Distinguish in between adhesive and also cohesive forcesDefine viscosity, surface tension, and also capillary riseDescribe the roles of intermolecular attractive forces in each of these properties/phenomena

When you to water a glass the water, or to fill a automobile with gasoline, girlfriend observe that water and gasoline circulation freely. Yet when you pour syrup on pancakes or add oil to a vehicle engine, you keep in mind that syrup and motor oil carry out not flow as readily. The viscosity the a fluid is a measure up of the resistance to flow. Water, gasoline, and other liquids that circulation freely have actually a short viscosity. Honey, syrup, motor oil, and other liquids that execute not flow freely, like those presented in figure 1, have greater viscosities. We deserve to measure viscosity by measuring the rate at i beg your pardon a metal ball drops through a fluid (the round falls more slowly v a more viscous liquid) or by measure the price at i beg your pardon a liquid flows with a small tube (more viscous liquids flow more slowly).

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Figure 1. (a) Honey and (b) motor oil are examples of liquids v high viscosities; they flow slowly. (credit a: alteration of work by Scott Bauer; credit transaction b: modification of occupational by David Nagy)

The IMFs in between the molecules of a liquid, the size and also shape the the molecules, and also the temperature determine how quickly a liquid flows. As Table 2 shows, the much more structurally complex are the molecule in a liquid and also the stronger the IMFs in between them, the more daunting it is for them to relocate past each other and the better is the viscosity of the liquid. Together the temperature increases, the molecules move an ext rapidly and also their kinetic energies are better able to conquer the pressures that host them together; thus, the viscosity of the liquid decreases.

SubstanceFormulaViscosity (mPa·s)
ethylene glycolCH2(OH)CH2(OH)16.1
motor oilvariable~50–500
Table 2. Viscosities of common Substances at 25 °C

The assorted IMFs in between identical molecule of a substance are instances of cohesive forces. The molecules within a liquid room surrounded by other molecules and also are attracted equally in every directions through the cohesive pressures within the liquid. However, the molecule on the surface of a liquid are attracted just by around one-half as many molecules. Because of the unbalanced molecule attractions top top the surface ar molecules, liquids contract to type a shape that minimizes the variety of molecules top top the surface—that is, the shape with the minimum surface ar area. A tiny drop the liquid has tendency to assume a spherical shape, as presented in number 2, because in a sphere, the ratio of surface ar area to volume is at a minimum. Larger drops are more greatly influenced by gravity, wait resistance, surface ar interactions, and so on, and as a result, are much less spherical.

Figure 2. Attractive forces result in a spherical water drop that minimizes surface area; cohesive forces hold the sphere together; adhesive forces keep the autumn attached to the web. (credit photo: change of occupational by “OliBac”/Flickr)

Surface tension is identified as the energy required to increase the surface ar area the a liquid, or the force required to boost the length of a liquid surface by a offered amount. This home results native the cohesive forces in between molecules at the surface ar of a liquid, and also it causes the surface ar of a fluid to behave favor a extended rubber membrane. Surface ar tensions of several liquids are presented in Table 3. Among common liquids, water exhibits a patent high surface stress and anxiety due to strong hydrogen bonding between its molecules. As a result of this high surface tension, the surface ar of water represents a fairly “tough skin” that have the right to withstand substantial force there is no breaking. A stole needle carefully placed on water will float. Part insects, like the one presented in number 3, also though they are denser than water, move on its surface since they room supported by the surface tension.

SubstanceFormulaSurface tension (mN/m)
ethylene glycolCH2(OH)CH2(OH)47.99
Table 3. surface Tensions of typical Substances in ~ 25 °C
Figure 3. Surface stress (right) stays clear of this insect, a “water strider,” from sinking right into the water.

The IMFs of attraction in between two different molecules are referred to as adhesive forces. Take into consideration what happens when water comes into contact with part surface. If the adhesive forces in between water molecules and the molecules of the surface space weak compared to the cohesive forces in between the water molecules, the water does not “wet” the surface. Because that example, water does not wet waxed surface or countless plastics such together polyethylene. Water creates drops on these surfaces because the cohesive forces within the autumn are better than the adhesive forces between the water and also the plastic. Water spreads out on glass because the adhesive force between water and glass is better than the cohesive pressures within the water. Once water is confined in a glass tube, that is meniscus (surface) has a concave shape due to the fact that the water wets the glass and creeps increase the next of the tube. ~ above the various other hand, the cohesive forces between mercury atoms space much higher than the adhesive forces between mercury and glass. Mercury because of this does no wet glass, and also it develops a convex meniscus when confined in a tube due to the fact that the cohesive pressures within the mercury often tend to draw it right into a drop (Figure 4).

Figure 4. differences in the loved one strengths of cohesive and adhesive forces an outcome in various meniscus forms for mercury (left) and water (right) in glass tubes. (credit: note Ott)

If you place one end of a file towel in spilled wine, as displayed in figure 5, the liquid wicks increase the file towel. A similar procedure occurs in a fabric towel as soon as you usage it to dry turn off after a shower. This are instances of capillary action—when a fluid flows in ~ a porous material because of the attraction that the fluid molecules come the surface of the material and also to other liquid molecules. The adhesive forces between the liquid and the porous material, combined with the cohesive pressures within the liquid, may be strong enough to move the fluid upward against gravity.

Figure 5. wine wicks increase a record towel (left) because of the solid attractions the water (and ethanol) molecule to the −OH teams on the towel’s cellulose fibers and also the strong attractions that water molecules to other water (and ethanol) molecules (right). (credit photo: alteration of job-related by note Blaser)

Towels soak increase liquids choose water since the yarn of a towel are made of molecule that are attracted to water molecules. Most towel towels are made of cotton, and file towels are usually made from record pulp. Both consist of of lengthy molecules the cellulose that contain many −OH groups. Water molecules are attracted to this −OH groups and kind hydrogen bonds with them, i beg your pardon draws the H2O molecule up the cellulose molecules. The water molecules are likewise attracted to each other, so huge amounts of water are drawn up the cellulose fibers.

Capillary activity can likewise occur when one finish of a little diameter tube is immersed in a liquid, as illustrated in number 6. If the fluid molecules are strongly attractive to the tube molecules, the liquid creeps increase the inside of the tube till the weight of the liquid and also the adhesive pressures are in balance. The smaller the diameter that the tube is, the higher the fluid climbs. That is partly by capillary activity occurring in plant cells referred to as xylem that water and dissolved nutrients are carried from the floor up through the roots and into a plant. Capillary activity is the communication for slim layer chromatography, a laboratory an approach commonly provided to separate little quantities the mixtures. You count on a consistent supply that tears to keep your eye lubricated and also on capillary action to pump tear liquid away.

Figure 6.

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depending upon the relative strengths that adhesive and cohesive forces, a liquid may rise (such as water) or fall (such as mercury) in a glass capillary tube. The extent of the climb (or fall) is straight proportional to the surface anxiety of the liquid and inversely proportional to the density of the liquid and also the radius that the tube.

The height to which a fluid will rise in a capillary pipe is established by several determinants as presented in the adhering to equation: