“Vaporization” redirects here. For the vaporization of a solid, see sublimation (chemistry).
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Hot tea condenses into visible droplets. Gaseous water is invisible, but the clouds of water droplets are evidence of evaporation.
Evaporation is the process by which molecules in a liquid state (e.g. water) spontaneously become gaseous (e.g. water vapor), without being heated to boiling point. It is the opposite of condensation. Generally, evaporation can be seen by the gradual disappearance of a liquid, when exposed to a significant volume of gas.
The reason a liquid evaporates is that its molecules are all in motion in nearly random directions and speeds, and the energy of that movement can be compared to the heat needed to boil that liquid. On average, the molecules do not have enough energy to escape from the liquid, or else the liquid would turn into vapor quickly. When the molecules collide, they transfer energy to each other in varying degrees, based on how they collide. Sometimes the transfer is so one-sided that one of the molecules ends up with enough energy to be considered past the boiling point of the liquid. If this happens near the surface of the liquid it may actually fly off into the gas and thus "evaporate".
Liquids that do not appear to evaporate visibly at a given temperature in a given gas (e.g. cooking oil at room temperature) have molecules that do not tend to transfer energy to each other in a pattern sufficient to frequently give a molecule the "escape velocity" - the heat energy - necessary to turn into vapor. However, these liquids are evaporating, it's just that the process is much slower and thus significantly less visible.
Evaporation is an essential part of the water cycle. Solar energy drives evaporation of water from oceans, lakes, moisture in the soil, and other sources of water. In hydrology, evaporation and transpiration (which involves evaporation within plant stomata) are collectively termed evapotranspiration.
Theory
- See also:
For molecules of a liquid to evaporate, they must be located near the surface, be moving in the proper direction, and have sufficient kinetic energy to overcome liquid-phase intermolecular forces.[1] Only a small proportion of the molecules meet these criteria, so the rate of evaporation is limited. Since the kinetic energy of a molecule is proportional to its temperature, evaporation proceeds more quickly at higher temperature. As the faster-moving molecules escape, the remaining molecules have lower average kinetic energy, and the temperature of the liquid thus decreases. This phenomenon is also called evaporative cooling. This is why evaporating sweat cools the human body. Evaporation also tends to proceed more quickly with higher flow rates between the gaseous and liquid phase and in liquids with higher vapor pressure. For example, laundry on a clothes line will dry (by evaporation) more rapidly on a windy day than on a still day.Three key parts to evaporation are heat, humidity and air movement.
Evaporative equilibrium
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If the evaporation takes place in a closed vessel, the escaping molecules accumulate as a vapor above the liquid. Many of the molecules return to the liquid, with returning molecules becoming more frequent as the density and pressure of the vapor increases. When the process of escape and return reaches an equilibrium,[1] the vapor is said to be "saturated," and no further change in either vapor pressure and density or liquid temperature will occur. For a system consisting of vapor and liquid of a pure substance, this equilibrium state is directly related to the vapor pressure of the substance, as given by the Clausius-Clapeyron relation:
where P1, P2 are the vapor pressures at temperatures T1, T2 respectively, and ΔHvap is the enthalpy of vaporization. The rate of evaporation in an open system is related to the vapor pressure found in a closed system. If a liquid is heated, when the vapor pressure reaches the ambient pressure the liquid will boil.
The ability for a molecule of a liquid to evaporate is largely based on the amount of kinetic energy an individual particle may possess. Even at lower temperatures, individual molecules of a liquid can potentially evaporate if they have more than the minimum amount of kinetic energy required for vaporization. Evaporation is when a liquid is changed to a gas.
Factors influencing the rate of evaporation
- Concentration of the substance evaporating in the air: If the air already has a high concentration of the substance evaporating, then the given substance will evaporate more slowly.
- Concentration of other substances in the air: If the air is already saturated with other substances, it can have a lower capacity for the substance evaporating.
- Flow rate of air: This is in part related to the concentration points above. If fresh air is moving over the substance all the time, then the concentration of the substance in the air is less likely to go up with time, thus encouraging faster evaporation. This is result of the boundary layer at the evaporation surface decreasing with flow velocity, decreasing the diffusion distance in the stagnant layer.
