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Critical Temperature and an essential Pressure Surface Tension
Viscosity Hydrogen Bonding & the Anomalous properties of Water

Critical Temperature and CriticalPressure

The obvious method to revolve a gas into a fluid is come cool it come a temperature listed below itsboiling point. Over there is another way of condensing a gas to type a liquid, however, whichinvolves raising the push on the gas. Liquids boil at the temperature at which thevapor push is equal to the pressure on the liquid from the surroundings. Increasing thepressure on a gas therefore effectively increases the boiling point of the liquid.

Suppose that we have water vapor (or steam) in a close up door container at 120oCand 1 atm. Since the temperature the the system is over the regular boiling point of water,there is no reason for the heavy steam to condense to form a liquid. Nothing happens as weslowly compress the container therebyraising the pressure on the gas until thepressure will 2 atm. At this point, the device is in ~ the boiling point of water, andsome that the gas will certainly condense to type a liquid. As quickly as the pressure on the gas exceeds2 atm, the vapor press of water in ~ 120oC is no longer large enough for theliquid come boil. The gas therefore condenses to type a liquid, as displayed in the figurebelow.


In theory, we should be able to predict the pressure at i beg your pardon a gas condenses in ~ agiven temperature through consulting a plot the vapor push vs. Temperature .In practice, every compound has a critical temperature (Tc).If the temperature of the gas is over the vital temperature, the gas can"t becondensed, nevertheless of the push applied.

The existence of a critical temperature was discovered by thomas Andrews in 1869 whilestudying the impact of temperature and pressure ~ above the behavior of carbon dioxide. Andrewsfound the he can condense CO2 gas into a liquid by raising the pressure onthe gas, as long as he kept the temperature below 31.0oC. At 31.0oC,for example, that takes a press of 72.85 atm come liquify CO2 gas. Andrew foundthat it was difficult to revolve CO2 into a liquid over this temperature, nomatter just how much push was applied.

Gases can"t it is in liquified at temperatures above the an important temperature since at thispoint the properties of gases and liquids end up being the same, and there is no basis on whichto distinguish in between gases and also liquids. The vapor press of a fluid at the criticaltemperature is dubbed the critical pressure (Pc). The vaporpressure of a liquid never ever gets larger than this an important pressure.

The crucial temperatures, an essential pressures, and boiling points of a variety of gasesare given in the table below. Over there is an evident correlation between the criticaltemperature and also boiling allude of this gases. These properties are related due to the fact that theyare both indirect measures of the pressure of attraction between particles in the gas phase.

Critical Temperatures, vital Pressures and also Boiling clues ofCommon Gases

Gas Tc(oC) Pc (atm) BP (oC)
He -267.96 2.261 -268.94
H2 -240.17 12.77 -252.76
Ne -228.71 26.86 -246.1
N2 -146.89 33.54 -195.81
CO -140.23 34.53 -191.49
Ar -122.44 48.00 -185.87
O2 -118.38 50.14 -182.96
CH4 -82.60 45.44 -161.49
CO2 31.04 72.85 -78.44
NH3 132.4 111.3 -33.42
Cl2 144.0 78.1 -34.03

The experimental values the the an important temperature and pressure of a substance deserve to beused to calculate the a and also b constants in the van der Waalsequation.


Surface Tension

There is a force of attraction in between molecules in liquids, and liquids can circulation untilthey take on the form that maximizes this force of attraction. Listed below the surface of theliquid, the force of cohesion (literally, "sticking together")between molecules is the same in all directions, as displayed in the figure below. Moleculeson the surface ar of the liquid, however, feel a net force of attraction that pulls lock backinto the body of the liquid. Together a result, the fluid tries to take it on the shape that hasthe smallest feasible surface area theshape that a sphere. The size of the force that controls the shape of the fluid iscalled the surface tension. The stronger the bonds between the moleculesin the liquid, the larger the surface ar tension.


There is likewise a force of adhesion (literally, "sticking")between a liquid and the walls of the container. Once the force of adhesion is much more thanhalf as huge as the pressure of cohesion between the liquid molecules, the liquid is stated to"wet" the solid. A great example that this phenomenon is the wetting of document bywater. The pressure of adhesion between file and water linked with the pressure of cohesionbetween water molecules describes why sheets that wet paper stick together.

Water wets glass because of the pressure of adhesion that results from interactionsbetween the positive ends the the polar water molecules and the negatively charged oxygenatoms in glass. Together a result, water forms a meniscus the curves increase ina small-diameter glass tube, as shown in the number below. (The ax meniscuscomes from the Greek word for "moon" and is provided to explain anything that has actually acrescent shape.) The meniscus that water develops in a buret results from a balance betweenthe pressure of adhesion pulling increase on the shaft of water to wet the walls of the glass tubeand the pressure of heaviness pulling under on the liquid.

Water climbs the walls of a small-diameter tube to type a meniscus the curves upward, conversely, mercury develops a meniscus the curves downward.

The force of adhesion between water and also wax is very small compared come theforce of cohesion in between water molecules. As a result, rain doesn"t adhere towax. It has tendency to type beads, or drops, through the smallest feasible surface area, therebymaximizing the force of cohesion in between the water molecules. The exact same thing wake up whenmercury is spilled on glass or poured right into a small glass tube. The pressure of cohesionbetween mercury atom is for this reason much larger than the pressure of adhesion between mercury andglass that the area of contact in between mercury and also glass is maintained to a minimum, with thenet result being the meniscus displayed in the above figure.


