Polarity and Interactions in Liquids: A Demonstration with Chloroform, Water, and Hexane, Exams of Chemistry

Download Polarity and Interactions in Liquids: A Demonstration with Chloroform, Water, and Hexane and more Exams Chemistry in PDF only on Docsity! LIKE DISSOLVES LIKE Kathryn M. Wagner, Princeton University Chemistry Department kmwagner@princeton.edu Brief description of demonstration Three clear liquids form three distinct layers in a cylinder. Iodine crystals sprinkled on the top layer sink and form pink solutions with the top and bottom layers but do not dissolve in the middle. When the liquids are mixed, two layers form: a pink layer on the bottom, and a colorless layer on top. When white potassium iodide crystals are added and the liquids are mixed again, the colorless layer turns yellow. Concepts illustrated: • Phases and phase boundaries (surfaces) • Density • Polar/non-polar (hydrophilic/hydrophobic) interactions • Solubility, miscibility • Chemical reaction • Extraction • [Solution and emulsion] Materials • Clear glass reaction cylinder or gas-washing bottle, at least 200 mm tall, with ground glass stopper (A cylindrical container is preferable to a separatory funnel for this experiment. The stopper must be non- reactive, and, to prevent a potentially dangerous pressure build-up, the stopper must be easily released. For a very small class, a large test tube with a suitable stopper is adequate. ) • Equal volumes of chloroform, water, and hexane (The volume of each liquid should be a little more than one-fourth the volume of the cylinder.) • Iodine crystals and small spatula • Potassium iodide crystals and medium spatula Preparation Work in a hood. Pour the chloroform into the reaction cylinder. Add the water and allow the liquids to separate completely. (If necessary, speed the process by holding the cylinder vertical and gently swirling the solution with a circular motion.) Tip the cylinder and pour the hexane slowly down the side to prevent mixing. Close the cylinder and set it aside, away from sources of heat, until time for the demonstration. “Like Dissolves Like,” page 2/5 Presentation To make the layers and the surfaces between them easier for a large audience to see, hold the cylinder against a white background and illuminate it with a strong light such as an overhead projector lamp. Choose the vocabulary and emphasize the points that fit your lesson. I use this demonstration in many different contexts and alter the presentation accordingly. • The procedure below uses the terms “non-polar”, “polar”, and “ionic” to describe the degree of charge separation within molecules or other chemically connected groups of atoms. If you are using this demonstration as part of a unit on biological or ecological systems, you may wish to substitute the terms “hydrophobic” for “non-polar” and “hydrophilic” for “polar” and “ionic”. • If you are using the demonstration to reinforce the concepts of density or phase, include the gas phase in the discussion. 1. Show the cylinder and ask students to describe its contents: the phases present in the cylinder and their number, relative density, state of matter, color, etc. • What evidence indicates that more than one liquid phase is present? [Surfaces are visible as lines between layers.] • [optional] Discuss phases, phase boundaries, or density in appropriate depth. 2. Tell students that the middle layer of liquid is water, a polar liquid, and that the surrounding liquids are non-polar. • [optional] Specifically identify the non-polar liquids as chloroform and hexane. 3. Identify iodine as a non-polar substance • [optional] Discuss the polarity of the I2 molecule or ask students to predict its polarity. 4. Ask students to predict what will happen when a non-polar substance is added to the cylinder. 5. .Sprinkle a few iodine crystals on top of the hexane and replace the stopper. • Keep the cylinder vertical so the layers don’t mix. • The iodine sinks immediately to the hexane/water interface. [Note: Usually, some of the iodine crystals sit at the hexane/water interface for a short time before sinking further. You may use this observation to discuss surfaces and surface tension.] 6. Swirl the liquids gently to break the surface tension between the layers and to allow the iodine to mix with each liquid as it sinks. Show the results [Iodine dissolves in the hexane and the chloroform to give violet-pink top and bottom layers surrounding a colorless water layer. (Molecular iodine does not dissolve to any appreciable extent in water.)]