- Concentration of other substances in the liquid: If the liquid contains other substances, it will have a lower capacity for evaporation.
- Temperature of the substance: If the substance is hotter, then evaporation will be faster.
- Inter-molecular forces: The stronger the forces keeping the molecules together in the liquid state the more energy that must be input in order to evaporate them.
- Surface Area: A substance which has a larger surface area will evaporate faster as there are more surface molecules which are able to escape.
Applications
When clothes are hung on a laundry line, even though the ambient temperature is below the boiling point of water, water evaporates. This is accelerated by factors such as low humidity, heat (from the sun), and wind. In an electric clothes dryer hot air is blown through clothes, allowing water to evaporate very rapidly.Combustion vaporization
If fuel droplets vaporize as they receive heat by mixing with the hot gases in the combustion chamber. Heat (energy) can also be received by radiation from any hot refractory wall of the combustion chamber.Film deposition
Thin films may be deposited by evaporating a substance and condensing it onto a substrate.
See also
- Crystallisation
- Desalination
- Distillation
- Drying
- Evaporation pan
- Evaporator
- Evapotranspiration
- Flash evaporation
- Heat of vaporization
- Latent heat
References
- Sze, Simon Min. Semiconductor Devices: Physics and Technology. ISBN 0-471-33372-7. Has an especially detailed discussion of film deposition by evaporation.
1. ^ Silberberg, Martin A. (2006). Chemistry, 4th edition, New York: McGraw-Hill, 431–434. ISBN 0-07-296439-1.
External links
From To Solid Liquid Gas Plasma Solid Solid-Solid Transformation Melting Sublimation - Liquid Freezing N/A Boiling/Evaporation - Gas Deposition Condensation N/A Ionization Plasma - - Recombination/Deionization N/A Sublimation of an element or compound is a transition from the solid to gas phase with no intermediate liquid stage. Sublimation is a phase transition that occurs at temperatures and pressures below the triple point (see phase diagram).
..... Click the link for more information.boiling point of a liquid is the temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid.[1][2][3][4]
..... Click the link for more information.Condensation is the change in matter of a substance to a denser phase, such as a gas (or vapor) to a liquid.[1] Condensation commonly occurs when a vapor is cooled to a liquid, but can also occur if a vapor is compressed (i.e.
..... Click the link for more information.water cycle.]]
The Earth's water is always in movement, and the water cycle, also known as the hydrologic cycle, describes the continuous movement of water on, above, and below the surface of the Earth.
..... Click the link for more information.Solar energy is energy from the sun. It supports life on Earth and drives the Earth's weather. Solar energy predominantly arrives in the form of infrared, visible and ultraviolet light, and is either returned back to space or is absorbed.
..... Click the link for more information.Earth's oceans
(World Ocean)- Arctic Ocean
- Atlantic Ocean
- Indian Ocean
- Pacific Ocean
- Southern Ocean
..... Click the link for more information.lake (from Latin ligacus) is a body of water or other liquid of considerable size contained on a body of land. A vast majority of lakes on Earth are fresh water, and most lie in the Northern Hemisphere at higher latitudes.
..... Click the link for more information.Water content or moisture content is the quantity of water contained in a material, such as soil (called soil moisture), rock, ceramics, or wood on a volumetric or gravimetric basis.
..... Click the link for more information.Hydrology (from Greek: Yδωρ, hudōr, "water"; and λόγος, logos, "knowledge") is the study of the movement, distribution, and quality of water throughout the Earth, and thus addresses both the hydrologic cycle and water resources.
..... Click the link for more information.Transpiration is the evaporation of water from the aerial parts of plants, especially leaves but also stems, flowers and roots. Leaf transpiration occurs through stomata, and can be thought of as a necessary "cost" associated with the opening of stomata to allow the diffusion of
..... Click the link for more information.Plantae
Haeckel, 1866[1]
Divisions
Green algae- Chlorophyta
- Charophyta
- Non-vascular land plants (bryophytes)
..... Click the link for more information.stomata) is a tiny opening or pore, found mostly on the underside of a plant leaf, and used for gas exchange. The pore is formed by a pair of specialized sclerenchyma cells known as guard cells which are responsible for regulating the size of the opening.
