January 15, 2008
Lab Partner: Caroline Main
Soap and Water
By observing the physical characteristics of many substances, they may be easily classified for
better identification as a result of their intermolecular forces acting upon them. There are four main
intermolecular forces which act upon molecules ranging from weakest to smallest: London dispersion
forces which act on all molecules, dipole – dipole forces which occur in polar substances, hydrogen
bonding, present in substances containing O-H, F-H and N-H bonds, and ionic bonding, occurring in ionic
substances. The result of these forces is varying levels physical characteristics including vapor pressure,
the point at which the rate of condensation of the gas and the rate of vaporization of the liquid equal one
another; capillary action, the ability of a substance to move against gravity up a narrow tube through
adhesion (the attraction of molecules with the surface of the tube) and cohesion (The attraction of
molecules with one another); surface tension, the tendency of molecules to minimize their surface area;
and viscosity, the resistance of a liquid to flow. Vapor pressure is witnessed in every day life whenever a
substance comes to a boil.
In order for the liquid to boil, the vapor pressure must equal the ambient
pressure, which can typically be determined through the use of a chart. Most of these properties are easily
observable through simple experiments. For example, the surface tension of a substance can be viewed
through simply noticing the “skin” on top of the surface. By dissolving things in the substance, the
surface tension of the substance is generally affected. Through measuring the capillary rise of the liquid,
the surface tension can be measured with the equation
γ = ½ (
, where h is the capillary rise, r
is the radius of the capillary tube, d is the density, and g is the acceleration due to gravity.
The intermolecular forces which act upon substances also help to determine which substances
dissolve in which. Because polar molecules have different, stronger intermolecular bonds than non polar
molecules, polar and non-polar molecules are immiscible, or can not dissolve in one another. Likewise,
polar and ionic molecules contain different levels of intermolecular forces and also are immiscible.
However, like molecules are miscible, or capable of dissolving in like substances, i.e. polar with polar.
Through this knowledge the process of creating soap has been formed. First, the non-polar fat is broken
down into its two polar constituents – fatty acid and glycerol by adding a strong base such as lye. Once
the fatty acid has been separated, the non-polar ends stick away from the water and the polar oxygen ends
stick toward the water, forming a sphere known as a micelle and the fat is emulsified, or covered with a