vapor pressure

(noun)

The pressure that a vapor exerts, or the partial pressure if it is mixed with other gases.

Related Terms

  • humidity
  • equilibrium

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.
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