Although you have been presented to few of the interactions that hold molecules together in a liquid, we have not yet debated the results of those interactions because that the bulk properties of liquids. We currently turn our fist to three distinct properties that liquids the intimately count on the nature of intermolecular interactions: surface tension, capillary action, and viscosity.

You are watching: Why do substances with high surface tension also tend to have high viscosities?


Surface Tension

We proclaimed in section 11.1 "The Kinetic Molecular summary of Liquids" that liquids tend to adopt the shapes of your containers. Why, then, do little amounts the water on a fresh waxed car form raised droplets instead of a thin, continuous film? The prize lies in a property called surface tension, which counts on intermolecular forces.

Figure 11.9 "A representation of Surface anxiety in a Liquid" presents a microscopic view of a fluid droplet. A usual molecule in the interior of the droplet is surrounding by other molecules the exert attractive pressures from all directions. Consequently, there is no net force on the molecule that would cause it to relocate in a certain direction. In contrast, a molecule on the surface experiences a net attraction toward the drop since there room no molecules on the external to balance the forces exerted by surrounding molecules in the interior. Since a sphere has actually the smallest feasible surface area for a given volume, intermolecular attractive interactions between water molecules cause the droplet to adopt a spherical shape. This maximizes the variety of attractive interactions and also minimizes the number of water molecule at the surface. For this reason raindrops are virtually spherical, and also drops of water top top a waxed (nonpolar) surface, which does not connect strongly with water, form round beads (see the chapter opener photo). A dirty auto is extended with a mixture that substances, several of which room polar. Attractive interactions in between the polar substances and also water cause the water to spread out right into a thin film rather of creating beads.


Figure 11.9 A representation of Surface stress in a Liquid

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Molecules at the surface ar of water suffer a network attraction to various other molecules in the liquid, i beg your pardon holds the surface of the mass sample together. In contrast, those in the inner experience uniform attractive forces..


The same phenomenon holds molecules with each other at the surface of a bulk sample of water, almost as if they formed a skin. When filling a glass through water, the glass deserve to be overfilled so the the level that the liquid actually extends above the rim. Similarly, a sewing needle or a paper clip have the right to be placed on the surface of a glass that water wherein it “floats,” even though steel is much denser 보다 water (part (a) in number 11.10 "The results of the High Surface tension of fluid Water"). Many insects take benefit of this residential or commercial property to to walk on the surface of swimming pool or ponds without sinking (part (b) in number 11.10 "The effects of the High Surface stress and anxiety of liquid Water").


Figure 11.10 The impacts of the High Surface tension of liquid Water

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(a) A paper clip can “float” ~ above water due to the fact that of surface tension. (b) Surface anxiety also enables insects such together this water strider to “walk top top water.”


Such phenomena space manifestations of surface tensionThe power required to boost the surface ar area that a liquid by a particular amount. Surface tension is measure up in devices of power per area (e.g., J/m2)., i m sorry is characterized as the energy required to increase the surface area that a fluid by a particular amount. Surface stress and anxiety is as such measured as power per unit area, such as joules per square meter (J/m2) or dyne every centimeter (dyn/cm), wherein 1 dyn = 1 × 10−5 N. The values of the surface tension of part representative liquids are provided in Table 11.4 "Surface Tension, Viscosity, Vapor push (at 25°C uneven Otherwise Indicated), and Normal Boiling point out of common Liquids". Note the correlation between the surface anxiety of a liquid and the toughness of the intermolecular forces: the stronger the intermolecular forces, the greater the surface tension. For example, water, v its solid intermolecular hydrogen bonding, has one that the highest surface tension values of any liquid, whereas low-boiling-point necessary molecules, i beg your pardon have reasonably weak intermolecular forces, have much reduced surface tensions. Mercury is an obvious anomaly, however its really high surface tension is due to the visibility of solid metallic bonding, which we will comment on in much more detail in thing 12 "Solids".


Table 11.4 surface ar Tension, Viscosity, Vapor press (at 25°C uneven Otherwise Indicated), and also Normal Boiling point out of usual Liquids

problem Surface anxiety (× 10−3 J/m2) Viscosity (mPa·s) Vapor push (mmHg) typical Boiling suggest (°C)
Organic Compounds
diethyl ether 17 0.22 531 34.6
n-hexane 18 0.30 149 68.7
acetone 23 0.31 227 56.5
ethanol 22 1.07 59 78.3
ethylene glycol 48 16.1 ~0.08 198.9
Liquid Elements
bromine 41 0.94 218 58.8
mercury 486 1.53 0.0020 357
Water
0°C 75.6 1.79 4.6
20°C 72.8 1.00 17.5
60°C 66.2 0.47 149
100°C 58.9 0.28 760

Adding soaps and detergents that disrupt the intermolecular attractions between adjacent water molecules can reduce the surface stress and anxiety of water. Due to the fact that they impact the surface ar properties that a liquid, soaps and also detergents are called surface-active agents, or surfactantsSubstances (surface-active agents), such as soaps and also detergents, that disrupt the attractive intermolecular interactions in between molecules that a polar liquid, thereby reducing the surface stress of the liquid.. In the 1960s, united state Navy researchers occurred a method of fighting fires aboard plane carriers making use of “foams,” which room aqueous remedies of fluorinated surfactants. The surfactants mitigate the surface stress of water below that of fuel, so the fluorinated systems is able come spread across the burn surface and also extinguish the fire. Together foams are now used universally to fight large-scale fires of organic liquids.


