(credit: Mark Ott)įor example, water does not wet waxed surfaces or many plastics such as polyethylene. Differences in the relative strengths of cohesive and adhesive forces result in different meniscus shapes for mercury (left) and water (right) in glass tubes. Surface Tensions of Common Substances at 25 ☌įigure 4. Some insects, like the one shown in Figure 3, even though they are denser than water, move on its surface because they are supported by the surface tension. A steel needle carefully placed on water will float. As a result of this high surface tension, the surface of water represents a relatively “tough skin” that can withstand considerable force without breaking. Among common liquids, water exhibits a distinctly high surface tension due to strong hydrogen bonding between its molecules. Surface tensions of several liquids are presented in Table 2. This property results from the cohesive forces between molecules at the surface of a liquid, and it causes the surface of a liquid to behave like a stretched rubber membrane. Surface tension is defined as the energy required to increase the surface area of a liquid, or the force required to increase the length of a liquid surface by a given amount. (credit: modification of work by “OliBac”/Flickr) Attractive forces result in a spherical water drop that minimizes surface area cohesive forces hold the sphere together adhesive forces keep the drop attached to the web. Larger drops are more greatly affected by gravity, air resistance, surface interactions, and so on, and as a result, are less spherical.įigure 2. A small drop of liquid tends to assume a spherical shape, as shown in Figure 2, because in a sphere, the ratio of surface area to volume is at a minimum. 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. However, the molecules on the surface of a liquid are attracted only by about one-half as many molecules. The molecules within a liquid are surrounded by other molecules and are attracted equally in all directions by the cohesive forces within the liquid. The various IMFs between identical molecules of a substance are examples of cohesive forces. 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. As Table 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. 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 a: modification of work by Scott Bauer credit b: modification of work by David Nagy) (a) Honey and (b) motor oil are examples of liquids with high viscosities they flow slowly. 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).įigure 1. Honey, syrup, motor oil, and other liquids that do not flow freely, like those shown in Figure 1, have higher viscosities. Water, gasoline, and other liquids that flow freely have a low viscosity. The viscosity of a liquid is a measure of its resistance to flow. 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. When you pour a glass of water, or fill a car with gasoline, you observe that water and gasoline flow freely. Describe the roles of intermolecular attractive forces in each of these properties/phenomena.Define viscosity, surface tension, and capillary rise.Distinguish between adhesive and cohesive forces.By the end of this section, you will be able to:
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