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From Wikipedia

Evaporation

Evaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs on the entire mass of the liquid. Evaporation is also part of the water cycle.

Evaporation is a type of phase transition; it is the process by which molecules in a liquidstate (e.g., water) spontaneously become gaseous (e.g., water vapor). In general, evaporation can be seen by the gradual disappearance of a liquid from a substance when exposed to a significant volume of gas. Vaporization and evaporation however, are not entirely the same processes.

On average, the molecules in a glass of water do not have enough heat energy to escape from the liquid. With sufficient heat, the liquid would turn into vapor quickly (see boiling point). 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 is 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 plantstomata) are collectively termed evapotranspiration. Evaporation is caused when water is exposed to air and the liquid molecules turn into water vapor, which rises up and forms clouds.

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. 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 temperatures. 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.

On a molecular level, there is no strict boundary between the liquid state and the vapor state. Instead, there is a Knudsen layer, where the phase is undetermined. Because this layer is only a few molecules thick, at a macroscopic scale a clear phase transition interface can be seen.

Evaporative equilibrium

If 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, 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:

\ln \left( \frac{ P_2 }{ P_1 } \right) = - \frac{ \Delta H_{ vap } }{ R } \left( \frac{ 1 }{ T_2 } - \frac{ 1 }{ T_1 } \right)

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. 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 based largely on the amount of kinetic energy an individual particle may possess. Even at lower temperatures, individual molecules of a liquid can 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.
Concentration of other s

Vapor

A vapor (American spelling) or vapour (see spelling differences) is a substance in the gas phase at a temperature lower than its critical point. This means that the vapor can be condensed to a liquid or to a solid by increasing its pressure without reducing the temperature.

For example, water has a critical temperature of 374°C (or 647 K), which is the highest temperature at which liquid water can exist. In the atmosphere at ordinary temperatures, therefore, gaseous water is known as water vapor and will condense to liquid if its partial pressure is increased sufficiently.

A vapor may co-exist with a liquid (or solid). When this is true, the two phases will be in equilibrium, and the gas pressure will equal the equilibrium vapor pressure of the liquid (or solid).

Properties

Vapor refers to a gas phase at a temperature where the same substance can also exist in the liquid or solid state, below the critical temperature of the substance. If the vapor is in contact with a liquid or solid phase, the two phases will be in a state of equilibrium. The term gas refers to a compressible fluid phase. Fixed gases are gases for which no liquid or solid can form at the temperature of the gas (such as air at typical ambient temperatures). A liquid or solid does not have to boil to release a vapor.

Vapor is responsible for the familiar processes of cloud formation and condensation. It is commonly employed to carry out the physical processes of distillation and headspace extraction from a liquid sample prior to gas chromatography.

The constituent molecules of a vapor possess vibrational, rotational, and translational motion. These motions are considered in the kinetic theory of gases.

Vapor pressure

The vapor pressure is the equilibrium pressure from a liquid or a solid at a specific temperature. The equilibrium vapor pressure of a liquid or solid is not affected by the amount of contact with the liquid or solid interface.

The normal boiling point of a liquid is the temperature at which the vapor pressure is equal to one atmosphere (unit).

For two-phase systems (e.g., two liquid phases), the vapor pressure of the system is the sum of the vapor pressures of the two liquids. In the absence of stronger inter-species attractions between like-like or like-unlike molecules, the vapor pressure follows Raoult's Law, which states that the partial pressure of each component is the product of the vapor pressure of the pure component and its mole fraction in the mixture. The total vapor pressure is the sum of the component partial pressures.

The physical chemistry behind distillation is based on manipulating the equilibrium occurring between the liquid and vapor phases of a molecule in solution.

Examples

Measuring vapor

Since it is in the gas phase, the amount of vapor present is quantified by the partial pressure of the gas. Also, vapors obey the barometric formula in a gravitational field just as conventional atmospheric gases do.

Vapors of flammable liquids

Flammable liquids do not burn when ignited. It is the vapor cloud above the liquid that will burn if the vapor's concentration is between the Lower Flammable Limit (LFL) and Upper Flammable Limit (UFL) of the flammable liquid.


Vapor-liquid equilibrium

Vapor-liquid equilibrium (sometimes abbreviated as VLE) is a condition where a liquid and its vapor (gas phase) are in equilibrium with each other, a condition or state where the rate of evaporation (liquid changing to vapor) equals the rate of condensation (vapor changing to liquid) on a molecular level such that there is no net (overall) vapor-liquid interconversion. Although in theory equilibrium takes forever to reach, such an equilibrium is practically reached in a relatively closed location if a liquid and its vapor are allowed to stand in contact with each other long enough with no interference or only gradual interference from the outside.

