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By Susan Godfrey
Carolina Teaching Partner
A quick demonstration is one of the best ways to motivate your high school chemistry students and pique their curiosity. Make each demo a mystery; give no explanations. Just tell your students it is the magic of chemistry. Later in the course, when you are teaching the topic illustrated by the demo, remind them of the demo. Students will remember and make the connection between the lesson and the demo, enhancing their grasp of the topic.
The following 4 demos require little in materials and equipment, but are great catalysts for learning. As with all labs and demos, try these out on your own before presenting them to the class. Always use sound lab safety practices when performing demos alone or in front of the class, including wearing personal protective equipment (PPE) such as eye protection, a lab apron or coat, and gloves.
Topic: Solubility—Polar and nonpolar liquids
A polar liquid (water) will not mix with a nonpolar liquid (paint thinner). Food coloring mixes with the alcohol and water. There are a number of practical illustrations of this which you may use.
Topic: Solubility
Polystyrene (Styrofoam®) is a long-chain hydrocarbon of very low polarity. Since “like dissolves like,” polar water will not dissolve nonpolar Polystyrene®. Acetone, a nonpolar solvent, will dissolve it.
Topic: Use of catalyst to speed up a chemical reaction
Hydrogen peroxide decomposes slowly into water and oxygen at room temperature. Light can speed up the process, so hydrogen peroxide is always stored in a brown or opaque bottle. A catalyst, such as potassium iodide, speeds up the reaction dramatically. Since it is an exothermic reaction, a great deal of heat is given off. Be sure to use a heat-resistant graduated cylinder and handle it with caution during the demo. The dishwashing liquid enhances the effect by making the oxygen bubbles visible.
Topic: Solubility
Before the demo, make up a solution of 10 mL methanol and 90 mL water in a beaker. Add a pinch of sodium chloride for color. Mix the solution well and pour it into a bottle labeled “Counterfeit Money Detector.”
Alcohol and sodium chloride are soluble in water. Since alcohol burns with a colorless flame, sodium chloride is added to produce a yellow flame. As long as the bill is wet, it will not reach its kindling temperature; therefore, it does not burn.
Continue doing demos throughout the year. They are very effective for introducing and reinforcing concepts. Choose demos that are fun to watch, but not practical for the students to do as labs. Do not do them instead of labs! It is essential for students to have the hands-on experience of doing labs themselves. Work safely—and motivate your students with great demos!