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Critical Temperature and critical Pressure Surface Tension
Viscosity Hydrogen Bonding & the Anomalous nature of Water

Critical Temperature and also CriticalPressure

The obvious method to turn a gas right into a fluid is come cool it come a temperature listed below itsboiling point. There is another method of condensing a gas to kind a liquid, however, whichinvolves increasing the pressure on the gas. Liquids cook at the temperature at which thevapor pressure is equal to the push on the liquid from the surroundings. Raising thepressure on a gas because of this effectively rises the boiling suggest of the liquid.

Suppose the we have water vapor (or steam) in a closeup of the door container at 120oCand 1 atm. Because the temperature the the system is over the typical boiling point of water,there is no reason for the vapor to condensation to type a liquid. Nothing happens as weslowly compress the container therebyraising the pressure on the gas till thepressure reaches 2 atm. In ~ this point, the device is in ~ the boiling suggest of water, andsome of the gas will condense to form a liquid. As shortly as the press on the gas exceeds2 atm, the vapor pressure of water at 120oC is no longer big enough for theliquid come boil. The gas thus condenses to form a liquid, as shown in the figurebelow.


In theory, we should have the ability to predict the pressure at i m sorry a gas condenses at 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 critical temperature, the gas can"t becondensed, regardless of the press applied.

The presence of a critical temperature was discovered by thomas Andrews in 1869 whilestudying the result of temperature and also pressure ~ above the behavior of carbon dioxide. Andrewsfound that he could condense CO2 gas right into a fluid by elevating the pressure onthe gas, as long as he kept the temperature listed below 31.0oC. At 31.0oC,for example, it takes a press of 72.85 atm to liquify CO2 gas. Andrews foundthat it was difficult to turn CO2 right into a liquid above this temperature, nomatter how much press was applied.

Gases can"t be liquified at temperatures above the critical temperature due to the fact that at thispoint the nature of gases and liquids end up being the same, and also there is no communication on whichto distinguish between gases and liquids. The vapor pressure of a fluid at the criticaltemperature is called the critical push (Pc). The vaporpressure the a liquid never ever gets bigger than this critical pressure.

The an important temperatures, an essential pressures, and also boiling point out of a number of gasesare offered in the table below. There is an evident correlation in between the criticaltemperature and also boiling allude of this gases. This properties room related due to the fact that theyare both indirect steps of the force of attraction between particles in the gas phase.

Critical Temperatures, an important Pressures and Boiling point out 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 that the an important temperature and also pressure the a substance can beused to calculation the a and b constants in the van der Waalsequation.


Surface Tension

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


There is likewise a pressure of adhesion (literally, "sticking")between a liquid and the walls of the container. As soon as the pressure of adhesion is an ext thanhalf as large as the pressure of cohesion between the liquid molecules, the fluid is said to"wet" the solid. A an excellent example the this phenomenon is the wetting of file bywater. The pressure of adhesion between file and water an unified with the force of cohesionbetween water molecules explains why sheets that wet paper stick together.

Water wets glass because of the pressure of adhesion that outcomes from interactionsbetween the positive ends the the polar water molecules and the negatively fee oxygenatoms in glass. Together a result, water forms a meniscus that curves upward ina small-diameter glass tube, as displayed in the number below. (The hatchet meniscuscomes indigenous the Greek word because that "moon" and also is provided to explain anything that has actually acrescent shape.) The meniscus the water forms in a buret outcomes from a balance betweenthe pressure of adhesion pulling up on the obelisk of water to wet the walls of the glass tubeand the pressure of heaviness pulling under on the liquid.

Water climbs the wall surfaces of a small-diameter tube to type a meniscus the curves upward, whereas mercury forms a meniscus that curves downward.

The pressure of adhesion between water and wax is very small compared come theforce that cohesion between water molecules. Together a result, rain doesn"t adhere towax. It tends to type beads, or drops, v the smallest possible surface area, therebymaximizing the force of cohesion in between the water molecules. The very same thing wake up whenmercury is spilled on glass or poured into a small glass tube. The force of cohesionbetween mercury atoms is so much larger than the force of adhesion between mercury andglass that the area the contact 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 of the resistance come flow. Engine oils room moreviscous than gasoline, because that example, and also the maple syrup used on pancakes is more viscousthan the vegetables oils used in salad dressings.

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



Viscosity is report in units called poise (pronounced "pwahz").The viscosity of water at room temperature is roughly 1 centipoise, or 1 cP. Gasoline hasa viscosity between 0.4 and 0.5 cP; the viscosity of wait is 0.018 cP.

