Evaporation

Examples of Evaporation in the following topics:

• Evaporation

  • This phenomenon is also called evaporative cooling.
  • This is why evaporating sweat cools the human body.
  • Evaporation is an essential part of the water cycle.
  • In hydrology, evaporation and transpiration (which involves evaporation within plant stomata) are collectively termed evapotranspiration.
  • (b) If the container is sealed, evaporation will continue until there is enough vapor density for the condensation rate to equal the evaporation rate.

• Humidity, Evaporation, and Boiling

  • The amount of water vapor in air is a result of evaporation or boiling, until an equilibrium is reached.
  • If a lid is placed over the container, as in (b), evaporation continues, increasing the pressure, until sufficient vapor has built up for condensation to balance evaporation.
  • This phenomenon is also called evaporative cooling.
  • This is why evaporating sweat cools the human body.
  • Thus, it can evaporate without limit at this temperature and pressure.

• Water’s Heat of Vaporization

  • Evaporation of water requires a substantial amount of energy due to the high heat of vaporization of water.
  • The fact that hydrogen bonds need to be broken for water to evaporate means that a substantial amount of energy is used in the process.
  • As the water evaporates, energy is taken up by the process, cooling the environment where the evaporation is taking place.
  • In many living organisms, including humans, the evaporation of sweat, which is 90 percent water, allows the organism to cool so that homeostasis of body temperature can be maintained.
  • (b) If the container is sealed, evaporation will continue until there is enough vapor density for the condensation rate to equal the evaporation rate.

• The Evaporating Atmosphere

  • At equilibrium, evaporation and condensation processes exactly balance and there is no net change in the volume of either phase.
  • At equilibrium, evaporation and condensation processes exactly balance and there is no net change in the volume of either phase.

• The Water (Hydrologic) Cycle

  • For example, when water evaporates, it takes up energy from its surroundings, cooling the environment.
  • The evaporation phase of the cycle purifies water, which then replenishes the land with fresh water.
  • This leads to the evaporation (water to water vapor) of liquid surface water and the sublimation (ice to water vapor) of frozen water, which deposits large amounts of water vapor into the atmosphere.
  • Rain eventually percolates into the ground, where it may evaporate again (if it is near the surface), flow beneath the surface, or be stored for long periods.
  • Water from the land and oceans enters the atmosphere by evaporation or sublimation, where it condenses into clouds and falls as rain or snow.

• Liquid to Gas Phase Transition

  • There are two types of vaporization: evaporation and boiling.
  • Evaporation occurs at temperatures below the boiling point, and occurs on the liquid's surface.
  • If the vapor pressure of the drop is greater than the partial pressure of vapor in the gas phase, the drop will evaporate.
  • Non-polar molecules (gray) evaporate or boil more quickly than polar molecules (blue and red).
  • This video looks at how vaporization and evaporation happens, and it addresses a big mistake that many people make when they deal with the liquid gas phase transition.

• Heat Pumps and Refrigerators

  • The basic components of a heat pump in are a condenser, an expansion valve, an evaporator and a compressor .
  • In the outdoor coils (the evaporator), heat transfer Qc occurs to the working fluid from the cold outdoor air, turning it into a gas.
  • The liquid then flows back through a pressure-reducing valve to the outdoor evaporator coils, being cooled through expansion.
  • (In a cooling cycle, the evaporator and condenser coils exchange roles and the flow direction of the fluid is reversed. )
  • A simple heat pump has four basic components: (1) condenser, (2) expansion valve, (3) evaporator, and (4) compressor.

• Vapor Pressure of Nonelectrolyte Solutions

  • A substance that evaporates quickly has high vapor pressure and is referred to as a volatile substance.
  • By definition, a non-volatile substance does not evaporate.
  • In a pure solvent, all of the liquid molecules on the surface can evaporate and transition to the gaseous phase.

• Solid to Gas Phase Transition

  • For some substances, such as carbon and arsenic, sublimation is much easier than evaporation.
  • Even ice has a measurable vapor pressure near its freezing point, as evidenced by the tendency of snow to evaporate in cold dry weather.

• Portals of Exit

  • When viruses are shed by an infected person through coughing or sneezing into the air, the mucus coating on the virus starts to evaporate.
  • Once this mucus shell evaporates the remaining viron is called a droplet nucleus or quanta.
  • The mucus evaporation rate is determined by the temperature and humidity inside the room.
  • The lower the humidity, the quicker the mucus shell evaporates thus allowing the droplet nuclei to stay airborne and not drop to the ground.