Viscosity of Liquids - Qualitative Test

The associated still images include pictures of the apparatus, labeled samples (from left to right: water, rubbing alcohol, ethylene glycol, vegetable oil), and the tip of pipet. The narrow point in the pipet is responsible for restriction of liquid flow.

Note: The viscosity of the liquids, not the pipets, is being tested.

The rate of flow of a liquid depends on the magnitude of the intermolecular forces and the shapes of the molecules. Each of the first three substances can form hydrogen-bonds. Rubbing alcohol molecules (2-propanol) are significantly larger than those of water, so their greater London forces result in a higher viscosity. Rubbing alcohol and ethylene glycol (HOCH₂CH₂OH) molecules will have similar London forces, but with two hydroxyl groups, ethylene glycol molecules form twice as many hydrogen bonds as those of 2-propanol, so ethylene glycol is more viscous. The molecules in vegetable oil form no hydrogen bonds, but their large size means that the London forces are very strong and the liquid is extremely viscous.