### Key Terms

- Avogadro’s number
- ${N}_{\text{A}},$ the number of molecules in one mole of a substance; ${N}_{\text{A}}=6.02\phantom{\rule{0.2em}{0ex}}\times \phantom{\rule{0.2em}{0ex}}{10}^{23}$ particles/mole

- Boltzmann constant
- ${k}_{\text{B}},$ a physical constant that relates energy to temperature and appears in the ideal gas law; ${k}_{\text{B}}=1.38\phantom{\rule{0.2em}{0ex}}\times \phantom{\rule{0.2em}{0ex}}{10}^{\mathrm{-23}}\phantom{\rule{0.2em}{0ex}}\text{J/K}$

- critical temperature
- ${T}_{\text{c}}$ at which the isotherm has a point with zero slope

- Dalton’s law of partial pressures
- physical law that states that the total pressure of a gas is the sum of partial pressures of the component gases

- degree of freedom
- independent kind of motion possessing energy, such as the kinetic energy of motion in one of the three orthogonal spatial directions

- equipartition theorem
- theorem that the energy of a classical thermodynamic system is shared equally among its degrees of freedom

- ideal gas
- gas at the limit of low density and high temperature

- ideal gas law
- physical law that relates the pressure and volume of a gas, far from liquefaction, to the number of gas molecules or number of moles of gas and the temperature of the gas

- internal energy
- sum of the mechanical energies of all of the molecules in it

- kinetic theory of gases
- theory that derives the macroscopic properties of gases from the motion of the molecules they consist of

- Maxwell-Boltzmann distribution
- function that can be integrated to give the probability of finding ideal gas molecules with speeds in the range between the limits of integration

- mean free path
- average distance between collisions of a particle

- mean free time
- average time between collisions of a particle

- mole
- quantity of a substance whose mass (in grams) is equal to its molecular mass

- most probable speed
- speed near which the speeds of most molecules are found, the peak of the speed distribution function

- partial pressure
- pressure a gas would create if it occupied the total volume of space available

- peak speed
- same as “most probable speed”

*pV*diagram- graph of pressure vs. volume

- root-mean-square (rms) speed
- square root of the average of the square (of a quantity)

- supercritical
- condition of a fluid being at such a high temperature and pressure that the liquid phase cannot exist

- universal gas constant
*R*, the constant that appears in the ideal gas law expressed in terms of moles, given by $R={N}_{A}{k}_{B}$

- van der Waals equation of state
- equation, typically approximate, which relates the pressure and volume of a gas to the number of gas molecules or number of moles of gas and the temperature of the gas

- vapor pressure
- partial pressure of a vapor at which it is in equilibrium with the liquid (or solid, in the case of sublimation) phase of the same substance