Capillary Action

Intermolecular forces additionally cause a phenomenon referred to as capillary actionThe propensity of a polar fluid to rise versus gravity right into a small-diameter glass tube., which is the tendency of a polar fluid to rise against gravity right into a small-diameter pipe (a capillary), as displayed in figure 11.11 "The Phenomenon the Capillary Action". When a glass capillary is put right into a food of water, water is drawn up into the tube. The height to i beg your pardon the water rises counts on the diameter of the tube and the temperature of the water but not on the edge at i beg your pardon the pipe enters the water. The smaller the diameter, the greater the liquid rises.


Figure 11.11 The Phenomenon of Capillary Action

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When a glass capillary is placed in liquid water, water rises up into the capillary. The smaller the diameter that the capillary, the greater the water rises. The elevation of the water does not count on the angle at i m sorry the capillary is tilted.


Capillary action is the net an outcome of two opposing sets of forces: cohesive forcesThe intermolecular forces that organize a fluid together., which space the intermolecular pressures that host a liquid together, and adhesive forcesThe attractive intermolecular forces between a liquid and also the substance consisting of the surface ar of a capillary., which space the attractive forces in between a liquid and the substance the composes the capillary. Water has both solid adhesion come glass, which includes polar SiOH groups, and strong intermolecular cohesion. When a glass capillary is put right into water, the surface ar tension as result of cohesive pressures constricts the surface ar area the water within the tube, if adhesion in between the water and also the glass create an upward pressure that maximizes the quantity of glass surface ar in call with the water. If the adhesive forces are stronger than the cohesive forces, as is the situation for water, climate the fluid in the capillary rises to the level whereby the downward force of gravity precisely balances this increase force. If, however, the cohesive forces are more powerful than the adhesive forces, together is the case for mercury and glass, the liquid pulls chin down right into the capillary listed below the surface of the mass liquid come minimize call with the glass (part (a) in number 11.12 "The results of Capillary Action"). The upper surface ar of a liquid in a tube is referred to as the meniscusThe upper surface of the liquid in a tube., and the shape of the meniscus depends on the relative strengths the the cohesive and also adhesive forces. In liquids such as water, the meniscus is concave; in liquids such together mercury, however, which have actually very solid cohesive forces and weak adhesion come glass, the meniscus is convex (part (b) in figure 11.12 "The impacts of Capillary Action").


Note the Pattern

Polar building material are attracted up a glass capillary and also generally have actually a concave meniscus.


Figure 11.12 The results of Capillary Action

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(a) This drawing illustrates the shape of the meniscus and the relative elevation of a mercury shaft when a glass capillary is put right into liquid mercury. The meniscus is convex and the surface ar of the liquid inside the pipe is lower than the level the the liquid outside the tube. (b) due to the fact that water adheres strongly to the polar surface ar of glass, it has actually a concave meniscus, conversely, mercury, which does not adhere to the glass, has a convex meniscus.


Fluids and also nutrients room transported up the trunk of plants or the trunks of tree by capillary action. Plants contain small rigid tubes created of cellulose, to which water has strong adhesion. Because of the strong adhesive forces, nutrients can be transported native the root to the tops of trees that are an ext than 50 m tall. Noodle towels are also made the cellulose; they absorb water because the tiny tubes act favor capillaries and also “wick” the water away from your skin. The humidity is soaked up by the entire fabric, not simply the layer in call with her body.


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Viscosity (η)The resistance the a liquid to flow. Is the resistance of a fluid to flow. Some liquids, such as gasoline, ethanol, and also water, flow an extremely readily and also hence have actually a low viscosity. Others, such as motor oil, molasses, and also maple syrup, flow very slowly and also have a high viscosity. The two most usual methods for analyzing the viscosity that a liquid room (1) to measure the time it takes because that a quantity of liquid to flow through a narrow vertical tube and (2) come measure the time it takes steel balls to loss through a given volume of the liquid. The greater the viscosity, the slow the liquid flows v the tube and also the steel balls fall. Viscosity is to express in devices of the poise (mPa·s); the greater the number, the higher the viscosity. The viscosities of some representative liquids are listed in Table 11.4 "Surface Tension, Viscosity, Vapor pressure (at 25°C unless Otherwise Indicated), and also Normal Boiling clues of typical Liquids" and show a correlation in between viscosity and also intermolecular forces. Since a fluid can flow only if the molecules can move previous one one more with minimal resistance, solid intermolecular attractive forces make the more challenging for molecule to relocate with respect come one another. The addition of a second hydroxyl group to ethanol, because that example, which produces ethylene glycol (HOCH2CH2OH), boosts the viscosity 15-fold. This impact is as result of the increased number of hydrogen bonds the can form between hydroxyl groups in nearby molecules, result in considerably stronger intermolecular attractive forces.

