No degree sign, no negatives. We write 300 K, not 300 °K, and say "300 kelvin", not "300 degrees kelvin". Because the scale starts at the lowest possible temperature, kelvin temperatures are never negative.

How the kelvin is defined. Since 2019 the kelvin has been defined by fixing the Boltzmann constant , k = 1.380 649 × 10⁻²³ J/K, which links temperature to the energy of particles. Before that, it was defined using the triple point of water — the single temperature (273.16 K) at which ice, liquid water and water vapour coexist.

Why chemists use kelvin. Many relationships in chemistry are only simple when temperature is measured from absolute zero. For example, at constant pressure, the volume of a gas is proportional to its kelvin temperature: doubling the kelvin temperature doubles the volume. Doubling a Celsius temperature has no such simple meaning. Gas laws, reaction rate equations and thermodynamics all require kelvin.

Step-by-step reasoning

To see why a kelvin scale is needed:

1. Consider gas at 10 °C and at 20 °C. Is the second "twice as hot"? It seems so from the numbers. 2. Convert to kelvin: 283 K and 293 K. 3. The particles' average energy has risen by only about 3.5%, not 100%. 4. Only the kelvin scale gives temperatures proportional to particle energy.

Visual explanation

Imagine a thermometer drawn beside two scales. On the right, the Celsius scale has 0 at the freezing point of water and runs into negative numbers below it. On the left, the kelvin scale has 0 at the very bottom of the tube, with no numbers below. The marks are equally spaced on both, but the zeros are 273.15 steps apart.

Real-world analogy

Measuring height from a table top gives the table-top a height of zero, even though it is well above the floor. Measuring from the floor gives the true height. The Celsius scale measures from a "table top" (freezing water); the kelvin scale measures from the true "floor", absolute zero.

Real-world example

The superconducting magnets in hospital MRI scanners are cooled with liquid helium to about 4 K (−269 °C). At such low temperatures some metals conduct electricity with zero resistance. Engineers and scientists working with these systems always quote kelvin temperatures.

Why?

Why is there a lowest temperature but no highest one? Temperature reflects particle motion. Motion can be reduced only until particles have their minimum energy, which sets a floor at 0 K. There is no similar upper limit on how much energy particles can gain, so temperatures can be extremely high, such as millions of kelvin inside stars.

Common misconception

"At absolute zero, particles stop completely." Even at the lowest possible energy, quantum theory says particles keep a small, unavoidable motion called zero-point energy. Absolute zero is the state of minimum energy, not zero motion.

Worked example

Question: A gas is at 27 °C. What is its temperature in kelvin, to the nearest kelvin? If its kelvin temperature is doubled at constant pressure, what happens to its volume?

Reasoning: T(K) = 27 + 273 = 300 K. At constant pressure, volume is proportional to kelvin temperature, so doubling to 600 K doubles the volume.

Answer: 300 K; the volume doubles.

Quick check

1. What is absolute zero on the Celsius scale? Answer: −273.15 °C (often rounded to −273 °C).

Exam focus

Always convert temperatures to kelvin in gas law and energy calculations. Remember that a temperature change is the same in K and °C. Do not write a degree sign with K, and note that negative kelvin temperatures are impossible — a negative answer signals an error.

Advanced insight

Physicists have cooled clouds of atoms to below a billionth of a kelvin using lasers and magnetic traps. At such temperatures, atoms can merge into a single quantum state called a Bose–Einstein condensate. The third law of thermodynamics states that absolute zero itself cannot be reached in a finite number of steps.

Summary

Temperature measures the average kinetic energy of particles. Absolute zero, −273.15 °C or 0 K, is the lowest possible temperature. The kelvin is the SI base unit of temperature, defined by the Boltzmann constant; its steps equal Celsius degrees but it starts at absolute zero, has no degree sign and has no negative values. Chemists use kelvin in gas laws and thermodynamics.

Practice questions

1. What does temperature measure in terms of particles? Answer: The average kinetic energy of the particles. 2. Why are there no negative temperatures on the kelvin scale? Answer: The scale starts at absolute zero, the lowest possible temperature, so nothing can be below 0 K. 3. A liquid is warmed from 20 °C to 35 °C. What is the temperature change in kelvin? Answer: 15 K, because kelvin and Celsius steps are the same size. 4. Correct the following: "Liquid nitrogen boils at 77 °K." Answer: It should read 77 K, with no degree sign.