physical chemistry - Boiling point and vapor pressure - Chemistry Stack Exchange
The boiling point of water, or any liquid, varies according to the surrounding atmospheric pressure. A liquid boils, or begins turning to vapor. The crucial factor is the relation between the pressure and the temperature of water A comparative table of vapor-pressure measurements for water (All of the .. however, he claimed that Tomlinson's result could not be trusted because it. “The relationship of total pressure (mbar) to the partial pressure (mbar) of water .. What is the relation between vapor pressure of water and negative pressure? However, we only trust them to protect from air when they are maintained.
When water is in the liquid form, its molecules have acquired enough heat to keep them moving more rapidly than those in ice. This increased motion is enough to overcome much of the electrical attraction between molecules and allow them to move about rather freely.
Since the molecules of water in the liquid state are not held in a rigid pattern, the water takes the shape of whatever container holds it.
When water exists as steam or vapor, its molecules are moving so swiftly—because of further increased heat—that attraction is fully overcome. Above sea level, where pressure is reduced, water boils at lower temperatures and freezes at higher temperatures. Density and Weight Water reaches its greatest density weight per unit volume at The density of pure water at This value is the basis for determining the specific gravity of a substance. The specific gravity of any substance is defined as the ratio of its density to the density of water at The density of gold, for example, is This means that gold is Substances with specific gravities greater than 1.
Each cubic foot of water weighs The weight of water, of course, causes pressure to increase with depth. At this rate the pressure a mile down in the ocean is more than 2, pounds per square inch. See also underwater diving. In fresh water and salt water, as the temperature descends to the freezing point, the movement of water molecules slows. By then the water has absorbed the same amount of heat that it lost while freezing. The amount of heat that is given off or absorbed without temperature change is called the latent heat of fusion.
It amounts to about 80 calories for each gram of water. Water expands by nearly one-tenth of its volume when it freezes. Thus, 1 cubic foot of water becomes 1. The ice therefore becomes less dense lighter than water at the same temperature, and the ice floats.
Freezing water expands with enormous force—up to tons per square inch depending on the rate of freeze and other factors. Unprotected water pipes often burst on cold nights because of this tremendous expansive force. Heavier water pipes would be useless because scientists have shown that a water-filled cast-iron vessel with sides many inches thick will still burst when the water freezes.
If faucets are allowed to run at a trickling rate, often the friction of the moving water produces enough heat to prevent pipe bursts. How Water Evaporates and Boils Heat transforms water from a liquid to a gas. All substances hold some heat, and their molecules are all in motion.
The molecules in liquid water do not move fast enough to escape. This constant escape of surface molecules is called evaporation. If the rise in temperature is great enough, even molecules deep beneath the surface will break loose from their neighbors and form bubbles of vapor. These bubbles then rise to the surface and fly away as steam. The temperature that is high enough to cause this activity is called the boiling point.
When liquid water turns to steam or vapor, the water absorbs heat without a rise in temperature. When two equal amounts of water turn into vapor, one slowly by ordinary evaporation and the other rapidly by boiling, the amount of heat finally absorbed by each is about equal.
In the absence of a flame or other applied-heat source, evaporating water draws heat from its surroundings. In doing so, it cools whatever is near it. People in warm climates often keep their water cool by placing it in a large canvas bag or a porous pottery jug, which becomes moist as some of the water seeps through it. As evaporation takes place from the moist surface, heat is drawn from water farther inside, and thus the water is cooled.
Water that turns into vapor has absorbed heat. Called the latent heat of vaporization, this is a useful property of water. Its effect is large. When a cubic foot of water at sea-level pressure boils away, it becomes about 1, cubic feet of steam. As the quickly moving water molecules fly off as steam, they can transfer considerable energy to surrounding objects. This energy is used in heating systemsin steam enginesand in turbines.
Atmospheric pressure influences the boiling point of water. When atmospheric pressure increases, the boiling point becomes higher, and when atmospheric pressure decreases as it does when elevation increasesthe boiling point becomes lower.
Pressure on the surface of water tends to keep the water molecules contained. As pressure increases, water molecules need additional heat to gain the speed necessary for escape. Pressure cookers work on this principle. Lowering the pressure lowers the boiling point because the molecules need less speed to escape. The low atmospheric pressure on high mountains lowers the boiling point to such an extent that water cannot get hot enough to boil eggs satisfactorily.
They later learned that hydrogen has three isotopes and oxygen has six isotopes. These nine isotopes can combine in a number of ways to form water molecules of different weights. The isotopes of hydrogen are far more important. Chemists call these isotopes protium single-weight hydrogendeuterium double-weight hydrogenand tritium triple-weight hydrogen. Protium combines with oxygen to form light water; deuterium and oxygen form heavy water; and tritium and oxygen produce superheavy water.
