Foating: A Chemical Phenomenon (page 2)
Design Your Own Experiment
- Does the purity of water affect its buoyancy? A hydrometer is an instrument used to measure the density of a liquid. This instrument rises and sinks in water depending on the amount of dissolved salt. Construct and demonstrate the buoyancy of a hydrometer in pure water and various concentrations of salty water.
- Insects do not float on the surface of water; they stand on it due to the phenomenon of surface tension. The surface of all liquids behaves differently than the interior. The molecules within the liquid are attracted to one another in all directions—sides, top, and below. At the surface, water molecules are attracted only to molecules to their sides and from below.
Make a simple hydrometer by cutting a 4-inch (10-cm) section from a drinking straw. With a permanent marking pen, draw lines dividing the length of the straw into ten even sections. Use modeling clay to plug up one end of the straw. Drop two or three steel BBs into the straw. Stand the straw, clay plug down, in a glass of distilled water. Use the lines marked on the straw to determine the height of the straw above the water's surface. Remove the hydrometer from the glass and add 1 teaspoon (5 ml) of salt to the water. Place the hydrometer. back in the liquid and determine the height of the straw above the water's surface (see Figure 20.3). Repeat this experiment adding 1 teaspoon (5 ml) of salt each time until a total of 6 teaspoons (30 ml) of salt have been added.
Since the surface molecules have fewer surrounding molecules, their force of attraction is more focused. Thus, the surface molecules exert a stronger attractive force on the molecules around them, resulting in a strong cohesion among the molecules. The surface molecules are pulled closer together, which produces a skinlike film over the liquid called surface tension. Demonstrate the surface tension of water by carefully laying a needle on the surface of water in a bowl. You could display photographs of the experiment along with pictures of insects standing on the surface of water.
Get the Facts
- Soap and detergents are surfactants (substances that accumulate at the surface of a solvent). Surfactants disrupt the surface forces of a liquid and thus decrease its surface tension. Find out more about the effect of surfactants on surface tension. Why do surfactants lower surface tension while solutes, such as sodium chloride, which diffuse homogeneously throughout water, raise the surface tension?
- The shape of raindrops and water flowing from a faucet are examples of how the force of surface tension pulls the liquid into a shape with the smallest possible surface area. Use chemistry and/or physics texts to find out more about the surface tension of water. What is a meniscus? Is it the same in all liquids?
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