Surface Tension and Surfanctants

What is surface tension?

Surface tension occurs when the surface of a liquid acts like a thin elastic sheet when it is in contact with an external force, such as air. The molecules at the surface of a liquid ‘cling’ together very tightly.

Surfactants:

  • Surfactants are compounds that lower the surface tension of a liquid

  • Surfactants have many commercial uses and depending on the use can be called by many names, including soaps and detergents 


Experiment #1 (Sink or Swim):

  • Demonstrates how adding soap to a glass of water weakens the surface tension.

Equipment:

  • Two glasses

  • Spoon

  • Water

  • Soap/washing up liquid

  • Pieces of paper

Procedure:

  1. Fill two glasses equally full with water

  2. In one of the glasses add about 30ml of soap/washing up liquid and gently stir the solution - you don’t want any bubbles.

  3. Create two identical balls of paper that can fit into your glasses of water

  4. Gently drop one paper ball into the plain water and the other into the glass with soapy water

  5. Observe what happens.

Was there a difference in how the paper balls behaved in the glasses?

  • Yes, in the soapy water, the ball dank whereas it floated in the regular water.

What effect did the soap have on the second glass of water?

  • The soap lowered the surface tension in the second glass of water (surfactant)


Experiment #2 (the strength of surface tension):

  • This experiment shows how surface tension can hold the weight of a paper clip.

Equipment:

  • 1 cup of water

  • Soap

  • 2 small paper clips

  • Paper towels

Procedure:

  1. Place the cup of water on a paper towel

  2. Take one paper clip and drop it into the cup. Observe if it floats or sinks.

  3. Tear off a piece of paper towel slightly larger than the size of the paper clip. Place this on top of the water.

  4. Carefully place the paper clip on the widest side of the piece of paper towel. Once the paper towel gets wet, obverse if it sinks or floats.

  5. Add a drop of soap to the cup of water.

Is the paper clip more or less dense than the water? Will it sink or float?

  • More dense - sinks

What was different about the paper clip that was dropped into the cup versus the one that floated?

  • The one that floated was on the paper towel but sank soon after.

What happened with the addition of soap?

  • With the addition of soap, the paper towel didn’t float because there was less surface tension due to the surfactant (soap).