Ordinary water found in nature consists mostly of the light variety and has the formula H2O. Heavy water is called deuterium oxide D2O by chemists. It is about 10 percent heavier than H2O. Only one part of heavy water is found in about 5, parts of ordinary water.
Heavy water can be separated from light water by evaporation, but chemists commonly use a more efficient process called electrolysis. Because D2O reacts more slowly to electrolysis than does H2O, the heavy water remains after the light water disappears.
Scientists use heavy water to slow down fast-moving neutrons in nuclear reactors. Superheavy water is called tritium oxide T2O. Little is known about its properties because it is difficult to separate and is highly unstable. Since tritium is radioactivescientists use traces of T2O to observe the effect of water upon various organic compounds. The radioactive tritium can be detected and followed by special instruments. Pure water is never found in nature because water is an excellent solvent for many minerals.
It also picks up bits of matter wherever it flows. Chemists must distill water to obtain pure water for delicate chemical processes. Anhydrous and dehydrated mean that water usually present in a substance has been removed. How Water Circulates Throughout the World Water must be readily available to support life and its activities.
At first thought it may seem that water is always available, since Earth is literally surrounded by water: The vast oceansalmost an unending source of water, cover some million square miles and contain some million cubic miles of water. Yet, with all this water, there are parts of Earth that are scorched and arid. The manner in which water circulates between Earth and the atmosphere determines where ample water supplies can be found and used.
The land surfaces would become lifeless deserts. Water, however, does not stagnate in the oceans. It is continually evaporated from the oceans and other bodies of water by the heat of the sun and blown by the winds across sea and land.
Thus an immense amount of water is always suspended in the atmosphere in the form of vapor. When certain weather conditions prevail in the atmosphere, some of the water vapor condenses into droplets of liquid water, ice crystals, or both—forming clouds.
When such clouds accumulate more moisture than they can hold, the water is returned to the land as rain or snow. This process of moving water out of the oceans, into the atmosphere, and back to the land and oceans is called the water cycleor hydrologic cycle. Sun, air, water, and the force of gravity work together to keep the water cycle going.
Major steps in the cycle include: Some water evaporates into the air from rivers, lakes, moist soil, and plants, but most of the water that moves over the surface of Earth comes from the oceans and eventually returns to the oceans. Surface Water and Groundwater The soil covering Earth acts as a giant sieve. Soil particles have tiny spaces between them that allow water to trickle down into the soil. When a heavy rainfall occurs, these tiny spaces in soil quickly fill with water, and the excess water, called surface water, runs over the top of the soil.
Such surface runoff flows as a thin, hardly noticeable sheet of water until it reaches a depression in the land, such as a gutter or a streambed, where the water can be contained. There, it no longer flows as a sheet of water but as a clear-cut channel of water, moving downward to the ocean.
Water that infiltrates the soil trickles slowly downward, or percolates, through pores and cracks in soil and rocks. Rock strata, or layers, and soil capable of holding water are called aquifers.
Eventually, the water reaches a level where it can go no farther because bedrock forms a base. As more and more water accumulates, the aquifer becomes saturated filled with water and cannot hold any more. Water held in aquifers is called groundwater. The depth at which groundwater is found varies because the hard bedrock base exists at varying levels.
Groundwater is a major source of fresh water. By means of wells, humans bring this water to the surface to satisfy their need for water. Some of the groundwater moves toward the surface of the soil by capillary action and is evaporated into the air.
11.5: Vapor Pressure
Plants draw their water from ground so moistened. Water is drawn through the roots of a plant to its leaves, from which it evaporates. This process is called transpiration. A fully grown oak tree may transpire about gallons liters of water a day. In summer an acre of corn maize transpires from 3, to 4, gallons 11, to 15, liters of water each day. The topmost level of groundwater is called the water table; below this level the soil is waterlogged. If a hole is dug deep enough in the soil, it may reach the water table.
The water table is not at the same level everywhere. It may be close to the surface in some places and hundreds of feet beneath the soil in others.
Sometimes a deep cut in the land will expose the water table. Then the groundwater runs off as a stream or river. Heavy rainfall can raise the water table. If the level becomes too high, damage can occur to plants. During times of sparse rainfall, the soil becomes extremely dry, and groundwater that seeps to the surface and evaporates is not replaced.
The water table then becomes lower. If much of the lost water is not soon replaced, a drought may occur. Water that is drawn from wells may affect the level of the water table in a given area. When groundwater is pumped to the surface, the water level in the well becomes slightly lower than the surrounding water table.
Groundwater then flows downward to the level of water in the well, causing a cone of depression in the water table. This lowers the water table slightly. If water is rapidly drawn from a number of wells in the same area, the water table may be lowered considerably.
