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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-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

Liquid bandage

Liquid bandage is a topical skin treatment for minor cuts and sores that is sold by several companies. The products are mixtures of chemicals which create a polymeric layer which binds to the skin. This protects the wound by keeping dirt and germs out, and keeping moisture in.

Design

Liquid bandage is typically a polymer dissolved in a solvent (commonly water or an alcohol), sometimes with an added antiseptic, although the alcohol in some brands may serve the same purpose. These products protect the wound by forming a thin film of polymer when the carrier evaporates. Polymers used may include polyvinylpyrrolidone (water based), pyroxylin/nitrocellulose or poly(methylacrylate-isobutene-monoisopropylmaleate) (alcohol based), and acrylate or siloxane polymers (hexamethyldisiloxane or isooctane solvent based). Other types of liquid bandages (more suited for use when the wound is actively bleeding), are based on cyanoacrylates. Although ethylcyanoacrylates are conventionally used in "superglue" adhesives, medical cyanoacrylates are based on octylcyanoacrylates, as they do not break down in the body to form toxic byproducts, as ethylcyanoacrylates do. Research is underway into acrylate copolymer based products, as there is less chance of gluing body parts together accidentally.

In addition to their use in replacing conventional bandages in minor cuts and scrapes, they have found use in surgical and veterinary offices, as they cause less trauma, and do not have to be removed like sutures (stitches) and staples do. Liquid bandages are increasingly finding use in the field of combat, where they can be used to rapidly stanch a wound until proper medical attention can be obtained. Liquid bandage has also been used to treat skin tags.

Brands and ingredients

Recent developments

A novel type of liquid bandage would incorporate amino acids to form peptide links directly with the skin. This product has potential to reduce bleeding during and after surgery.


Liquid and digits

Liquid and digits is a type of gestural, interpretive, rave and urbanstreet dance that sometimes involve aspects of pantomime. The term invokes the word liquid to describe the fluid-like motion of the dancer's body and appendages and digits to refer to illusions constructed with the dancer's fingers. Liquid dancing has many moves in common with popping and waving. The exact origins of the dances are uncertain, although they came out of either popping, raves, or both sometime from the 1970s to 1990s. The dance is typically done to a variety of electronic dance music genres from trance to drum and bass to glitch hop, depending on the dancer's musical taste.

Origins

Since the spontaneous rise and propagation of Liquid throughout the rave culture in the 1980’s and early 1990’s, the root origins of the dance have ultimately remained a source of contention between both those involved directly with the dance as well as those outside of the immediate culture. In fact, even the time frame is difficult to pinpoint. Sightings of the dance range all the way back to the early and mid 1970’s. While some argue that the dance evolved spontaneously from combining elements in the rave culture, others still contend that the dance is merely an extension of existing ideas from other art forms. Scores of these artists (Funk Stylists, Glowstickers Contact Jugglers, Mimes, & The Unknown) attended raves regularly all throughout the 1980’s and 1990’s. In the wake of the decline of the original rave scene, liquid has become a standing part of a worldwide club culture and the underground street dancing movement.

B-boys and funk stylists generally contend that liquid dancing is a development of waving, a technique in popping. Liquid dancing covers many of the same fundamentals as popping and it is fully possible (and common) for dancers to combine the styles, further blurring the distinction between the two. The defining difference is liquid dancing concentrating on smooth movements while popping is characterized by jerky pops (hits) and contractions.

In 2000, a group of liquid dancers from throughout the northeastern United States formed the Liquid Pop Collective (LPC). The name later caused some confusion since some thought the LPC did a dance called "liquid popping" but the name was chosen because many members did both liquid and popping. In Philadelphia, they began performing at events run by Reflective Multimedia, a collective of DJs and visual artists. After performing for a bit, they noticed other people in the clubs who they did not know starting to do liquid and digits. Before this, those that were interested in liquid generally knew each other. The LPC was concerned that these newcomers to the dance did not have anyone teaching them. They thought about how funk styles flourished without any direct teachers and came to the conclusion that they needed to develop a standard vocabulary for the dance. Around this time, the LPC put a video (now available [http://www.youtube.com/watch?v=vBIfCQJi32U on YouTube]) of one of the members, Eric, liquid dancing on Napster. The video spread and people wanted to learn the dance. So, the LPC decided to make an instructional video by the name of All Access Liquid and Digitz, Volume 01 (no other volumes were made) which defined the concepts that are the foundation of liquid and digits and had performances of four members. They sold about 2000 VHS tapes through their now-defunct website lpclabs.com and shipped to all over the world. The LPC has since broken up and no one is shipping the video, although it has all been uploaded [http://www.youtube.com/view_play_list?p=CC8F51A66444D195 to YouTube].

