Characteristic properties of Gases

The gaseous state has several characteristic physical properties that set it apart from solids and liquids: gases are far less dense (lighter) than solids or liquids of the same substance; they have no fixed volume or shape of their own and simply expand to fill whatever container holds them; their molecules are widely separated and in continuous, random motion, so gas molecules constantly collide with the container walls, and this is what produces gas pressure, exerted equally in every direction; intermolecular forces in a gas are the weakest of the three states; gases diffuse readily, i.e. two or more gases spontaneously and uniformly intermix; and gases are highly compressible because most of the volume they occupy is actually empty space between molecules. Only eleven of the periodic table's 118 known elements exist as gases under normal conditions. A gas sample is fully described by four measurable properties. Mass ($m$) is measured in kilograms (SI) and relates to the number of moles by $n = m/M$, or via the Avogadro number ($n = N/N_A$, $N_A = 6.022\times10^{23}$ mol$^{-1}$), or via $n = V/22.414$ L at STP. Volume ($V$), the space the gas occupies, is measured in litres, millilitres, cm$^3$, dm$^3$ or m$^3$ (SI unit m$^3$); $1$ L $= 1000$ mL $= 1000$ cm$^3 = 1$ dm$^3$. Pressure ($P$) is force per unit area, $P = F/A$; its SI unit is the pascal (Pa $=$ N m$^{-2}$), and $1$ bar $= 1.00\times10^{5}$ Pa, while $1$ atm $= 76$ cm Hg $= 760$ mm Hg $= 760$ torr $= 101.325$ kPa; gas pressure is measured with a manometer, atmospheric pressure with a barometer. Temperature ($T$), measured on the Celsius or Kelvin scale ($T$(K) $= t$°C $+ 273.15$; SI unit kelvin), determines …