Because the molecules closest to the walls of a small-diameter tube adhere to theglass, viscosity measures the price at which molecule in the middle of the stream ofliquid or gas circulation past this external layer of much more or less stationary molecules. Viscositytherefore relies on any factor that deserve to influence the ease through which molecule slip pasteach other. Liquids tend to become an ext viscous as 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 the AnomalousProperties that Water

We are so familiar with the nature of water the it is daunting to appreciate theextent to which its behavior is unusual. most solids expand when they melt. Water expands when the freezes. many solids are more dense than the equivalent liquids. Ice (0.917 g/cm3) is not as dense as water. Water has actually a melting point at least 100oC greater than supposed on the communication of the melting points of H2S, H2Se, and H2Te. Water has a boiling point almost 200oC greater than intended from the boiling point out of H2S, H2Se, and also H2Te. Water has the biggest surface anxiety of any type of common liquid except liquid mercury. Water has an unusually huge viscosity. Water is great solvent. It deserve to dissolve compounds, such as NaCl, that are insoluble or only slightly dissolve in other liquids. Water has an abnormally high heat capacity. The takes an ext heat to raise the temperature of 1 gram that water through 1oC than any other liquid. These anomalous properties 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 charge to give positive andnegative poles. The force of attraction between a positively fee hydrogen atom on onewater molecule and also the negatively charged oxygen atom on an additional gives climb to anintermolecular bond, as displayed in the number below. This dipole-dipole communication betweenwater molecule is recognized as a hydrogen bond.


Hydrogen bonds room separated from other examples of van der Waals forces due to the fact that theyare unusually strong: 10-12 kJ/mol. The hydrogen bonds in water are an especially importantbecause that the dominant function that water dram in the 4476mountvernon.comistry of living systems. Hydrogenbonds room not restricted to water, however.

Hydrogen-bond donors encompass substances the contain fairly polar H-Xbonds, such together NH3, H2O, and also HF. Hydrogen-bond acceptors includesubstances that have actually nonbonding pairs of valence electrons. The H-X bond need to bepolar to create the partial confident charge top top the hydrogen atom that enables dipole-dipoleinteractions come exist. As the X atom in the H-X bond i do not care lesselectronegative, hydrogen bonding between molecules becomes less important. Hydrogenbonding in HF, for example, is much stronger than in one of two people H2O or HCl.

The hydrogen bonds between water molecules in ice create the open structure shown inthe number below. Once ice melts, few of these bonds are broken, and also this structurecollapses to form a liquid the is about 10% denser. This unusual residential property of water hasseveral important consequences. The expansion of water once it freezes is responsible forthe cracked of concrete, which develops potholes in streets and highways. However it also meansthat ice floats on top of rivers and also streams. The ice that forms each winter as such hasa opportunity to melt during the summer.

The structure of ice. Note that the hydrogen atoms are closer to one of the oxygen atoms than the various other in each of the hydrogen bonds.

See more: Wallis Currie-Wood Measurements, Wallis Currie

The figure listed below shows another repercussion of the stamin of the hydrogen binding inwater. There is a steady boost in boiling point in the series CH4, GeH4,SiH4, and also SnH4. The boiling point out of H2O and also HF,however, space anomalously big because of the solid hydrogen bonds in between molecules inthese liquids. If this doesn"t seem important, try to imagine what life would be choose ifwater boiled at -80oC.

The surface ar tension and viscosity the water are also related come the toughness of thehydrogen bonds in between water molecules. The surface stress and anxiety of water is responsible forthe capillary activity that brings water up v the root equipment of plants. It is alsoresponsible for the performance with i beg your pardon the wax the coats the surface of leaves canprotect tree from too much loss the water through evaporation.

The unusually large heat capacity of water is also related come the toughness of thehydrogen bonds between water molecules. Noþeles that rises the activity of watermolecules, and also therefore the temperature of water, have to interfere with the hydrogen bondsbetween these molecules. The fact that that takes therefore much energy to disrupt this bondsmeans that water deserve to store enormous quantities of heat energy. Return the water in lakesand rivers gets warmer in the summer and also cooler in the winter, the large heat capacity ofwater boundaries the variety of temperatures that would otherwise threaten the life thatflourishes in this environment. The warmth capacity the water is additionally responsible because that theocean"s capacity to act together a thermal reservoir that moderates the swings in temperaturethat happen from winter to summer.