Viscosity is a measure up of the resistance come flow. Engine oils are moreviscous 보다 gasoline, for example, and also the maple syrup offered on pancakes is more viscousthan the vegetables oils used in salad dressings.

Viscosity is measure up by identify the rate at which a fluid or gas flows v asmall-diameter glass tube. In 1844 Jean luigi Marie Poiseuille proved that the volume offluid (V) that flows down a small-diameter capillary pipe per unit the time (t)is proportional to the radius of the rube (r), the pressure pushing the fluiddown the pipe (P), the size of the tube (l), and also the viscosity that thefluid (



Viscosity is report in units dubbed poise (pronounced "pwahz").The viscosity that water in ~ room temperature is approximately 1 centipoise, or 1 cP. Gasoline hasa viscosity in between 0.4 and also 0.5 cP; the viscosity of waiting is 0.018 cP.

Because the molecules closest come the wall surfaces of a small-diameter tube adhere come theglass, viscosity actions the rate at which molecules in the middle of the currently ofliquid or gas flow past this external layer of more or less stationary molecules. Viscositytherefore depends on any type of factor that deserve to influence the ease with which molecule slip pasteach other. Liquids have tendency to become more viscous together the molecules end up being larger, or as theamount the intermolecular bonding increases. They end up being less viscous as the temperatureincreases. The viscosity of water, because that example, decreases indigenous 1.77 cP in ~ 0oCto 0.28 cP at 100oC.

Hydrogen Bonding and also the AnomalousProperties that Water

We are so acquainted with the nature of water that it is daunting to evaluate theextent come which its habits is unusual. most solids increase when castle melt. Water increases when that freezes. most solids are much more dense 보다 the matching liquids. Ice cream (0.917 g/cm3) is no as dense as water. Water has a melting allude at least 100oC greater than intended on the communication of the melting points of H2S, H2Se, and also H2Te. Water has a boiling suggest almost 200oC higher than intended from the boiling points of H2S, H2Se, and H2Te. Water has actually the largest surface anxiety of any type of common liquid other than liquid mercury. Water has actually an unusually big viscosity. Water is great solvent. It deserve to dissolve compounds, such as NaCl, that space insoluble or only slightly soluble in various other liquids. Water has an uncommonly high heat capacity. It takes an ext heat come raise the temperature that 1 gram of water through 1oC than any other liquid. This anomalous nature all result from the strong intermolecular bond in water.Water is ideal described as a polarmolecule in which over there is a partial separation of fee to provide positive andnegative poles. The pressure of attraction in between a positively charged hydrogen atom ~ above onewater molecule and also the negatively fee oxygen atom on one more gives climb to anintermolecular bond, as presented in the number below. This dipole-dipole communication betweenwater molecule is well-known as a hydrogen bond.


Hydrogen bonds room separated native other instances of van der Waals forces since theyare unusually strong: 10-12 kJ/mol. The hydrogen bond in water are an especially importantbecause of the dominant duty that water plays in the sirhenryjones-museums.orgistry of life systems. Hydrogenbonds room not restricted to water, however.

Hydrogen-bond donors incorporate substances the contain fairly polar H-Xbonds, such as NH3, H2O, and HF. Hydrogen-bond acceptors includesubstances that have actually nonbonding bag of valence electrons. The H-X bond have to bepolar to produce the partial hopeful charge top top the hydrogen atom that allows dipole-dipoleinteractions to exist. As the X atom in the H-X bond becomes lesselectronegative, hydrogen bonding in between molecules becomes less important. Hydrogenbonding in HF, because that example, is much stronger 보다 in either H2O or HCl.

The hydrogen bonds between water molecule in ice produce the open up structure displayed inthe figure below. As soon as ice melts, several of these bonds are broken, and also this structurecollapses to kind a liquid the is about 10% denser. This unusual property of water hasseveral vital consequences. The development of water once it freezes is responsible forthe cracking of concrete, which forms potholes in streets and highways. However it additionally meansthat ice floats on top of rivers and streams. The ice cream that develops each winter therefore hasa opportunity to melt during the summer.

The framework of ice. Keep in mind that the hydrogen atoms room closer to among the oxygen atoms than the various other in each of the hydrogen bonds.

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The figure below shows another repercussion of the strength of the hydrogen bonds inwater. There is a steady boost in boiling allude in the series CH4, GeH4,SiH4, and also SnH4. The boiling clues of H2O and also HF,however, space anomalously large because the the strong hydrogen bonds in between molecules inthese liquids. If this doesn"t it seems ~ important, try to imagine what life would be choose ifwater boiled at -80oC.

The surface tension and viscosity of water are also related come the stamin of thehydrogen bonds between water molecules. The surface anxiety of water is responsible forthe capillary activity that bring water up v the root equipment of plants. The is alsoresponsible because that the efficiency with i m sorry the wax the coats the surface ar of pipeline canprotect tree from too much loss that water v evaporation.

The unusually large heat volume of water is likewise related come the toughness of thehydrogen bonds between water molecules. Noþeles that increases the motion of watermolecules, and also therefore the temperature that water, must interfere v the hydrogen bondsbetween these molecules. The fact that that takes for this reason much energy to disrupt these bondsmeans that water have the right to store enormous quantities of heat energy. Back the water in lakesand rivers it s okay warmer in the summer and cooler in the winter, the large heat volume ofwater limits the variety of temperatures that would certainly otherwise threaten the life thatflourishes in this environment. The warmth capacity the water is additionally responsible because that theocean"s ability to act as a thermal reservoir that moderates the swings in temperaturethat take place from winter come summer.