1.1.3—Temperature and kinetic energy

Temperature on the Kelvin scale is proportional to the average translational kinetic energy of particles. At the same Kelvin temperature, samples of different gases have the same average kinetic energy, although lighter particles have a higher typical speed than heavier particles. Use absolute temperature, not degrees Celsius, when comparing kinetic energies or temperature ratios.

Worked example — convert before interpreting

For $25.0\,^\circ\mathrm{C}$, substitute into the conversion: $T=25.0+273.15=298.15\,\mathrm{K}$, reported as $298.2\,\mathrm{K}$ to one decimal place. The Kelvin value is the absolute temperature used for kinetic-energy comparisons; a rise of $10\,\mathrm{K}$ is the same temperature interval as a rise of $10\,^\circ\mathrm{C}$.

Read a heating curve by first deciding whether the substance is within one state or changing state. On a sloping section, supplied energy increases average kinetic energy, so temperature rises. On a horizontal phase-change section at constant pressure, energy is still absorbed, but it is used to overcome intermolecular attractions and change particle arrangement; temperature and average kinetic energy stay constant until that change is complete. Cooling reverses the energy flow.

A 20 K temperature interval has the same size as a 20 °C interval, but 20 °C is not an absolute temperature of 20 K. Do not infer that heavier gas particles have greater average kinetic energy at the same temperature, and do not interpret a heating-curve plateau as a period when no energy is transferred.