Absolute zero (0 K, or −273 °C) is the lowest possible temperature — the point at which particles have minimum internal energy and cannot transfer any heat to a colder object. The Kelvin scale starts at absolute zero and uses degree-sized steps equal to Celsius, so converting between the two requires only adding or subtracting 273.
Why does absolute zero exist?
Temperature is a measure of the average kinetic energy of the particles in a substance. The faster the particles move, the higher the temperature. When you cool something down, you remove kinetic energy from its particles and they slow down.
Absolute zero (0 K) is the theoretical lower limit: the temperature at which particles have the minimum possible kinetic energy (their zero-point energy in quantum mechanics, though at GCSE we treat this as zero kinetic energy). It is impossible for particles to have negative kinetic energy, so it is impossible for any substance to be colder than 0 K.
Absolute zero corresponds to −273 °C (more precisely, −273.15 °C).
No substance in the universe has ever been cooled to exactly 0 K — the third law of thermodynamics states it is unattainable in a finite number of steps — but scientists have cooled gases to within a billionth of a kelvin above absolute zero.
What is the Kelvin temperature scale?
The Kelvin scale is the SI unit of thermodynamic temperature. It was devised by William Thomson (Lord Kelvin) in the mid-19th century to create an absolute temperature scale that starts at the coldest possible temperature.
Key facts:
- The Kelvin is the SI base unit of temperature — symbol K (no degree symbol).
- 0 K = absolute zero = −273 °C.
- The size of one kelvin is identical to the size of one degree Celsius.
- Water freezes at 273 K (0 °C); water boils at 373 K (100 °C) at standard pressure.
How do you convert between Kelvin and Celsius?
The conversion is straightforward because the scales are offset by exactly 273:
| To convert | Formula |
|---|---|
| Celsius → Kelvin | T(K) = T(°C) + 273 |
| Kelvin → Celsius | T(°C) = T(K) − 273 |
Worked examples:
- Convert 25 °C to kelvin: 25 + 273 = 298 K
- Convert 500 K to Celsius: 500 − 273 = 227 °C
- Convert −40 °C to kelvin: −40 + 273 = 233 K
- Absolute zero in Celsius: 0 − 273 = −273 °C
Important: in all gas law calculations at GCSE (and beyond), temperature must be in kelvin. Using Celsius gives incorrect answers because 0 °C is not zero temperature on an absolute scale.
How does the Kelvin scale link to the gas laws?
The gas laws describe how the pressure, volume and temperature of a fixed mass of gas are related. The temperature must be in kelvin for these relationships to be directly proportional:
Pressure–temperature law (constant volume): $$\frac{P_1}{T_1} = \frac{P_2}{T_2}$$
If temperature doubles (in kelvin), pressure doubles — a directly proportional relationship. This makes physical sense: doubling the absolute temperature doubles the kinetic energy of the gas particles and therefore doubles the frequency and force of their collisions with the container walls.
Volume–temperature law (Charles' Law) (constant pressure): $$\frac{V_1}{T_1} = \frac{V_2}{T_2}$$
If temperature doubles in kelvin, volume doubles.
Worked example:
A gas at 300 K and 150 kPa is heated to 450 K at constant volume. What is the new pressure?
$$P_2 = P_1 \times \frac{T_2}{T_1} = 150 \times \frac{450}{300} = 150 \times 1.5 = \textbf{225 kPa}$$
What is the evidence for absolute zero?
Graphical evidence: if you plot the pressure of a fixed volume of gas against its Celsius temperature, you get a straight line. Extrapolating this line to the x-axis (zero pressure) gives a temperature of approximately −273 °C for all gases tested. Since pressure is zero when the particles have no kinetic energy to exert force on the walls, this extrapolated point represents the theoretical zero of kinetic energy — absolute zero.
| Gas | Extrapolated zero-pressure temperature |
|---|---|
| Oxygen | −273 °C |
| Nitrogen | −273 °C |
| Hydrogen | −273 °C |
| Carbon dioxide | −273 °C |
All gases converge on the same value — strong evidence that this is a fundamental temperature limit rather than a property of any particular gas.
Frequently asked questions
Why must temperature be in kelvin for gas law calculations?
The gas laws express directly proportional relationships: doubling the temperature doubles the pressure (at constant volume). This proportionality only holds if you measure temperature from true zero. If you used Celsius, you would be measuring from an arbitrary zero (the freezing point of water). Consider a gas at 10 °C heated to 20 °C: the Celsius temperature doubled, but the actual kinetic energy has barely changed — the kelvin temperatures are 283 K and 293 K, a ratio of 1.04, not 2. Using Celsius in gas law equations gives wildly wrong answers for any temperature near 0 °C.
Has absolute zero ever been achieved?
Absolute zero has never been reached and, according to the third law of thermodynamics, cannot be reached in a finite number of cooling steps. However, scientists have cooled systems to extraordinarily close — within a few billionths of a kelvin (nanokelvin). At these temperatures, gases condense into exotic quantum states such as Bose–Einstein condensates, where particles lose their individual identities and behave as a single quantum entity. These experiments confirm that absolute zero is a real physical limit approached asymptotically, not a boundary that can be crossed.
Why does gas pressure drop as temperature decreases?
Gas pressure arises from the force of gas molecules colliding with the walls of the container. At higher temperature, molecules move faster (higher kinetic energy) and collide with the walls more frequently and with more force — producing higher pressure. When temperature falls, molecular speeds decrease, collisions become less frequent and less forceful, and pressure drops. At absolute zero, particles would have no kinetic energy and would not move at all — producing zero pressure. This is the physical interpretation behind the pressure–temperature graphs that extrapolate to −273 °C.
What is meant by the "internal energy" of a substance?
The internal energy of a substance is the sum of all the kinetic energies and potential energies of its particles. At a given temperature, particles in a gas have a range of speeds and kinetic energies — temperature measures the average. Potential energies arise from the forces between particles (significant in solids and liquids). Heating a substance increases its internal energy: either the kinetic energy rises (temperature rises) or the potential energy rises (during a change of state, where temperature stays constant while bonds between particles are broken). At absolute zero, the internal energy is at its minimum — no more can be removed.
For Socratic GCSE physics with Professor Newton — predicting gas behaviour from particle kinetic energy through the Kelvin scale to absolute zero — visit aitutors.me.