vapor pressure

Examples of vapor pressure in the following topics:

• Humidity, Evaporation, and Boiling

  • The capacity of air to hold water vapor is based on vapor pressure of water.
  • Then equilibrium has been achieved, and the vapor pressure is equal to the partial pressure of water in the container.
  • The vapor pressure of water at 100ºC is 1.01×105 Pa, or 1.00 atm.
  • (b) As the temperature rises, water vapor enters the bubble because its vapor pressure increases.
  • (c) At 100ºC, water vapor enters the bubble continuously because water's vapor pressure exceeds its partial pressure in the bubble, which must be less than 1.00 atm.

• Evaporation

  • With sufficient heat, however, the liquid would quickly turn into vapor.
  • Three key parts to evaporation are heat, atmospheric pressure (determines the percent humidity) and air movement .
  • Evaporation also tends to proceed more quickly with higher flow rates between the gaseous and liquid phases and in liquids with higher vapor pressure.
  • This vapor density and the partial pressure it creates are the saturation values.
  • They depend only on the vapor pressure of water.

• Latent Heat

  • Similarly, energy is needed to vaporize a liquid, because molecules in a liquid interact with each other via attractive forces.
  • Even more energy is required to vaporize water; it would take 2256 kJ to change 1 kg of liquid water at the normal boiling point (100ºC at atmospheric pressure) to steam (water vapor).
  • A phase change we have neglected to mention thus far is sublimation, the transition of solid directly into vapor.
  • The opposite case, where vapor transitions directly into a solid, is called deposition.
  • The system is constructed so that no vapor evaporates while ice warms to become liquid water, and so that, when vaporization occurs, the vapor remains in of the system.

• The Evaporating Atmosphere

  • Likewise, every once in a while a vapor molecule collides with the liquid surface and condenses into the liquid.
  • At room temperature and pressure, the water jar reaches equilibrium when the air over the water has a humidity of about 3%.
  • At 100 °C and atmospheric pressure, equilibrium is not reached until the air is 100% water.
  • The water vapor in it changes phases.
  • Collisions between water molecules in the atmosphere allows some to condense and some to remain in vapor.

• Speed of Sound

  • The general value given for the speed of sound is the speed of a sound wave in air, at sea level, at normal atmospheric pressure; that number is 344 m/s.
  • The vapor cone is made just before it reaches the speed of sound and is caused by a sudden drop in air pressure.

• Phase Changes and Energy Conservation

  • These amounts of energy are the molar heat of vaporization and molar heat of fusion.
  • Using the graph, if you know the pressure and temperature you can determine the phase of water.
  • The solid lines—boundaries between phases—indicate temperatures and pressures at which the phases coexist (that is, they exist together in ratios, depending on pressure and temperature).
  • As the pressure increases, the boiling temperature rises steadily to 374º C at a pressure of 218 atm.
  • Note that water changes states based on the pressure and temperature.

• Momentum Transfer and Radiation Pressure Atom

  • Radiation pressure is the pressure exerted upon any surface exposed to electromagnetic (EM) radiation.
  • Radiation pressure is the pressure exerted upon any surface exposed to electromagnetic (EM) radiation.
  • Perhaps one of the most well know examples of the radiation pressure would be comet tails.
  • Newton's Second Law tells us that force equals rate of change of momentum; thus during each second, the surface experiences a force (or pressure, as pressure is force per unit area) due to the momentum the photons transfer to it.
  • As a comet approaches the inner Solar System, solar radiation causes the volatile materials within the comet to vaporize and stream out of the nucleus.

• Convection

  • The specific heat of air is a weighted average of the specific heats of nitrogen and oxygen, which is c=cp≅1000 J/kg⋅C (note that the specific heat at constant pressure must be used for this process).
  • If the water vapor condenses in liquid droplets as clouds form, heat is released in the atmosphere (this heat release is latent heat)  .
  • Water vapor carried in by convection condenses, releasing tremendous amounts of energy, and this energy allows air to become more buoyant (warmer than its surroundings) and rise.
  • Cumulus clouds are caused by water vapor that rises because of convection.

• Gauge Pressure and Atmospheric Pressure

  • Pressure is often measured as gauge pressure, which is defined as the absolute pressure minus the atmospheric pressure.
  • Gauge pressure is a relative pressure measurement which measures pressure relative to atmospheric pressure and is defined as the absolute pressure minus the atmospheric pressure.
  • Most pressure measuring equipment give the pressure of a system in terms of gauge pressure as opposed to absolute pressure.
  • For example, tire pressure and blood pressure are gauge pressures by convention, while atmospheric pressures, deep vacuum pressures, and altimeter pressures must be absolute.
  • Explain the relationship among absolute pressure, gauge pressure, and atmospheric pressure

• Measurements: Gauge Pressure and the Barometer

  • In practice, pressure is most often measured in terms of gauge pressure.
  • Gauge pressure is the pressure of a system above atmospheric pressure.
  • Gauge pressure is much more convenient than absolute pressure for practical measurements and is widely used as an established measure of pressure.
  • Barometers are devices used to measure pressure and were initially used to measure atmospheric pressure.
  • Many modern pressure measuring devices are pre-engineered to output gauge pressure measurements.