VLE data introduction

The concentration of a vapor in contact with its liquid, especially at equilibrium, is often in terms of vapor pressure, which could be a partial pressure (part of the total gas pressure) if any other gas(es) are present with the vapor. The equilibrium vapor pressure of a liquid is usually very dependent on temperature. At vapor-liquid equilibrium, a liquid with individual components (compounds) in certain concentrations will have an equilibrium vapor in which the concentrations or partial pressures of the vapor components will have certain set values depending on all of the liquid component concentrations and the temperature. This fact is true in reverse also; if a vapor with components at certain concentrations or partial pressures is in vapor-liquid equilibrium with its liquid, then the component concentrations in the liquid will be set dependent on the vapor concentrations, again also depending on the temperature. The equilibrium concentration of each component in the liquid phase is often different from its concentration (or vapor pressure) in the vapor phase, but there is a correlation. Such VLE concentration data is often known or can be determined experimentally for vapor-liquid mixtures with various components. In certain cases such VLE data can be determined or approximated with the help of certain theories such as Raoult's Law, Dalton's Law, and/or Henry's Law.

Such VLE information is useful in designing columns for distillation, especially fractional distillation, which is a particular specialty of chemical engineers. Distillation is a process used to separate or partially separate components in a mixture by boiling (vaporization) followed by condensation. Distillation takes advantage of differences in concentrations of components in the liquid and vapor phases.

In mixtures containing two or more components where their concentrations are compared in the vapor and liquid phases, concentrations of each component are often expressed as mole fractions. A mole fraction is number of moles of a given component in an amount of mixture in a phase (either vapor or liquid phase) divided by the total number of moles of all components in that amount of mixture in that phase.

Binary mixtures are those having two components. Three-component mixtures could be called ternary mixtures. There can be VLE data for mixtures with even more components, but such data becomes copious and is often hard to show graphically. VLE data is often shown at a certain overall pressure, such as 1 atm or whatever pressure a process of interest is conducted at. When at a certain temperature, the total of partial pressures of all the components becomes equal to the overall pressure of the system such that vapors generated from the liquid displace any air or other gas which maintained the overall pressure, the mixture is said to boil and the corresponding temperature is the boiling point (This assumes excess pressure is relieved by letting out gases to maintain a desired total pressure). A boiling point at an overall pressure of 1 atm is called the normal boiling point.

Thermodynamic description of vapor-liquid equilibrium

The field of thermodynamics describes when vapor-liquid equilibrium is possible, and its properties. Much of the analysis depends on whether the vapor and liquid consist of a single component, or if they are mixtures.

Pure (single-component) systems

If the liquid and vapor are pure, in that they consist of only one molecular component and no impurities, then the equilibrium state between the two phases is described by the following equations:

P^{liq} = P^{vap}\,;
T^{liq} = T^{vap}\,; and
\tilde{G}^{liq} = \tilde{G}^{vap}

where P^{liq}\, and P^{vap}\, are the pressures within the liquid and vapor, T^{liq}\, and T^{vap}\, are the temperatures within the liquid and vapor, and \tilde{G}^{liq} and \tilde{G}^{vap} are the molar Gibbs free energies (units of energy per amount of substance) within the liquid and vapor, respectively. In other words, the temperature, pressure and molar Gibbs free energy are the same between the two phases when they are at equilibrium.

An equivalent, more common way to express the vapor-liquid equilibrium condition in a pure system is by using the concept of fugacity. Under this view, equilibrium is described by the following equation:

f^{\,liq}(T_s,P_s) = f^{\,vap}(T_s,P_s)

where f^{\,liq}(T_s,P_s) and f^{\,vap}(T_s,P_s) are the fugacities of the liquid and vapor, respectively, at the system temperature T_s\, and pressure P_s\,. Using fugacity is often more convenient for calculation, given that the fugacity of the liquid is, to a good approximation, pressure-independent, and it is often convenient to use the quantity \phi=f/P\,, the dimensionless fugacity coefficient, which is 1

Vapor-liquid-solid method

The vapor-liquid-solid method (VLS) is a mechanism for the growth of one-dimensional structures, such as nanowires, from chemical vapor deposition. Growth of a crystal through direct adsorption of a gas phase on to a solid surface is generally very slow. The VLS mechanism circumvents this by introducing a catalytic liquid alloy phase which can rapidly adsorb a vapor to supersaturation levels, and from which crystal growth can subsequently occur from nucleated seeds at the liquid-solid interface. The physical characteristics of nanowires grown in this manner depend, in a controllable way, upon the size and physical properties of the liquid alloy.

