14.2: Surface Tension and Viscosity

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Learning Objectives

Explain how the surface tension and viscosity of a liquid relates to intermolecular forces.

The next time you are by a still body of water, take a close look at what is scooting along on the surface. You may see insects seemingly floating on top of the water. These creatures are known by a variety of names including water skaters, water striders, pond skaters, and other equally descriptive names. They take advantage of a property called surface tension to stay above the water and not sink. The force they exert downward is less than the forces exerted among the water molecules on the surface of the pond, so the insect does not penetrate beneath the surface of the water.

Surface Tension

Molecules within a liquid are pulled equally in all directions by intermolecular forces. However, molecules at the surface are pulled downwards and sideways by other liquid molecules, but not upwards away from the surface. The overall effect is that the surface molecules are pulled into the liquid, creating a surface that is tightened like a film (Figure \(\PageIndex{1A}\)). The surface tension of a liquid is a measure of the elastic force in the liquid's surface. Liquids that have strong intermolecular forces, like the hydrogen bonding in water, exhibit the greatest surface tension. Surface tension allows objects that are denser than water, such as the paper clip shown in B in the figure below, to nonetheless float on its surface. It is also responsible for the beading up of water droplets on a freshly waxed car, because there are no attractions between the polar water molecules and the nonpolar wax.

alt — Figure \(\PageIndex{1}\): (A) Molecules at the surface of a liquid are pulled downwards into the liquid, creating a tightened surface. (B) Surface tension allows a paper clip to float on water's surface.

Other liquids, such as diethyl ether, do not demonstrate strong surface tension interactions. The intermolecular forces for the ether are the relatively weak dipole-dipole interactions that do not draw the molecules together as tightly as hydrogen bonds would.

Viscosity

Because its molecules can slide around each other, a liquid has the ability to flow. The resistance to such flow is called the viscosity. Liquids which flow very slowly, like glycerin or honey, have high viscosities. Those like ether or gasoline, which flow very readily, have low viscosities.

Viscosity is governed by the strength of intermolecular forces and especially by the shapes of the molecules of a liquid. Liquids whose molecules are polar or can form hydrogen bonds are usually more viscous than similar nonpolar substances. Honey, mostly glucose and fructose (see image below) is a good example of a liquid which owes its viscosity to hydrogen bonding.

Structure diagram of glucose and fructose. Text reads that both have 5 hydrogen bonds.

Liquids containing long molecules are invariably very viscous. This is because the molecular chains get tangled up in each other like spaghetti—in order for the liquid to flow, the molecules must first unravel. Fuel oil, lubricating grease, and other long-chain alkane molecules are quite viscous for this reason. Glycerol, CH₂OHCHOHCH₂OH, is viscous partly because of the length of the chain but also because of the extensive possibilities for hydrogen bonding between the molecules. The video below shows several different long chained oils, each progressively more viscous.

The viscosity of a liquid always decreases as temperature increases. As the molecules acquire more energy, they can escape from their mutual attraction more readily. Below is a video that demonstrates this effect with a household liquid: honey.

Warning: the video has loud background music.

The surface tension of a liquid is a measure of the elastic force in the liquid's surface.
Liquids with strong intermolecular forces have higher surface tensions than liquids with weaker forces.
Intermolecular forces are also responsible for a liquid viscosity, the stronger the forces the more viscous the liquid.
Temperature can also affect viscosity, with higher temperatures lowering the viscosity of a liquid.

Contributions & Attributions

This page was constructed from content via the following contributor(s) and edited (topically or extensively) by the LibreTexts development team to meet platform style, presentation, and quality:

Marisa Alviar-Agnew (Sacramento City College)
Henry Agnew (UC Davis)
Derived from page titled 10.7: Viscosity is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Ed Vitz, John W. Moore, Justin Shorb, Xavier Prat-Resina, Tim Wendorff, & Adam Hahn.