The water table may rise again when sufficient rainfall occurs or when there is a decrease in the amount of water taken from wells. Water Movement Both groundwater and surface water move downslope. Some groundwater may become trapped in hard rock. It remains there—under pressure because groundwater above the trapped water weighs down upon it. Wells drilled into the pool of trapped water release the water, and it rushes to the surface without being pumped. Such wells are called artesian wells.
Normally, groundwater moves slowly down sloping land, spreading and flattening itself in porous soil. It eventually empties into permanent, steadily flowing streams, which in turn drain into large rivers that flow into the ocean. Water-supply systems provide water for irrigation, homes, businesses, industry, and waste removal.
Water is also necessary for public needs, such as fire fighting, hydrant flushing, and street cleaning. City water-supply systems usually include works for the collection, transmission, purification, storage, and distribution of water. Caio do Valle Some cities get water by pumping it from a lake, from a river, or from ponds.
Other communities pump their water from wells. Storage reservoirs or dams are sometimes constructed at or near points of water collection to ensure a dependable supply of water. Many reservoirs have multiple uses, including public water supply, irrigation, navigation, hydroelectric power, flood control, and recreation.
Water is often transported to waterworks by canals, aqueducts, or tunnels. Pipelinesthrough which water flows either by gravity or under pressure, are also used. Another method of obtaining fresh water is by desalting seawater, commonly referred to as desalination. Desalination facilities are usually located along coastal areas. Before water is distributed for use, it is usually treated to make it hygienically safe, attractive, and palatable.
The pumping station, which regulates the amount of water distributed, and the water-treatment system are called waterworks. This figure includes water used for such purposes as fire fighting, waste disposal, street cleaning, and industry. Most cities cannot pay cash to build expensive waterworks, so they issue bonds to raise the money.
To repay these bonds and maintain the water system, cities once taxed property owners. Today, most cities require meters in each building and charge the user for the amount of water used. Major improvements and additions to the system are frequently financed by revenue bonds, which are paid for by the water users. Few cities, however, can find a supply of such water. Sewage or barnyard wastes may carry disease-causing organisms into the water supply. Untreated industrial wastes often pollute the supply.
The water may contain mud, silt, and dissolved minerals. Waterworks remove such impurities before sending the water into the mains. Waterworks process the water in different ways, depending on the water source and the intended use. Before purification, water is usually pumped through coarse screens that catch large objects. Pumps then force the screened water into a mixing tank. There, chemicals called coagulants are stirred into the water. The coagulants combine with bacteria, mud, and silt to form sticky clumps called flocs.
Then the water passes into deep, broad sedimentation tanks, or settling basins.
19th-century theories of boiling
As the water passes slowly through the tanks, the flocs settle to the bottom. They are removed from the tank bottom by mechanical scrapers. Water from the sedimentation tanks is filtered through sand or other porous material. The filter catches all remaining suspended matter. Rapid sand filters are most commonly used. Sand is spread from 24 to 36 inches 61 to 91 centimeters deep in the filter basin, which may cover several acres.
The filter sand does more than mechanically strain the water. Gradually the impurities form a jellylike surface mat on the sand. Bacteria and suspended matter stick to the surface mat as water passes through. Reversing the water flow washes away the accumulated wastes.
Some filtration plants use finely crushed anthracite, or hard coal, as a filter in place of sand. Twenty-five years later Marcettested out the adhesion hypothesis more rigorously. This prediction was borne out in his tests, since the lowest boiling temperature he could ever obtain with the insertion of metal pieces was Again as predicted, Marcet achieved boiling at When the bottom and sides of the vessel were covered with a thin layer of gomme laque, boiling took place at It is, however, on the basis of that fact, generally assumed to be exactly true, that physicists made a choice of the temperature of water boiling in a metallic vessel as one of the fixed points of the thermometric scale.
Marcet's beautiful confirmations seemed to show beyond any reasonable doubt the correctness of the pressure-balance theory modified by the adhesion hypothesis. However, two decades later Dufourvoiced strong dissent on the role of adhesion. Since he observed extreme superheating of water drops removed from solid surfaces by suspension in other liquids, he argued that simple adhesion to solid surfaces could not be the main cause of superheating.
Instead Dufour stressed the importance of the ill-understood molecular actions at the point of contact between water and other substances: It seems to me beyond doubt that heat alone, acting on water without the joint action of alien molecules, can only produce its change of state well beyond what is considered the temperature of normal ebullition.
Boiling was made possible at the point of pressure-balance, but some further factor was required for the breaking of equilibrium, unstable as it may be. Heat alone could serve as the further facilitating factor, but only at a much higher degree than the normal boiling point. Dufour also made the rather subtle point that the vapor pressure itself could not be a cause of vapor-production, since the vapor pressure was only a property of "future vapor," which did not yet exist before boiling actually set in.
