Surface Tension 

Definition:  is an effect within the surface layer of a liquid that causes that layer to behave as an elastic sheet. This effect allows insects (such as the water strider) to walk on water.  Water Tension also allows small metal objects such as needles, razor blades, or foil fragments to float on the surface of water.

What Causes Surface Tension:  The cohesive forces between molecules down into a liquid are shared with all neighboring atoms. Those on the surface have no neighboring atoms above, and exhibit stronger attractive forces upon their nearest neighbors on the surface. 

How is Surface Tension Measured:  Surface tension is typically measured in dynes/cm.  The surface tension of water is 72 dynes/cm at 25°C.  It would take a force of 72 dynes to break a surface film of water 1 cm long. The surface tension of water decreases significantly with temperature.

Hot water is a better cleaning agent because the lower surface tension makes it a better "wetting agent" to get into pores. Soaps and detergents further lower the surface tension.

Soap Power

What you'll need

1 index card
Scissors
A baking dish (or sink full of water)
Liquid dish detergent
Your science journal

What to do

1. From an index card, cut out a boat like this. Make it about 2 1/2 inches long and 1 1/2 inches wide.
2. Place the boat gently on the water in the dish.
3. Pour a little detergent into the notch in the end of the boat.
   What happens?
   If you repeat the experiment, wash out the baking dish carefully each time you use detergent, or your boat won't go.

Your boat should zip across the water. Water molecules are strongly attracted to each other and stick close together, especially on the surface. This creates a strong but flexible "skin" on the water's surface that we call surface tension. Adding soap disrupts the arrangement of the water molecules and breaks the skin, making the boat go forward.

Reference:

U.S Department of Education

http://www.ed.gov/pubs/parents/Science/soap.html