Summary:
p,v,T are state variables fully describing the actual state of
pure matter. The state equation f(p,v,T)=0 has for ideal gases (no repulsive or
attractive forces between molecules, negligible volume of molecules) the simple form
pV=nRT,
p[Pa], V[m3], n [mole], R=8.314 [J.mole -1 .K -1], T [K]. For real gases van der Waals or Redlich-Kwong's equations, and for liquids Tait's equation, can be used.
Total pressure p in a mixture of gases is the sum of partial pressures p i of
individual components (Dalton's law), and this conclusion can also be used for real
gases with acceptable accuracy (partial pressures can be computed from a state
equation, assuming that V is the whole volume of mixture, and ni is the amount of the
component). A similar rule holds for partial volumes (Amagat's law of additivity of
partial volumes) and for real gases the additivity of volumes used to be an even
better approximation than Dalton's law of the additivity of pressures.
Each gas is characterised by a critical point (p c, T c
, v c). The critical
temperature is the highest temperature allowing liquefaction. For T>T c only a
gaseous phase exists, no matter how large the pressure is. Tc for water is 374 0C, for
carbon dioxide 31 0C (therefore it is possible to liquefy CO2 at room temperature by
compression), for air -141 0 C, and for helium -268 0 C.
·
@TEC: 3. 3.2003
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