Viscosity of Liquids - 1-Pentanol

The relative viscosity of 1-pentanol can be measured using a stopwatch, and compared to other liquids.

To measure relative viscosity an experiment is shown where the rate of flow of six different liquids through a Mohr pipet is determined. The experiment is set up so that the times at which the liquid reaches the 0 mL and 3 mL marks on the pipet may be observed. The rate of flow depends on the size and shape of the molecules as well as on the types of intermolecular forces involved.


With the associated videos, it is possible to determine the relative viscosities of the water, hexane, octane, 2,2,4-trimethylpentane (isooctane) and 1-pentanol from measurements of the times when the meniscus passes the zero and 3 mL marks on the pipet. Viscosity is inversely proportional to the elapsed time.

The rate of flow of a liquid depends on the magnitude of the intermolecular forces and the shapes of the molecules. All molecules experience London forces, due to the polarizability of the electron cloud. Since hexane is smaller than octane, its intermolecular forces are weaker and it flows more rapidly. Although the sizes of the octane and 2,2,4-trimethylpentane (called isooctane because it is an isomer of octane) molecules are similar, the more compact isooctane molecule experiences smaller forces than the straight-chain n-octane molecule. In general, branch-chained isomers are less viscous and have lower boiling points than straight-chain isomers.

Although water molecules are much smaller than hexane molecules, water is more viscous than hexane. This is due to the hydrogen-bonding in water, a much stronger intermolecular attraction than the London force. Compare the hexane and 1-pentanol molecules. Both have similar sizes and shapes, so the London forces should be similar. However, 1-pentanol is much more viscous, due to the hydrogen-bonding between the -OH groups at the end of the molecules.

  • Design and Demonstration
    • Frederick Mattes Hastings College, Hastings, NE 68902
  • Video
    • Jerrold J. Jacobsen University of Wisconsin - Madison, Madison, WI 53706
  • Text
    • Frederick Mattes Hastings College, Hastings, NE 68902
    • Kelly Houston Jetzer University of Wisconsin - Madison, Madison, WI 53706
    • David Phillips Wabash College, Crawfordsville, IN 47933
    • Nancy S. Gettys University of Wisconsin - Madison, Madison, WI 53706