User:WindOwl/SurfaceTension

Objective
To observe an unusual surface property of water that results from hydrogen bonding.

Background
Surfactants are usually organic compounds that contain both a hydrophillic "head" and a hydrophobic "tail", and therefore are soluble in organic compounds and water. The 'head' binds with the surface of the water through hydrogen bonds, and the tail pushes apart individual molecules of water, causing the surface to be weakened. The term 'surfactant' comes from "surface active agent". Commons surfactants include soaps (also called "fatty acid salts"and Surfactants reduce the surface tension of water. Surface tension is the tension of the surface layer of liquids. High surface tension allows objects to rest on the surface of a liquid, such as water, without breaking the surface.

Materials

 * shallow dish or Petri dish
 * water
 * paper clip
 * rubber band, approximately 2 inches in diameter
 * micropipets or droppers (2)
 * vegetable oil
 * liquid dish detergent

Procedure

 * 1) Thoroughly clean and dry the dish.
 * 2) Fill your dish almost full with water. Dry your hands.
 * 3) Being careful not to break the surface, gently place the paper clip on the water. Observe what happens.
 * 4) Add a drop of detergent to the surface of the water
 * 5) Repeat steps 1 and 2.
 * 6) Gently place the open rubber band on the water.
 * 7) Slowly add oil drop by drop onto the water encircled by the rubber band until the water is covered with a layer of oil. Observe for 15 seconds.
 * 8) Allow one drop of dish detergent to fall onto the center of the oil layer. Observe the system for 15 seconds.

Paper Clip
When placed gently on the water, the paper clip will float. When the detergent is added, the paper clip sinks.

Rubber Band
When placed gently on top of the water, the rubber band floats. As the oil is added, it spreads out inside the rubber band. It does not mix with the water or leave the perimeter of the rubber band. When the detergent is added the oil spreads throughout the petri dish and mixes with the water. The rubber band sinks below the surface of the water.

Paper Clip
When the paper clip is initially placed gently on the water, it does not sink because it does not break the surface tension of the water. When the soap is added, it acts as a surfactant, reducing the surface tension of the water, and the paper clip sinks because the surface tension can no longer support its weight.

Rubber band
Just like the paper clip, the rubber band does not initially break the water's surface tension. The oil spreads out inside the rubber band without mixing with the water because it is a hydrophobe.