Absolute Zero: Temperature and Molecular Motion

Temperature is a measure of molecular motion, and the Kelvin scale starts where that motion — in the ideal-gas picture — would stop: 0 K = -273.15 °C. Drag the temperature slider and the molecules in the box slow down as the square root of T, while the Charles-law cylinder beside them shrinks linearly towards zero volume at 0 K. Real matter never gets there: the third law forbids reaching 0 K in a finite number of steps, and quantum zero-point motion remains even at the limit, so what you see below 4 K, and below each gas's boiling point, is an ideal-gas extrapolation rather than a real gas.

Phase notegas · Distance above absolute zero293.15 K above absolute zero
Temperature  T(K) = T(°C) + 273.15
293.15 K
20.00 °C · 68.00 °F
RMS molecular speed  vrms = sqrt(3RT/M)
510.9 m/s
Charles volume ratio  V / V0 = T / 273.15
1.073
of the volume at 0 °C (ideal gas)
Mean kinetic energy (3/2)kT 6.071e-21 J
Thermal energy kT 25.26 meV
Temperature T293.15 K
Gas
N2 · M = 28.013 g/mol · boils 77.36 K
Tip: nitrogen molecules average 510.9 m/s at room temperature and 262.5 m/s at their own boiling point — halving the absolute temperature divides the speed by 1.414, not by 2. Swap to helium and the same 293.15 K gives 1352 m/s, because vrms falls as the square root of the molar mass.

vrms: root-mean-square speed — not the mean speed, and not the most probable speed · (3/2)kT: translational, per molecule · kT: k = 8.617333262e-5 eV/K (CODATA 2018)

Boiling points at 1 atm: helium 4.22 K, nitrogen 77.36 K, oxygen 90.19 K; carbon dioxide sublimes at 194.7 K. At T = 0 the ideal-gas model gives zero speed and zero kinetic energy; real matter keeps quantum zero-point motion, and the third law forbids reaching 0 K in a finite number of steps.