Historical background

The VLS mechanism was proposed in 1964 as an explanation for silicon whisker growth from the gas phase in the presence of a liquid gold droplet placed upon a silicon substrate. The explanation was motivated by the absence of axial screw dislocations in the whiskers (which in themselves are a growth mechanism), the requirement of the gold droplet for growth, and the presence of the droplet at the tip of the whisker during the entire growth process.

Introduction

The VLS mechanism is typically described in three stages:

  • Preparation of a liquid alloy droplet upon the substrate from which a wire is to be grown
  • Introduction of the substance to be grown as a vapor, which adsorbs on to the liquid surface, and diffuses in to the droplet
  • Supersaturation and nucleation at the liquid/solid interface leading to axial crystal growth

Experimental technique

The VLS process takes place as follows:

  1. A thin (~1-10 nm) Au film is deposited onto a silicon (Si) wafer substrate by sputter deposition or thermal evaporation.
  2. The wafer is annealed at temperatures higher than the Au-Si eutectic point, creating Au-Si alloy droplets on the wafer surface (the thicker the Au film, the larger the droplets). Mixing Au with Si greatly reduces the melting temperature of the alloy as compared to the alloy constituents. The melting temperature of the Au:Si alloy reaches a minimum (~363 °C) when the ratio of its constituents is 4:1 Au:Si, also known as the Au:Si eutectic point.
  3. Lithography techniques can also be used to controllably manipulate the diameter and position of the droplets (and as you will see below, the resultant nanowires).
  4. One-dimensional crystalline nanowires are then grown by a liquid metal-alloy droplet-catalyzed chemical or physical vapor deposition process, which takes place in a vacuum deposition system. Au-Si droplets on the surface of the substrate act to lower the activation energy of normal vapor-solid growth. For example, Si can be deposited by means of a SiCl4:H2 gaseous mixture reaction (chemical vapor deposition), only at temperatures above 800 °C, in normal vapor-solid growth. Moreover, below this temperature almost no Si is deposited on the growth surface. However, Au particles can form Au-Si eutectic droplets at temperatures above 363 °C and adsorb Si from the vapor state (due to the fact that Au can form a solid-solution with all Si concentrations up to 100%) until reaching a supersaturated state of Si in Au. Furthermore, nanosized Au-Si droplets have much lower melting points (ref) due to the fact that the surface area-to-volume ratio is increasing, becoming energetically unfavorable, and nanometer-sized particles act to minimize their surface energy by forming droplets (spheres or half-spheres).
  5. Si has a much higher melting point (~1414 °C) than that of the eutectic alloy, therefore Si atoms precipitate out of the supersaturated liquid-alloy droplet at the liquid-alloy/solid-Si interface, and the droplet rises from the surface. This process is illustrated in figure 1.

Typical features of the VLS method

  • Greatly lowered reaction energy compared to normal vapor-solid growth.
  • Wires grow only in the areas activated by the metal catalysts and the size and position of the wires are determined by that of the metal catalysts.
  • This growth mechanism can also produce highly anisotropic nanowire arrays from a variety of materials.

Requirements for catalyst particles

The requirements for catalyst are:

  • It must form a liquid solution with the crystalline material to be grown at the nanowire growth temperature.
  • The solid solubility of the catalyzing agent is low in the solid and liquid phases of the substrate material.
  • The equilibrium vapor pressure of the catalyst over the liquid alloy be small so that the droplet does not vaporize, shrink in volume (and therefore radius), and decrease the radius of the growing wire until, ultimately, growth is terminated.
  • The catalyst must be inert (non-reacting) to the reaction products (during CVD nanowire growth).
  • The vapor-solid, vapor-liquid, and liquid-solid interfacial energies play a key role in the shape of the droplets and therefore must be examined before choosing a suitable catalyst; small contact angle between the droplet and solid are more suitable for large area growth, while large contacts angles result in smaller (decreased radius) whisker formations.
  • The solid-liquid interface must be well-defined crystallographically in order to produce highly directional growth of nanowires. It is also important to point out that the solid-liquid interface cannot, however, be completely smooth. Furthermore, if the solid liquid interface was atomically smooth, atoms near the interface trying to attach to the solid would have no place to attach to until a new island nucleates (atoms attach at step ledges), leading to an extremely long growth rate. Therefore, “roughâ€� solid surfaces, or surfaces containing a large number of surface atomic steps (ideally 1 atom wide, for large growth rates) are needed for depositing atoms to attach and nanowire growth to proceed.

