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Figure \(\PageIndex{4}\): Differences in the relative strengths of cohesive and adhesive forces result in different meniscus shapes for mercury (left) and water (right) in glass tubes. The data were obtained with a differential capillary-rise method and cover the entire liquid range from the triple-point temperature to the critical temperature. The various IMFs between identical molecules of a substance are examples of cohesive forces. Surface tension, capillarity, surface charge Exercise 1:1 At 20 C the surface tensions for water and n-octane are 72.8 mN/m and 21.8 mN/m, respectively, and the interfacial tension 50.8 mN/m. The water molecules are also attracted to each other, so large amounts of water are drawn up the cellulose fibers. Textbook content produced by OpenStax College is licensed under a Creative Commons Attribution License 4.0 license. Larger drops are more greatly affected by gravity, air resistance, surface interactions, and so on, and as a result, are less spherical. Have feedback to give about this text? This procedure can be easily done because of capillary action, the ability of a liquid to flow up a small tube against gravity, as shown in Figure \(\PageIndex{7}\). However, the molecules on the surface of a liquid are attracted only by about one-half as many molecules. 7.1: Surface Tension, Viscosity, and Capillary Action, [ "article:topic", "showtoc:no", "license:ccby" ]. Because of the unbalanced molecular attractions on the surface molecules, liquids contract to form a shape that minimizes the number of molecules on the surface—that is, the shape with the minimum surface area. Missed the LibreFest? (credit: Mark Ott). The intermolecular forces between molecules in the liquid state vary depending upon their chemical identities and result in corresponding variations in various physical properties. Click here. Figure \(\PageIndex{6}\): Depending upon the relative strengths of adhesive and cohesive forces, a liquid may rise (such as water) or fall (such as mercury) in a glass capillary tube. As Table \(\PageIndex{1}\) shows, the more structurally complex are the molecules in a liquid and the stronger the IMFs between them, the more difficult it is for them to move past each other and the greater is the viscosity of the liquid. Water molecules are attracted to these −OH groups and form hydrogen bonds with them, which draws the H2O molecules up the cellulose molecules. We can measure viscosity by measuring the rate at which a metal ball falls through a liquid (the ball falls more slowly through a more viscous liquid) or by measuring the rate at which a liquid flows through a narrow tube (more viscous liquids flow more slowly). If the adhesive forces between water molecules and the molecules of the surface are weak compared to the cohesive forces between the water molecules, the water does not “wet” the surface. This is the situation for water rising in a glass tube. We will not concern ourselves with calculating capillary height in this course. c) The spreading coefficient for n-octane on water. Video \(\PageIndex{2}\): Ice floating in water is one of the unique properties of water. Honey, syrup, motor oil, and other liquids that do not flow freely, like those shown in Figure \(\PageIndex{1}\), have higher viscosities.

Equations for calculating the surface tension of these n-alkanes as a function of temperature are presented. On the other hand, the cohesive forces between mercury atoms are much greater than the adhesive forces between mercury and glass. A small drop of liquid tends to assume a spherical shape, as shown in Figure \(\PageIndex{2}\), because in a sphere, the ratio of surface area to volume is at a minimum. For example, water does not wet waxed surfaces or many plastics such as polyethylene. When the weight of the liquid in the tube generates a downward force equal to the upward force associated with capillary action, the liquid stops rising. Figure \(\PageIndex{7}\):: Blood is collected for medical analysis by capillary action, which draws blood into a small diameter glass tube. Figure \(\PageIndex{1}\): (a) Honey and (b) motor oil are examples of liquids with high viscosities; they flow slowly. The extent of the rise (or fall) is directly proportional to the surface tension of the liquid and inversely proportional to the density of the liquid and the radius of the tube. It is partly by capillary action occurring in plant cells called xylem that water and dissolved nutrients are brought from the soil up through the roots and into a plant. Capillary action is the basis for thin layer chromatography, a laboratory technique commonly used to separate small quantities of mixtures. (credit a: modification of work by Scott Bauer; credit b: modification of work by David Nagy). As a result of this high surface tension, the surface of water represents a relatively “tough skin” that can withstand considerable force without breaking. When water is confined in a glass tube, its meniscus (surface) has a concave shape because the water wets the glass and creeps up the side of the tube. A steel needle carefully placed on water will float. Unit 7: Intermolecular and Intramolecular Forces in Action, 7.1: Surface Tension, Viscosity, and Capillary Action (Problems), http://cnx.org/contents/85abf193-2bd...a7ac8df6@9.110, Distinguish between adhesive and cohesive forces, Define viscosity, surface tension, and capillary rise, Describe the roles of intermolecular attractive forces in each of these properties/phenomena, Adelaide Clark, Oregon Institute of Technology, Crash Course Chemistry: Crash Course is a division of. Watch the recordings here on Youtube! The LibreTexts libraries are Powered by MindTouch® and are supported by the Department of Education Open Textbook Pilot Project, the UC Davis Office of the Provost, the UC Davis Library, the California State University Affordable Learning Solutions Program, and Merlot. Most cloth towels are made of cotton, and paper towels are generally made from paper pulp. But when you pour syrup on pancakes or add oil to a car engine, you note that syrup and motor oil do not flow as readily. b) The cohesion for both liquids. If you place one end of a paper towel in spilled wine, as shown in Figure \(\PageIndex{5}\), the liquid wicks up the paper towel. Water spreads out on glass because the adhesive force between water and glass is greater than the cohesive forces within the water. - For surface tension components (e.g. Figure \(\PageIndex{3}\): Surface tension (right) prevents this insect, a “water strider,” from sinking into the water. The liquid itself is held together by its own cohesive forces. Unless otherwise noted, LibreTexts content is licensed by CC BY-NC-SA 3.0. For more information contact us at info@libretexts.org or check out our status page at https://status.libretexts.org. Video \(\PageIndex{1}\): An overview of intermolecular forces in action as surface tension, viscosity, and capillary action. In the present study, the simultaneous and accurate determination of liquid viscosity and surface tension of the n-alkanes n-hexane (n-C6H14), n-octane (n-C8H18), n-decane (n-C10H22), and n-hexadecane (n-C16H34) by surface light scattering (SLS) in thermodynamic equilibrium is demonstrated. The IMFs between the molecules of a liquid, the size and shape of the molecules, and the temperature determine how easily a liquid flows. (credit photo: modification of work by Mark Blaser). Download for free at http://cnx.org/contents/85abf193-2bd...a7ac8df6@9.110). Legal. The smaller the diameter of the tube is, the higher the liquid climbs. octane: C 8 H 18: 21.14: ethylene glycol: CH 2 (OH)CH 2 (OH) 47.99: Among common liquids, water exhibits a distinctly high surface tension due to strong hydrogen bonding between its molecules. As the temperature increases, the molecules move more rapidly and their kinetic energies are better able to overcome the forces that hold them together; thus, the viscosity of the liquid decreases.

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