Dufour's critique was cogent, but he did not get very far in advancing an alternative. He was very frank in admitting that there was insufficient understanding of the molecular forces involved see the last pages of Dufouresp. Therefore the principal effect of his work was to demolish the adhesion hypothesis without putting in a firm positive alternative.
There were two main attempts to fill this theoretical vacuum. One was a revival of Cavendish's and De Luc's ideas about the importance of open surfaces in enabling a liquid to boil. According to Cavendish's "first principle of boiling," the conversion of water at the boiling point into steam was only assured if the water was in contact with air or vapor.
And De Luc had noted that air bubbles in the interior of water would serve as sites of vapor-production. For De Luc this phenomenon was an annoying deviation from true boiling, but it came to be regarded as the definitive state of boiling in the new theoretical framework. One crucial step in this development was taken by Marcel Emile Verdet, whose work is discussed briefly in the discussion of superheating.
Following the basic pressure-balance theory, he defined the "normal" point of boiling as the temperature at which the vapor pressure was equal to the external pressure, agreeing with Dufour that at that temperature boiling was made "possible, but not necessary. He theorized, somewhat tentatively, that boiling was not provoked by all solid surfaces, but only by "unwettable" surfaces that also possessed microscopic roughness.
On those surfaces, capillary repulsion around the points of irregularity would create small pockets of empty space, which could serve as sites of evaporation. There would be no air or steam in those spaces initially, but it seemed sensible that a vacuum should be able to serve the same role as gaseous spaces in enabling evaporation.
Does water's boiling point change with altitude? Americans aren't sure | Pew Research Center
If such an explanation were tenable, then not only Dufour's observations but all the observations that seemed to support the adhesion hypothesis could be accounted for. For the exposition of Verdet's view, I follow Gernez The information about Gernez is taken from M. Geison's entry on Pasteur in the Dictionary of Scientific Biography, In papers published in andGernez reported that common boiling could always be induced in superheated water by the insertion of a trapped pocket of air into the liquid by means of a small glass instrument.
A tiny amount of air was sufficient for this purpose, since boiling tended to be self-perpetuating once it began. Gernezthought that at least half a century had been wasted due to the neglect of De Luc's work: In Gernez's view a full understanding of boiling could be achieved by a consistent and thorough application of De Luc's idea, a process initiated by Donny, Dufour, and Verdet among others.
Donnyhad given a new theoretical definition of boiling as evaporation from interior surfaces: First, he stated that such boiling started at a definite temperature, which could be called "the point of normal ebullition.
Here one should also allow a possible role of Verdet's empty spaces created by capillary forces, and of internal gases produced by chemical reactions or electrolysis the latter effect was demonstrated by Dufour Gernez's mopping-up bolstered the pressure-balance theory of boiling quite sufficiently; the presence of internal gases was the crucial enabling condition for boiling, and together with the balance of pressure it constituted a sufficient condition as well.
The theoretical foundation of boiling now seemed quite secure. There were, however, more twists to come in the theoretical debate on boiling. While Verdet and Gernez were busily demonstrating the role of gases, a contrary view was being developed by Charles Tomlinson in London.
Tomlinson believed that the crucial enabling factor in boiling was not gases, but small solid particles. Tomlinson's argument was based on some interesting experiments that he had carried out with superheated liquids. Building on previous observations that inserting solid objects into a superheated liquid could induce boiling, Tomlinsonshowed that metallic objects lost their vapor-liberating power if they were chemically cleaned to remove all specks of dust.
In order to argue conclusively against the role of air, he lowered a small cage made out of fine iron-wire gauze into a superheated liquid, and showed that no boiling was induced as long as the metal was clean. The cage was full of air trapped inside, so Tomlinson inferred that there would have been visible production of vapor if air had really been the crucial factor. Tomlinson's theory and experiments attracted a good deal of attention, and a controversy ensued.
It is not clear to me whether and how this argument was resolved. As late asthe second edition of Thomas Preston's well-informed textbook on heat reported: Some experiments were less ambiguous, but still not decisive. For example, Gernez acknowledged in his attack on Tomlinson that the latter's experiment with the wire-mesh cage would clearly be negative evidence regarding the role of air; however, he claimed that Tomlinson's result could not be trusted because it had not been replicated by anyone else.
Like Dufour earlier, Gernez, also scored a theoretical point by denigrating as unintelligible Tomlinson's concept of a liquid near boiling as a supersaturated solution of its own vapor, though he was happy to regard a superheated liquid as a supersaturated solution of air. The Tomlinson-Gernez debate on the theory of boiling is fascinating to follow, but its details were not so important for understanding the fixity of the steam point.