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.

Demo 1: Ocean in a Bottle

Topic: Solubility—Polar and nonpolar liquids

Materials

• Clear Plastic Bottle (1- or 2-L soda bottles work well)
• Rubbing Alcohol
• Paint Thinner
• Food Coloring (blue works great)
• Water

Procedure

1. Pour the materials into the bottle as follows: about half of the total volume should be paint thinner, a fourth rubbing alcohol, and a fourth water. Leave an inch or 2 of space at the top of the bottle for expansion. Add food coloring to create the desired color. Glue or tape the lid on the bottle to prevent spills.
2. Hold the bottle horizontally and tilt it gently to get the waves-on-the-ocean effect. Let students shake the bottle vigorously to see if they can make the contents stay mixed. Keep the bottle on your desk throughout the year for a great reminder of an important concept.

Explanation

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.

Demo 2: Dissolving Polystyrene Cup

Topic: Solubility

Materials

• Styrofoam® Cup
• Large Beaker
• Small Beaker
• Acetone

Procedure

1. Pour some acetone into the small beaker.
2. Show your students an intact Styrofoam® cup. Make sure they see it from all angles. Hold the cup over the large beaker.
3. Pour the small beaker of acetone into the cup. The bottom of the cup will fall out and the acetone will go into the large beaker below.
4. Put the remainder of the cup into the acetone and watch it disappear. Remember, no explanations—save them for later!

Explanation

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.

Demo 3: Rapid Decomposition of Hydrogen Peroxide

Topic: Use of catalyst to speed up a chemical reaction

Materials

• Hydrogen Peroxide, 30%
• Potassium Iodide
• Dishwashing Liquid
• Food Coloring (optional)
• 1-L Graduated Cylinder (heat resistant)
• Spoon

Procedure

1. Pour 30 to 50 mL of 30% hydrogen peroxide into a heat-resistant 1-L graduated cylinder.
2. Add a few drops of dishwashing liquid (and a few drops of food coloring, if desired).
3. Tell your students that you are going to add a magic potion.
4. Add 2 or 3 g of potassium iodide with a spoon, and then stand back to watch the reaction. The cylinder will get hot—use caution. You may use a glowing splint to test the gas given off. The splint will flame up in the presence of oxygen.

Explanation

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.

Demo 4: The Burning Dollar Bill

Topic: Solubility

Materials

• Methanol
• Sodium Chloride
• Matches or Lighter
• Tongs
• Beaker
• Bottle
• Sink
• Water

Procedure

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.”

1. Tell your students you heard that there is counterfeit money circulating around the school. You know they would not want to be in possession of any of those bills, so you have prepared a solution that will detect them.
2. Ask for a volunteer to have a dollar bill tested with your Counterfeit Money Detector. If you feel brave, ask for a 10 or 20! After you get the bill in hand, explain that if the bill is counterfeit, it will burn after soaking in your solution and if it is not, it will not burn. Tell the volunteer not to worry; after all, no one wants to be in possession of counterfeit money.
3. Fill the sink with enough water to submerge the bill.
4. Soak the bill in the Counterfeit Money Detector. Make sure to thoroughly wet the bill to prevent it from actually burning during the demo.
5. Hold the bill with tongs and light it with a match or lighter over the sink. The water in the Counterfeit Money Detector prevents the bill from burning, while the methanol and salt in it give a nice yellow flame. Dowse the bill in the sink after everyone has had a chance to observe the flames, or sooner if the bill’s edges start to darken.

Explanation

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!