Gases, Vapours, Liquids

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Gases, Vapours, Liquids

• PV NRT Ideal gas law
• P pressure (absolute)
• V volume
• N number of mols
• R universal gas constant
• T temperature (absolute)
• Most gases behave this way at low pressures
  (e.g. atmospheric) and high temperature (e.g.
  much higher than the boiling temperature of the
  substance)

• V V/N specific volume (volume per
  mole)
• PV RT
• 1 mol of any gas occupies 22.4 L under
  standard conditions 1atm, 0 C (273 K)

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Pressure (Force/area)

• Force rate of change of momentum
• Gas molecules hit a surface and bounce back with
  the same speed but opposite direction change in
  momentum
• Force/area is the number of gas molecules
  hitting the area per unit time
• Consider the force (hence pressure) as the sum of
  the effects of different types of molecules
• P pa pb pc .
• pa partial pressure due to molecules of type
  a
• e.g. Patm pN2 pO2
• 1 atm 0.79 atm 0.21 atm

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Partial volumes

• Consider the types of molecules in a gas to be
  segregated, but with no physical barrier between
  them, i.e. they are all at the same pressure P
• V total va vb vc .
• va partial volume due to gas molecules of
  type a
• e.g. 100 L air 79 L N2 21 L O2
• Physically not realistic.
• Conceptually similar to partial pressure

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P- v -T behaviour of gases(on P vs v coordinates)

• As temperature decreases, P- v -T behaviour
  deviates from (P)(v) constant
• At sufficiently low temperature it is possible to
  condense the gas in the liquid form
• Define Vapour condensible gas (i.e. a
  substance in the gas phase, that can be condensed
  by cooling at constant pressure) e.g. H2O

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States of matter

• 1 Subcooled (compressed) liquid
• 2 Saturated liquid
• 3 Saturated vapour
• 4 Vapour-liquid mixture
• 5 Superheated vapour (gas?)

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2 phase (vapour and liquid) region

• Temperature constant for a given pressure (vice
  versa)
• A, B, C constants specific to each substance
• p saturation pressure (vapour pressure) , for
  a given temperature

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Vapour liquid mixture

• Let x fraction vapour

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Antoine expression for vapour pressure on P-T
coordinates

• Saturation
• temperature
• Condensation
• temperature
• Boiling point
• (temperature)

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Nomenclature

• Water H2O liquid
• Steam pure H2O vapour
• Water vapour H2O vapour mixed with other gas
  (air)