Viscosity

Submitted by ChemPRIME Staff on Thu, 12/16/2010 - 13:35

Because its molecules can slide around each other, a liquidA state of matter in which the atomic-scale particles remain close together but are able to change their positions so that the matter takes the shape of its container has the ability to flow. The resistance to such flow is called the viscosityThe resistance of a liquid to flow when subjected to shear stress.. Liquids which flow very slowly, like glycerin or honey, have high viscosities. Those like etherAn organic compound containing the functional group R-O-R'. or gasoline which flow very readily have low viscosities. The following video shows a qualitative test of viscosities.

In the video, the flow of four liquids down four different pipets is shown. From left to right, the liquids are water, rubbing alcoholAn organic compound containing the functional group -OH., vegetable oil, and ethylene glycol. The viscosity of increases as one goes from water to ethylene glycol.

Viscosity is governed by the strength of intermolecular forces and especially by the shapes of the molecules of a liquid. Liquids whose molecules are polarDescribes a molecule that has separated, equal positive and negative charges that consitute a positive and a negative pole; such a molecule tends to assume certain orientations more than others in an electric field. or can form hydrogen bonds are usually more viscous than similar nonpolarDescribes a molecule with no net permanent dipole; this can occur when there is no separation of centers of positive and negative electrical charge or because there are bond dipoles that cancel each others' effects. A polar molecule will assume certain orientations more than others in an electric field. substances. ConcentratedIncreased the concentration of a mixture or solution (verb). Having a large concentration (adjective). sulfuric acidIn Arrhenius theory, a substance that produces hydrogen ions (hydronium ions) in aqueous solution. In Bronsted-Lowry theory, a hydrogen-ion (proton) donor. In Lewis theory, a species that accepts a pair of electrons to form a covalent bond., H2SO4, is a good example of a liquid which owes its viscosity to hydrogen bonding. 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 alkaneA hydrocarbon containing only single bonds between carbon atoms. 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 ordering of the video in terms of these traits make sense. Water is the smallest molecule, and so despite its ability to readily hydrogen bond, it is the least viscous. The main constituent of rubbing alcohol is isopropanol, which retains and ability to hydrogen bond, and is also larger than water. Vegetable oil is made up of lipids, which have long, hydrocarbonA compound containing only the elements carbon and hydrogen. chains, and so is more viscous due to this effect. Ethylene glycol, while not as long a molecule as the lipids in vegetable oil, is polar, and has a greater ability to hydrogen bond, due to having two hydroxyl groups. This makes it the most viscous of the liquids tested.

The viscosity of a liquid always decreases as temperatureA physical property that indicates whether one object can transfer thermal energy to another object. increases. As the molecules acquire more energyA system's capacity to do work., they can escape from their mutual traction more readily. Long-chain molecules can also wriggle around more freely at a higher temperature and hence disentangle more quickly.