There is additionally a correlation in between viscosity and molecular shape. Liquids consisting of long, flexible molecules have tendency to have higher viscosities 보다 those written of much more spherical or shorter-chain molecules. The much longer the molecules, the easier it is because that them to come to be “tangled” with one another, making the more complicated for lock to relocate past one another. London dispersion forces additionally increase v chain length. Because of a mix of these 2 effects, long-chain hydrocarbons (such together motor oils) are extremely viscous.


Motor oils and other lubricants demonstrate the practical importance of regulating viscosity. The oil in an automobile engine must successfully lubricate under a wide selection of conditions, indigenous subzero starting temperatures come the 200°C that oil have the right to reach in one engine in the warm of the Mojave Desert in August. Viscosity decreases rapidly with increasing temperatures because the kinetic power of the molecule increases, and greater kinetic energy permits the molecule to get rid of the attractive pressures that avoid the fluid from flowing. Together a result, an oil that is thin sufficient to it is in a good lubricant in a cold engine will end up being too “thin” (have as well low a viscosity) to be effective at high temperatures. The viscosity the motor oil is described by an SAE (Society the Automotive Engineers) rating ranging from SAE 5 come SAE 50 for engine oils: the lower the number, the lower the viscosity. So-called single-grade oils deserve to cause major problems. If they are viscous enough to occupational at high operation temperatures (SAE 50, for example), climate at low temperatures, they can be therefore viscous that a auto is complicated to start or an engine is not effectively lubricated. Consequently, most contemporary oils space multigrade, v designations such as SAE 20W/50 (a grade offered in high-performance sporting activities cars), in which case the oil has actually the viscosity of one SAE 20 oil in ~ subzero temperature (hence the W because that winter) and the viscosity of an SAE 50 oil in ~ high temperatures. These properties are completed by a careful blend of additives that modulate the intermolecular interaction in the oil, thereby controlling the temperature dependence of the viscosity. Plenty of of the commercially easily accessible oil additive “for enhanced engine performance” are extremely viscous products that boost the viscosity and also effective SAE rating the the oil, but overusing these additives can reason the same problems experienced with very viscous single-grade oils.


Example 5

Based on the nature and also strength that the intermolecular cohesive forces and also the probable nature that the liquid–glass adhesive forces, suspect what will occur when a glass capillary is put into a manufacturer of SAE 20 engine oil. Will certainly the oil be traction up right into the tube by capillary action or thrust down below the surface ar of the fluid in the beaker? What will be the form of the meniscus (convex or concave)? (Hint: the surface ar of glass is lined v Si–OH groups.)

Given: substance and composition that the glass surface

Asked for: behavior that oil and the form of meniscus

Strategy:

A recognize the cohesive pressures in the motor oil.

B identify whether the forces connect with the surface of glass. Indigenous the toughness of this interaction, guess the actions of the oil and the form of the meniscus.

Solution:

A engine oil is a nonpolar liquid consisting largely of hydrocarbon chains. The cohesive pressures responsible because that its high boiling allude are almost solely London dispersion forces between the hydrocarbon chains. B together a liquid cannot form strong interactions with the polar Si–OH teams of glass, so the surface of the oil inside the capillary will be reduced than the level of the liquid in the beaker. The oil will have actually a convex meniscus similar to that of mercury.

Exercise

Predict what will occur when a glass capillary is put right into a manufacturer of ethylene glycol. Will the ethylene glycol be pulled up right into the pipe by capillary activity or propelled down listed below the surface of the fluid in the beaker? What will certainly be the form of the meniscus (convex or concave)?

Answer: Capillary action will pull the ethylene glycol up into the capillary. The meniscus will be concave.


Summary

Surface tension is the power required to boost the surface area the a liquid by a offered amount. The more powerful the intermolecular interactions, the better the surface tension. Surfactants space molecules, such together soaps and detergents, that mitigate the surface stress and anxiety of polar liquids like water. Capillary action is the phenomenon in i m sorry liquids rise up into a small tube dubbed a capillary. The results once cohesive forces, the intermolecular forces in the liquid, room weaker than adhesive forces, the attraction in between a liquid and also the surface ar of the capillary. The form of the meniscus, the upper surface ar of a fluid in a tube, additionally reflects the balance between adhesive and also cohesive forces. The viscosity the a fluid is that is resistance to flow. Liquids the have solid intermolecular forces tend to have high viscosities.


