Surface tension is a property of liquids. Liquids have a limited ability – called surface tension - to resist an external force. For example, when a liquid forms a lens-shaped meniscus in a graduated cylinder, the surface tension of the liquid is resisting the force of gravity. Because of surface tension, we observe objects float on liquids and insects walk on the surface of lakes, even though they may be far denser than the liquid. Surface tension is the product of Van der Waal forces. Since Van der Waal forces exist between the molecules of a single substance such as water, they are called inter-molecular forces. This force causes the molecules on the surface of a liquid to cling together, almost as if they were acting like a sheet of plastic wrap.
How do temperature and salinity affect surface tension?
- 2 clear glass 250 ml beakers (other types of transparent glassware can also be
- 1 gallon of distilled water
- Aluminum foil cut into small squares measuring approximately 1 cm x 1 cm
- White rice (size of grain does not matter)
- Access to refrigerator
- Access to a hotplate or microwave
- Table salt
- Graph paper
- Measuring spoon
- Label one beaker “NaCl” (NaCl stands for sodium chloride - otherwise known as salt). Label the other beaker “pure water.”
- Fill both beakers with 250 ml of distilled water.
- Add 3 tablespoons of table salt to the beaker labeled “NaCl.” Stir until the salt is dissolved.
- Put both beakers in the fridge. After a half hour check the temperature of the liquids. Keep them refrigerated until the temperature of the water drops to 15° C.
- Float a piece of aluminum foil on the surface of the water. See if you can place individual grains of rice on top of the foil. You may want to use a tweezers to grasp the rice grains. How many rice grains can be placed on the foil in the pure water beaker before they sink? How many grains can be placed on the foil in the saltwater beaker before they sink? Write down your results.
- Let the beakers sit on a counter until the water is room temperature. Repeat step5.
- Using a hot plate, heat the water until it is 35°C. If you do not have a hotplate, use a microwave, heating the water for 5-second intervals and checking the temperature after every 5 seconds. Repeat step 5. Try this again when the water is 40°C, 45°C, 50°C, and 55°C.
- Graph your results. The y-axis of your graph can represent temperature and the x- axis can represent number of rice grains. Surface tension is less if fewer grains of rice are needed to upset the aluminum. Which has less surface tension – saline or pure water? How does temperature affect surface tension?