Techniques, concepts, and construction

Liquid dancers use a variety of techniques rhythmically strung together to create an illusion of continuous flow that corresponds to the music.

Hand flow

Hand flow is the most commonly used technique in Liquid dancing and simultaneously the easiest to grasp. It consists of curling the fingers of one hand and following them with the straight fingers of the opposite hand. The wrists, elbows, and shoulders may be involved to extend the motion. A Liquid dancer's personal style is defined by his or her individual approach to hand flow, and how it fits into their dance as a whole.

Rails

Rails, often a heavy focus in liquid, are characterized by the moving of the arms along a set path or "rail".

Waves

Waves maintain the illusion that a wave is passing through one's body by the isolation and alternating tensing and relaxing of one part of the body at a time at a steady speed in a constant direction.

Traces

During a trace, one's hand follows the path of a wave going through one's body. The hand moves at the same speed and in the same direction as the wave.

Contours

This technique entails the hands following exactly the outline of an object, be it real or imaginary. Most commonly the hands follow the outline of one's own body.

Threads

This style maintains the illusion that one is pulling parts of their body through holes created by the positioning of other body parts, typically arms. An example of this would be holding one's shoulder to create a closed loop which the other arm goes through. These are performed at the same speed as the flow of the liquid and waves to maintain an illusion of continuity.

Splits

This technique is characterized by the hands moving independently of each other while maintaining the illusion of a fluid relationship between each other. This is typically accomplished by misaligning the hands but using the same finger motions as regular handflow.

Builds

Builds are identified by the manipulation of imaginary objects in a manner similar to pantomime. These moves can be combined with video editing to show the imaginary object being manipulated as the person dances.

Remotes

Using one part of the body as a remote control for another is referred to as a remote. For example, pulling a hand up while simultaneously lifting a leg as if they are connected by a string is a remote.

Levers

Using one body part to create the illusion of applying a driving force to rotate another body part around a hinge. Typically done with a hand driving the opposite hand + forearm around the opposite elb


From Encyclopedia

liquid

liquid one of the three commonly recognized states in which matter occurs, i.e., that state, as distinguished from solid and gas, in which a substance has a definite volume but no definite shape. Properties of Liquids In general, liquids show expansion on heating, contraction on cooling; water, however, does not follow the rule exactly. A liquid changes at its boiling point to a gas and at its freezing point, or melting point , to a solid. The boiling point is especially important because, since liquids change their states at different temperatures, those in a mixture can be separated from one another by raising the temperature of the mixture gradually so that each component in turn undergoes vaporization at its boiling point. This process is known as fractional distillation. Liquids, like gases, exhibit the property of diffusion. When two miscible liquids (i.e., they mix without separation) are poured carefully into a container so that the denser one forms a separate layer on the bottom, each will diffuse slowly into the other until they are thoroughly mixed. Liquids, like gases, differ from solids in that they are fluids, that is, they flow into the shape of a containing vessel. Liquids exert pressure on the sides of a containing vessel and on any body immersed in them, and pressure is transmitted through a liquid undiminished and in all directions. Liquids exert a buoyant force on an immersed body equal to the weight of the liquid displaced by the body (see Archimedes' principle and specific gravity ). Unlike gases, liquids are very nearly incompressible, and for that reason are useful in such devices as the hydraulic press. Liquids are useful as solvents. No one liquid can dissolve all substances; each takes into solution only certain specific substances. Molecular Structure of Liquids The molecules (or atoms or ions) of a liquid, like those of a solid (and unlike those of a gas), are quite close together; however, while molecules in a solid are held in fixed positions by intermolecular forces, molecules in a liquid have too much thermal energy to be bound by these forces and move about freely within the liquid, although they cannot escape the liquid easily. Although the molecules of a liquid have greater cohesion than those of a gas, it is not sufficient to prevent some of those at the free surface of the liquid from bounding off (see evaporation ). On the other hand, the cohesive forces between the molecules at the surface of a mass of liquid and those within cause the free surface to act somewhat like a stretched elastic membrane; it tends to draw inward toward the center of the liquid mass, to draw the liquid into the shape of a sphere, thus exhibiting the phenomenon known as surface tension . A liquid is said to "wet" a solid substance when the attractive force between the molecules of the liquid and those of the solid is great enough to hold the liquid's molecules at the solid surface. For example, water "wets" glass since its molecules cling to glass surfaces, whereas mercury does not since the adhesive force between its molecules and those of glass is not strong enough to hold them together. Capillarity is an example of surface tension and adhesion acting at the same time.