Key Takeaway

surface ar tension, capillary action, and viscosity are unique properties of liquids that count on the nature that intermolecular interactions.

How is the atmosphere of molecules on the surface of a liquid droplet various from that of molecule in the internal of the droplet? exactly how is this difference related come the principle of surface ar tension?


A mosquito is able come walk throughout water there is no sinking, but if a couple of drops of laundry detergent are included to the water, the insect will sink. Why?


Explain just how soaps or surfactants decrease the surface stress and anxiety of a liquid. Just how does the meniscus of one aqueous equipment in a capillary readjust if a surfactant is added? illustrate your answer with a diagram.


Of CH2Cl2, hexane, and ethanol, which has the lowest viscosity? Which has actually the highest possible surface tension? describe your reasoning in every case.


At 25°C, cyclohexanol has a surface tension of 32.92 mN/m2, conversely, the surface stress and anxiety of cyclohexanone, i beg your pardon is very similar chemically, is just 25.45 mN/m2. Why is the surface anxiety of cyclohexanone for this reason much less than the of cyclohexanol?


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What is the partnership between

surface tension and temperature? viscosity and temperature?

Explain your answers in regards to a microscopic picture.


What two opposing pressures are responsible for capillary action? just how do these forces determine the shape of the meniscus?


Which of the complying with liquids will have actually a concave meniscus in a glass capillary? define your reasoning.

pentane diethylene glycol (HOCH2CH2OCH2CH2OH) carbon tetrachloride

Adding a soap or a surfactant to water disrupts the attractive intermolecular interactions between water molecules, in order to decreasing the surface ar tension. Because water is a polar molecule, one would expect that a soap or a surfactant would additionally disrupt the attractive interaction responsible because that adhesion of water to the surface of a glass capillary. As presented in the sketch, this would certainly decrease the height of the water pillar inside the capillary, and also making the meniscus much less concave.


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As the frameworks indicate, cyclohexanol is a polar substance that can engage in hydrogen bonding, much like methanol or ethanol; consequently, that is intended to have a greater surface tension due to stronger intermolecular interactions.


Cohesive pressures are the intermolecular pressures that host the molecule of the liquid together, while adhesive forces are the attractive forces between the molecules of the liquid and the walls of the capillary. If the adhesive forces are more powerful than the cohesive forces, the liquid is traction up into the capillary and the meniscus is concave. Whereas if the cohesive forces are stronger than the adhesive forces, the level that the liquid within the capillary will certainly be reduced than the level exterior the capillary, and the meniscus will be convex.


Viscous substances frequently consist of molecule that room much much longer than castle are vast and whose frameworks are frequently rather flexible. Together a result, the molecules often tend to become tangled with one an additional (much prefer overcooked spaghetti), which decreases the rate at i m sorry they can move through the liquid.


The viscosities of 5 liquids in ~ 25°C are given in the complying with table. Describe the observed patterns in viscosity.


link Molecular Formula Viscosity (mPa·s)
benzene C6H6 0.604
aniline C6H5NH2 3.847
1,2-dichloroethane C2H4Cl2 0.779
heptane C7H16 0.357
1-heptanol C7H15OH 5.810

The adhering to table provides values for the viscosity, boiling point, and surface anxiety of four substances. Examine these data very closely to check out whether the data because that each compound room internally constant and allude out any type of obvious errors or inconsistencies. Describe your reasoning.

See more: Which One Of The Following Items Is Not Generally Used In Preparing A Statement Of Cash Flows


compound Viscosity (mPa·s at 20°C) Boiling suggest (°C) Surface tension (dyn/cm in ~ 25°C)
A 0.41 61 27.16
B 0.55 65 22.55
C 0.92 105 36.76
D 0.59 110 28.53

Surface stress and anxiety data (in dyn/cm) for propanoic acid (C3H6O2), and also 2-propanol (C3H8O), as a duty of temperature, are provided in the adhering to table. Plot the data because that each compound and also explain the differences in between the 2 graphs. Based on these data, i beg your pardon molecule is much more polar?


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The plots that surface tension versus temperature for propionic acid and also isopropanol have basically the very same slope, however at every temperatures the surface stress of propionic mountain is around 30% better than because that isopropanol. Due to the fact that surface tension is a measure of the cohesive forces in a liquid, these data imply that the cohesive pressures for propionic mountain are substantially greater 보다 for isopropanol. Both substances covers polar molecules with similar molecular masses, and also the most necessary intermolecular interactions are likely to be dipole–dipole interactions. Consequently, these data imply that propionic acid is more polar 보다 isopropanol.