Exp 5. Determination the Surface Tension of Liquids

Exp 5. Determination the Surface Tension of Liquids

Theory:
measurement the surface tension of different types of liquids by using a capillary tube method .
Surface tension is a physical property of liquid, its equal to the amount of force per unit length (dyne/cm) or the energy per unit area (erg/cm2), but when referring to energy per unit of area must use the term surface energy.
Interface refers to the boundary between two or more phases exist together.
Cohesive force is the force of attraction between molecules of the same kind or of the same substance. Cohesive forces are the intermolecular forces such as those from hydrogen bonding and Van der Waals forces which cause a tendency in liquids to resist separation.
Adhesive force is the attractive forces between unlike molecules; they are caused by forces acting between two substances, such as mechanical forces (sticking together) and electrostatic forces (attraction due to opposing charges).



Methods used to measure the surface tension:
Capillary tube method: When wet the walls of the capillary tube by the liquid, this liquid rise inside the capillary tube due to the attempt to reduce the surface area, this liquid continues rise until to occur equilibrium between the surface tension force and the force of earth gravity.
?=dhgr/2cos?
When:
?: surface tension.
r: radius of capillary tube.
d: density of liquid.
h: high of liquid.
g: earth gravity.
cos?: Angle adhesion between the liquid and the capillary tube wall
which is usually too small for many liquids including water
against the glass and then van cos? = 1.