"Viscosity" Return to list of pracs

Aim: This is to illustrate the property of liquid thickness - "Viscosity".

Equipment: The equipment includes...  

• various liquids: water, vegetable oil, golden syrup, cream, dishwashing liquid, honey, treacle and lemonade;
• containers for pouring liquids.

Method: Place each in a jar across the front of the class. Pose the question "Which liquid is the thinkest?" Compare the liquids in terms of thickness: water, vegetable oil, golden syrup, cream, dishwashing liquid, honey, treacle, lemonade. Place them in a line in order of thickness.

Background knowledge: Viscosity is the measure of "thickness" of a liquid. The measure of viscosity is to do with the interaction of molecules with each other on the microscopic level. If the molecules that make up the mixture are very long then on the microscopic level they may tangle easily. This would make the mixture harder to pour and therefore increase the viscosity. If there were greater bonds between molecules then the same thing will happen. Bonds are forces that exist as a result of molecules having charges. Sometimes molecules may have a positive end and a negative end on them (in much the same way as static electricity makes clothes cling to you). As a result of this the positive end of one molecules attracts the negative end of another therefore creating a bond. This force of attraction tends to stop external forces from "tearing" these two molecules apart. The greater the forces the less likely movement away from each other is possible.

In most of the substances above the size of the molecules is most important. Liquids like water and lemonade are made up of molecules that are no more than 3 atoms joined together. Some of the others (eg. vegetable oil) can have 20-60 atoms joined together ... they are massive molecules with respect to the sizes that we are concerned with.