Take the Water Tension Test
Shared by Girl Day Presenting Partner: Halliburton
Recommended Grades: 6th, 7th, 8th
The Challenge: Will adding soap to water increase or decrease the surface tension of water?
What is Surface Tension?: The property of the surface of a liquid that allows it to resist an external force, due to the cohesive nature of the water molecules.
Recommended Materials: Two pennies, A medicine dropper (if no dropper, use a straw), Dish Soap, Two cups, Tap water
Steps:
1. Fill one cup with tap water and fill the second cup with tap water and a small amount of dish soap (5-10mls), when stirring the soap and water together try not to create any bubbles.
2. Fill the medicine dropper with the cup that ONLY contains water.
3. Now carefully add one drop of water at a time to the top of the penny. Hold the medicine dropper just above the top of the penny (not touching it) so each new drop has to fall a short distance before it merges with the drop on the penny. You can write down the number of drops you add if you like. How many drops of water do you think will fit on top of the penny? Watch the drop on top of the penny carefully as it grows. It should keep getting bigger and bigger until it touches the edges of the penny. Keep adding drops (refill your medicine dropper as necessary) one at a time. How big does the drop on the penny get before it finally spills over the edges?
4. Fill the medicine dropper with the cup that contains water AND soap.
5. Now, repeat the experiment using soapy water. Do you think you will be able to add more drops or less before the liquid spills over the sides of the penny? Again, slowly add one drop at a time. How big does the drop of water on top of the penny get before it breaks and flows over the edges? Was it more or fewer drops than the first penny?
STEM Connections: The molecules in water (hydrogen & oxygen) are cohesive and tend to stick together very strongly. By adding a surfactant, a substance that tends to reduce the surface tension of a liquid in which it is dissolved (in this case, dish soap) allows the water to move and flow more freely. At Halliburton, we study the effect a surfactant has on surface tension with the chemicals we produce and use.
Observations & Results: You should find that plain tap water produces a much larger, stable drop of water on top of the penny than the soapy water does. This is because plain tap water has higher surface tension, so the surface is "stronger" and can hold together a larger drop. Adding soap lowers the water’s surface tension so the drop becomes weaker and breaks apart sooner. Making water molecules stick together less is what helps soaps clean dishes and clothes more easily.
Activity Video
Check out this video to see how the surface tension of water is affected by adding a surfactant (dish soap). Did the dish soap increase or decrease the surface tension?
About Halliburton
Founded in 1919, Halliburton is one of the world's largest providers of products and services to the energy industry. With more than 40,000 employees, representing 140 nationalities in more than 70 countries, the company helps its customers maximize value throughout the lifecycle of the reservoir – from locating hydrocarbons and managing geological data, to drilling and formation evaluation, well construction and completion, and optimizing production throughout the life of the asset.