..... Click the link for more information.Evapotranspiration (ET) is a term used to describe the sum of evaporation and plant transpiration from the earth's land surface to atmosphere. Evaporation accounts for the movement of water to the air from sources such as the soil, canopy interception, and waterbodies.
..... Click the link for more information.molecule is defined as a sufficiently stable electrically neutral group of at least two atoms in a definite arrangement held together by strong chemical bonds.[1][2] In organic chemistry and biochemistry, the term molecule
..... Click the link for more information.kinetic energy of an object is the extra energy which it possesses due to its motion. It is defined as the work needed to accelerate a body of a given mass from rest to its current velocity.
..... Click the link for more information.SWEAT is an OLN/TSN show hosted by Julie Zwillich that aired in 2003-2004.
Each of the 13 half-hour episodes of SWEAT features a different outdoor sport: kayaking, mountain biking, ice hockey, beach volleyball, soccer, windsurfing, rowing, Ultimate, triathlon,
..... Click the link for more information.Vapor or vapour (see spelling differences) is the gas phase component of another state of matter (e.g. liquid or solid) which does not completely fill its container. It is distinguished from the pure gas phase by the presence of the same substance in another state of matter.
..... Click the link for more information.In physics, density is mass m per unit volume V—how heavy something is compared to its size. A small, heavy object, such as a rock or a lump of lead, is denser than a lighter object of the same size or a larger object of the same weight, such as pieces of
..... Click the link for more information.Pressure (symbol: p) is the force per unit area applied on a surface in a direction perpendicular to that surface.
Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.
..... Click the link for more information.thermodynamic equilibrium when it is in thermal equilibrium, mechanical equilibrium, and chemical equilibrium. The local state of a system at thermodynamic equilibrium is determined by the values of its intensive parameters, as pressure, temperature, etc.
..... Click the link for more information.Vapor pressure, also known as vapour pressure, is the pressure of a vapor in equilibrium with its non-vapor phases. All liquids and solids have a tendency to evaporate to a gaseous form, and all gases have a tendency to condense back into their orignal form (either liquid
..... Click the link for more information.The Clausius-Clapeyron relation, named after Rudolf Clausius and Émile Clapeyron, is a way of characterizing the phase transition between two states of matter, such as solid and liquid.
..... Click the link for more information.The enthalpy of vaporization, (symbol ), also known as the heat of vaporization or heat of evaporation, is the energy required to transform a given quantity of a substance into a gas.
..... Click the link for more information.Boiling, a type of phase transition, is the rapid vaporization of a liquid, which typically occurs when a liquid is heated to its boiling point, the temperature at which the vapor pressure of the liquid is equal to the pressure exerted on the liquid by the surrounding environmental
..... Click the link for more information.kinetic energy of an object is the extra energy which it possesses due to its motion. It is defined as the work needed to accelerate a body of a given mass from rest to its current velocity.
..... Click the link for more information.In chemistry, concentration is the measure of how much of a given substance there is mixed with another substance. This can apply to any sort of chemical mixture, but most frequently the concept is limited to homogeneous solutions, where it refers to the amount of
..... Click the link for more information.In physics and fluid mechanics, a boundary layer is that layer of fluid in the immediate vicinity of a bounding surface. In the Earth's atmosphere the planetary boundary layer is the air layer near the ground affected by diurnal heat, moisture or momentum transfer to or from the
..... Click the link for more information.Area is the measure of how much exposed area any two dimensional object has. It is expressed in square units, and is calculated by adding together the areas of all the faces of the object.Area formulas
Note: For 2D figures, the surface area and the area are the same.
..... Click the link for more information.A clothes dryer or tumble dryer is a household appliance that is used to remove the residual moisture from a load of clothing and other textiles, generally shortly after they are cleaned in a washing machine.
..... Click the link for more information.drop or droplet is a small volume of liquid, bounded completely or almost completely by free surfaces.Surface tension
The simplest way to form a drop is to allow liquid to flow slowly from the lower end of a vertical tube of small diameter.
..... Click the link for more information.