From Yahoo Answers

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:I need to make a lab on how intermolecular forces affect the rate of evaporation. I have to write up the methods and equipment but i don't know how to carry out the experiment. Can anyone give me some idea about how to measure the rate of evaporation of liquids?

Answers:If it were me, I would label an open container (beaker, coffee cup, glass, bowl...) and weigh it. Add the test liquid and reweigh to get the starting weight of the liquid. Record this weight along with the time. Let the open container set and reweigh it from time to time, recording the weights and times. Test a variety of liquids using similar containers. Use an empty container as a control. Set them in various places to see if this changes things. Use a thermometer and record temperatures to see if this is a factor. If any of the liquids are flammable or toxic, take suitable precautions. This would measure the evaporation rates. Do you have some way of measuring the intermolecular forces? I hope this helps you get started. Think about it, write something up and present it to your teacher and ask for advice. Good Luck!! (Try to have some fun.)

Question:I am doing a Weather and Climate lab and I am confused by this question? Can someone please explain to me what it would be? This is the whole question: Hurricanes gain their energy through evaporation of warm ocean waters. How does the subsidence of dry air affect evaporation rate? How does wind speed affect evaporation rate? also there is part 2 to this question: How do sea-surface temperatures affect evaporation rate? What could cause variations in sea-surface temperatures ahead of a hurricane's path?

Answers:I have read twice your question without really understanding. The third time I came to think that "evaporation rate" is the evaporation rate of sea water at the surface. If that is correct then a subsidence of dry air, i.e. the sinking of air at a low relative humidity level, will - of course - increase evaporation. Likewise, the higher the wind speed, the greater the evaporation. This is because, right over the surface of the water, some molecules mix with the air until it is 100 percent saturated. More evaporation can't occur until that air is replaced by new air and the faster it happens, and the drier the air is, the faster evaporation occurs. Of course, since warmer air can contain more moisture than cold one, the warmer the more evaporation too. But I don't think that the adiabatic warming of a subsidence has much to say for the air right above the sea. A subsidence, as it happens in a high pressure, is something that happens very slowly and I can't imagine it has any effect of the rate of sea water evaporation. It is said that hurricanes happen in the southern part of the North Atlantic when the temperature of the surface of the sea reaches 27 C. This usually occur by the end of the summer, in August. I don't see why the sea surface temperature should vary ahead of a hurricane but I am not a specialist, only a sailor and I know that off the Cape Verde islands, where hurricanes are born, the weather is boringly the same in the NE'ly Trade winds. I know, I have been there. The reason hurricanes occur when the sea water is that high is not only because of that but because the weather is not about temperature but the difference of temperature. While it varies a lot at the surface between the poles and the equator, at the tropopause, the top of our stroposphere, the temperature is pretty much the same, around -55 C. The only difference is that the tropopause is nearly twice as high at the equator than the poles. But it is the great difference of temperature between the surface and altitude that is responsible for the hurricanes and all other extreme tropical cyclones. The reason it is born over the sea is that moisture is the fuel of a convection since the wet adiabatic lapse rate is only half that of the dry air. Moist air keeps rising and lowers even further the pressure.

Question:I know there are probably a number of factors that can affect the rate of evaporation but is there a general rule of thumb or formula I can use to calculate it? Here's the environment: freshwater concrete fishpond about 2.5 feet deep (assume near vertical walls). The (circular) surface is roughly 10 feet in diameter--which make approx 80 square feet. Assuming an average temp of about 75 degrees, how many inches of evaporation loss should I expect in a 24 hr period? (I need a rough answer so I can determine if the pond is leaking or if it's just evaporation that is causing the bulk of the daily water level drop.)

Answers:Here are a couple of sources to get you going. Edit- If you're trying to find out if your pond is leaking here's something you can try--- Place a five gallon bucket on the second pool step and fill it to match the water level of the pool (probably have to put concrete block in bucket fore you fill it with water to help keep in place). Over the course of 2-3 days the bucket will lose water from evaporation and gain water from rainfall at the same rate as your pool! As long as there is no splash-out or backwashing during that time, if the pool level drops more than the bucket level then you have a leak. Source- http://wiki.answers.com/Q/How_do_you_measure_water_evaporation