The Foundations of Chemistry

VISCOSITY

Viscosityis the resistance to flow of a liquid. Honey has a high viscosity at room temper-
ature, and freely flowing gasoline has a low viscosity. The viscosity of a liquid can be
measured with a viscometer such as the one in Figure 13-7.
For a liquid to flow, the molecules must be able to slide past one another. In general,
the stronger the intermolecular forces of attraction, the more viscous the liquid is.
Substances that have a great ability to form hydrogen bonds, especially involving several
hydrogen-bonding sites per molecule, such as glycerine (margin), usually have high viscosi-
ties. Increasing the size and surface area of molecules generally results in increased
viscosity, due to the increased dispersion forces. For example, the shorter-chain hydro-
carbon pentane (a free-flowing liquid at room temperature) is less viscous than dodecane
(an oily liquid at room temperature). The longer the molecules are, the more they can
get “tangled up” with one another, and the harder it is for them to flow.

pentane, C 5 H 12

viscosity0.215

centipoise at 25°C

dodecane, C 12 H 26

viscosity1.38

centipoise at 25°C

As temperature increases and the molecules move more rapidly, their kinetic energies
are better able to overcome intermolecular attractions. Thus, viscosity decreases with
increasing temperature, as long as no changes in composition occur.

SURFACE TENSION

Molecules below the surface of a liquid are influenced by intermolecular attractions from

all directions. Those on the surface are attracted only toward the interior (Figure 13-8);
these attractions pull the surface layer toward the center. The most stable situation is one

13-4

HHC

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

HHC

H

H

C

H

H

C

H

H

C

H

H

C

H

H

13-3

Viscosity945 centipoise at 25°C

C

H

C C

OH

HH

H

OH

H

OH

glycerine

13-4 Surface Tension 493

See the Saunders Interactive

General Chemistry CD-ROM,

Screen 13.11, Properties of Liquids (4):

Surface Tension/Capillary Action/

Viscosity.

The poiseis the unit used to express

viscosity. The viscosity of water at

25°C is 0.89 centipoise.

stronger higher

attractive mn viscosity

forces

increasing lower

temperature mn viscosity





Figure 13-7 The Ostwald viscometer, a device used

to measure viscosity of liquids. The time it takes for a

known volume of a liquid to flow through a small neck

of known size is measured. Liquids with low viscosities

flow rapidly.

Figure 13-8 A molecular-level

view of the attractive forces

experienced by molecules at and

below the surface of a liquid.

Calibration
marks