Growth mechanism

Catalyst droplet formation

The materials system used, as well as the cleanliness of the vacuum system and therefore the amount of contamination and/or the presence of oxide layers at the droplet and wafer surface during the experiment, both greatly influence the absolute magnitude of the forces present at the droplet/surface interface and, in turn, determine the shape of the droplets. The shape of the droplet, i.e. the contact angle (β0, see Figure 4) can, be modeled mathematically, however, the actual forces present during growth are extremely difficult to measure experimentally. Nevertheless, the shape of a catalyst particle at the surface of a crystalline substrate is determined by a balance of the forces of surface tension and the liquid-solid interface tension. The radius of the droplet varies with the contact angle as:

R=\frac{r_\mathrm{o}}{\sin(\beta_\mathrm{o})},

where r0is the radius of the contact area and β0 is defined by a modified You


From Yahoo Answers

Question:Water vapor is passed through a bed packed with activated carbon. Activated carbon adsorbent has a wide pore size distribution. Which equations are to be used to calculate the saturation and equilbrium values of vapor pressure respectively, given the temperature, the pore radius and the surface tension? In terms of both the pressures, when will the vapor condense inside the pores?

Answers:Let us consider a pore with radius r. Vapor in this pore adsorbs in multilayer. This is a equilibrium : Surface S + vapor V <=> adsorbed water-surface complex X Multilayer adsorption stops at the moment the equilibrium vapor pressure is reached. Then the rate of adsorption is equal the rate of desorption. This is called capillary condensation. The equilibrium vapor pressure is always smaller than the saturation vapor pressure because reaching the latter would result in a condensation of water and liquid water falls on the surface not necessarily with a accompanied adsorption. This would be a simple liquid- vapor equilibrium. Capillary condensation can be estimated with the Kelvin equation: ln (Pv/Psat)= -2 *C*vL/RT Pv: equilibrium vapor pressure Psat: saturation vapor pressure. You get it from a steam table. Boiling point T -> P=Psat : surface tension liquid-vapor. Consult tables. vL: molar volume of liquid C: Curvature = 2/r if you assume the geometry of your pores to be cylindrical. The Kelvin equation shows that for capillary condensation Pv must be
Question:THE QUESTION IS HERE!!! the vapor phases of liquids such as acetone & alcohol are more flammable than their liquid phases. for flammable liquids, what is the relationship between evaporation rate and the likelihood that the liquid will burn? this question is so f'd up!! helppp pleaz!

Answers:Flammable liquids evaporate and their vapours form part of the surrounding atmosphere. As the concentration of vapours increase the air vapour mixture can get to a concentration called the Lower Explosive Limit abbreviated "LEL". Once the vapour concentration is above the "LEL" any energy discharge in the are can trigger an explosion. However if no explosion was to occur the concentration could increase to go past the "UEL" or upper explosive limit and no explosion will occur, simply because there is insufficient oxygen to burn the vapours. Read the link provided. The rate of evaporation depends on the partial pressure of the material in the surrounding atmosphere. Also evaporation causes cooling, so as the liquid vaporises it drops temperature and slows down the rate of evaporation. When the partial pressure of a compound is equal to the atmospheric pressure in the vicinity, the compound has reached its boiling point. The greater the evaporation rate the more likely you will attain the LEL and cause a small explosion which will result in the burning of the liquid phase of the compound as well. If however you do not provide the initial energy you can get to the UEL and the compound will not burn since insufficient oxygen will be present. An external energy source is required to start most combustion reactions. Sodium and phosphorus can spontaneously oxidise at room temperatures and cause fires.

Question:Both sublimation and evaporation bring material into the vapor phase. How do the two methods of separation differ? 2. Both maphthalene and para-dichlorobenzene are effective moth repellents. What property of these chemicals allows them to be used in mothballs for the protection of wollen clothing? 3. Snow in the winter can slowly disappear even when the temperature is freezing. How do you account for this observation? 4.A sample of French fried potatoes weighing 150g was extracted with the volatile organic solvent hexane. The recovered cooking oil weighed 12.3g. What methods could be used to separate and isolate the cooking oil after the extraction? What was the percent oil in the potatoes?

Answers:boyles law for gases. temperature - pressure - volume sublimation : the substance goes from solid phase directly to gas phase. evaporation : the substance goes from liquid phase to gas phase. http://www.grc.nasa.gov/WWW/K-12/airplane/glussac.html http://www.grc.nasa.gov/WWW/K-12/airplane/boyle.html

Question:For my 8th rgade science fair project, I'm testing to see if the acidity of water affects the formation of [NaCl] stalactites. I have to add different concentrations of citric acid to each setup, which is salt water in plastic cups with a string between them. The strings asorbs the water, the water evaporates, and the salt crystals are left behind, forming "Stalactites". I was wondering if the different amounts of citric acid will affect its rate of evaporation? If so, how? Thanks!!

Answers:Yes. The addition of any soluble, nonvolatile solute will lower the vapor pressure of the solution. Thus the evaporation rate will also slow down.

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