Evaporation is the process by which molecules in a liquid state (e. In Chemistry, a molecule is defined as a sufficiently stable electrically neutral group of at least two Atoms in a definite arrangement held together by Liquid is one of the principal States of matter. A liquid is a Fluid that has the particles loose and can freely form a distinct surface at the boundaries of g. water) spontaneously become gaseous (e. Water is a common Chemical substance that is essential for the survival of all known forms of Life. This page is about the physical properties of gas as a state of matter g. water vapor). General properties of water vapor Evaporation/sublimation Whenever a water molecule leaves a surface it is said to have evaporated It is the opposite of condensation. Condensation is the change of the physical state of aggregation (or simply state of matter from gaseous phase into liquid phase Generally, evaporation can be seen by the gradual disappearance of a liquid when exposed to a significant volume of gas.
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 for a molecule near the surface that it ends up with enough energy to escape.
Liquids that do not 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 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. 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 Solar energy drives evaporation of water from oceans, lakes, moisture in the soil, and other sources of water. Solar energy is the Light and radiant heat from the Sun that powers Earth 's Climate and Weather and sustains Life An ocean (from Greek, ''Okeanos'' (Oceanus) is a major body of saline water, and a principal component of the Hydrosphere. A lake (from Latin lacus) is a Terrain feature (or Physical feature) a body of Liquid on the surface of a world that is localized to the Water content or moisture content is the quantity of Water contained in a material such as Soil (called soil moisture) rock, In hydrology, evaporation and transpiration (which involves evaporation within plant stomata) are collectively termed evapotranspiration. Hydrology (from Greek Yδωρ hudōr, "water" and λόγος logos, "study" is the study of the movement distribution and quality of Transpiration is the Evaporation of water from the aerial parts of Plants especially leaves but also stems Flowers and Roots Plants are living Organisms belonging to the kingdom Plantae. In Botany, a stoma (also stomate; plural stomata) is a tiny opening or pore found mostly on the underside of a Plant Leaf Evapotranspiration (ET is a term used to describe the sum of Evaporation and Plant Transpiration from the earth's land surface to Atmosphere
Contents |
Theory
- See also: Kinetic theory
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. Kinetic theory (or kinetic theory of gases) attempts to explain Macroscopic properties of Gases such as pressure temperature or volume by considering In Chemistry, a molecule is defined as a sufficiently stable electrically neutral group of at least two Atoms in a definite arrangement held together by The kinetic energy of an object is the extra Energy which it possesses due to its motion [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. Vapor pressure (also known as equilibrium vapor pressure or saturation vapor pressure) is the Pressure of a Vapor in equilibrium 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
If evaporation takes place in a closed vessel, the escaping molecules accumulate as a vapor above the liquid. A vapor or vapour (see Spelling differences) is a substance in the Gas phase at a Temperature lower than its Critical temperature Many of the molecules return to the liquid, with returning molecules becoming more frequent as the density and pressure of the vapor increases. The density of a material is defined as its Mass per unit Volume: \rho = \frac{m}{V} Different materials usually have different Pressure (symbol 'p' is the force per unit Area applied to an object in a direction perpendicular to the surface 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. In Thermodynamics, a thermodynamic system is said to be in thermodynamic equilibrium when it is in thermal equilibrium Mechanical equilibrium, and Vapor pressure (also known as equilibrium vapor pressure or saturation vapor pressure) is the Pressure of a Vapor in equilibrium 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, ΔHvap is the enthalpy of vaporization, and R is the universal gas constant. Vapor pressure (also known as equilibrium vapor pressure or saturation vapor pressure) is the Pressure of a Vapor in equilibrium The Clausius-Clapeyron relation, named after Rudolf Clausius and Émile Clapeyron, is a way of characterizing the Phase transition between two phases of The enthalpy of vaporization, (symbol \Delta{}_{v}H also known as the heat of vaporization or heat of evaporation, is the Energy required Relationship with the Boltzmann constant The Boltzmann constant kB (often abbreviated k) may be used in place of the gas constant by working 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. Vapor pressure (also known as equilibrium vapor pressure or saturation vapor pressure) is the Pressure of a Vapor in equilibrium Boiling (also called ebullition) a type of Phase transition, is the rapid vaporization of a Liquid, which typically occurs when a liquid
The ability for a molecule of a liquid to evaporate is largely based on the amount of kinetic energy an individual particle may possess. The kinetic energy of an object is the extra Energy which it possesses due to its motion 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.
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. In Chemistry, concentration is the measure of how much of a given substance there is mixed with another substance
- 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. In Physics and Fluid mechanics, a boundary layer is that layer of Fluid in the immediate vicinity of a bounding surface
- Concentration of other substances in the liquid (impurities): 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 one must get to escape.
- Surface area: A substance which has a larger surface area will evaporate faster as there are more surface molecules which are able to escape. Surface area is the measure of how much exposed Area an object has
In the US, the National Weather Service measures the actual rate of evaporation from a standardized "pan" open water surface outdoors, at various locations nationwide. Others do likewise around the world. The US data is collected and compiled into an annual evaporation map. [1] The measurements range from under 30 to over 120 inches per year. Formulas for calculating the rate of evaporation from a water surface such as a swimming pool of can be found here[2] and here[3]
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 a clothes dryer hot air is blown through the clothes, allowing water to evaporate very rapidly. A clothes dryer or tumble dryer is a household appliance that is used to remove the moisture from a load of Clothing and other Textiles, generally
Combustion vaporization
Fuel droplets vaporize as they receive heat by mixing with the hot gases in the combustion chamber. A drop or droplet is a small volume of Liquid, bounded completely or almost completely by Free surfaces Surface tension 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. Evaporation is a common method of Thin film deposition. The source material is evaporated in a Vacuum. Thin films are thin material layers ranging from fractions of a Nanometre to several Micrometres in thickness Thin-film deposition is any technique for depositing a Thin film of material onto a substrate or onto previously deposited layers
See also
- Atmometer (evaporimeter)
- Crystallisation
- Desalination
- Distillation
- Drying
- Evaporator
- Evapotranspiration
- Flash evaporation
- Heat of vaporization
- Latent heat
- Pan evaporation
References
- Sze, Simon Min. The atmometer or evaporimeter is a scientific instrument used for measuring the rate of Evaporation from a wet surface to the Atmosphere and Crystallization is the (natural or artificial process of formation of solid Crystals precipitating from a homogeneous --> identical Solution Desalination, desalinization, or desalinisation refers to any of several processes that remove excess salt and other Minerals from Water Distillation is a method of separating Mixtures based on differences in their volatilities in a boiling liquid mixture Drying is a Mass transfer process resulting in the removal of water Moisture or moisture from another Solvent, by Evaporation from a Solid Within a Downstream processing system several stages are used to further isolate and purify the desired product Evapotranspiration (ET is a term used to describe the sum of Evaporation and Plant Transpiration from the earth's land surface to Atmosphere Flash (or partial Evaporation is the partial Vaporization that occurs when a saturated liquid stream undergoes a reduction in pressure by passing through The enthalpy of vaporization, (symbol \Delta{}_{v}H also known as the heat of vaporization or heat of evaporation, is the Energy required In Thermochemistry, latent heat is the amount of Energy in the form of Heat released or absorbed by a substance during a change of phase Pan evaporation is a measurement that combines or integrates the effects of several climate elements temperature humidity solar radiation and wind Semiconductor Devices: Physics and Technology. ISBN 0-471-33372-7. Has an especially detailed discussion of film deposition by evaporation.
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
Dapyx Software network: MP3 Explorer | Ebook Manager | Zenithic