The Macroscopic View

The resistance of a liquid to flow is called its viscosity. Viscous liquids like syrup and shampoo flow slowly. Less viscous liquids like water and gasoline flow quickly. The viscosity of a liquid usually depends on its temperature. Viscosity generally decreases as the temperature increases. Viscosity generally increases as the temperature decreases.

The Microscopic View

The viscosity of a liquid is related to the ease with which the molecules can move with respect to one another. Thus the viscosity of a liquid depends on the:

• strength of attractive forces between molecules, which depend on their composition, size, and shape.
• the kinetic energy of the molecules, which depend on the temperature.

An Example of the Effect of Composition

Compare the structures of ethyl alcohol and ethylene glycol and explain why ethylene glycol has the higher viscosity.

		= C = O = H
ethyl alcohol A low viscosity liquid at 25oC.	ethylene glycol A high viscosity liquid at 25oC.

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An Example of the Effect of Structure

Compare the structure and composition of n-pentane and octadecane and explain why octadecane has the higher viscosity.

		= C = H
n-pentane A low viscosity liquid at 25oC.	octadecane A high viscosity liquid at 25oC.

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The Effect of Temperature

Explain why an increase in temperature generally results in a decrease in viscosity.

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