Lesson 5:

Concepts conveyed | Materials | Procedure | Benefits | Resource

Concepts conveyed:

The purpose of this demonstration is to illustrate characteristics that indicate changes in chemical and physical properties of matter.

Part 1: Chemical changes

Four easily observable indicators of a chemical change or reaction are the formation of a precipitate (solid product), the rapid change in viscosity (resistance to flow),the formation of a gaseous product (effervescence), and the occurrence of a color change. These four phenomena are demonstrated in the test for the presence of CO2 gas, the formation of slime, the reaction of baking soda and vinegar, and the discoloration of fruit. (Note: These indicators of a chemical reaction do not have to be present for a chemical change to have occurred.)

Part 2: Physical changes

When substances become part of a mixture, whether homogeneous or heterogeneous, they might undergo a physical change. What constitutes a physical change is considered. The characteristics of a physical change are demonstrated by discussing mixtures that students bring to class that are of their own choosing.

Materials:

(for one group of students)

Part 1: Chemical changes

A. Test for Presence of CO₂: Formation of Precipitate

• 1 tablespoon of lime (used in making pickles)
• tablespoon
• 2 glass quart jars with lids
• 1 plastic drinking straw
• 1 pint glass jar
• 1 glass stirring rod

B. SLIME: Rapid change in Viscosity

• 40 mL of a PVA (polyvinyl alcohol) solution in a small clear plastic cup. (Prepare the solution by combining 80 g of PVA in 1.7 L of water. Some heating may be required.)
• 1, 100 mL graduated cylinder
• 4 mL of sodium tetraborate solution in another plastic cup. (Prepare this solution as a 4% solution by weight; for example, combine 4 g of Borax in 100 mL of water)
• 1 box of food coloring (red, blue, yellow, green)
• 1 glass stirring rod
• 1, 200 mL beaker

C. Baking Soda and Vinegar: Formation of Gaseous Product

• 1 tablespoon of baking soda
• 1 cup of vinegar
• 1, 200 mL beaker
• measuring cup
• tablespoon
• Note: Effervescence is observable at many different ratios of baking soda and vinegar. Some starting quantities are suggested above.

D. Discoloration of fruit: Occurrence of a Color Change

• 2 unpeeled apples
• 2 unpeeled bananas
• 1 sharp knife (to cut apple)

Part 2: Physical changes

No materials need to be prepared. Students provide mixtures of their own choosing.

Procedure:

Part 1: Chemical Changes

A. Test for presence of CO₂: Formation of Precipitate
Two days before the lecture, prepare the solution for testing carbon dioxide. Fill one of the quart-size glass jars with water. Add one tablespoon of lime and stir with a glass stirring rod.
Secure the lid and allow the solution to stand overnight. The next day, decant (pour off) the clear liquid into a second glass quart jar, making sure not to pour any of the lime that has settled on the bottom of the jar. Keep the jar closed. This limewater will be used to test for carbon dioxide in the class demonstration.

For the class demonstration, fill the pint-size glass jar half-full with the limewater. Ask for one student volunteer to exhale into the limewater using the plastic drinking straw. He/she will continue to exhale into the liquid until a distinctive change is observed. (The saturated solution of limewater turns from clear to a milky color.) Explain to the students that limewater always turns milky when carbon dioxide is mixed with it. The limewater combines with the carbon dioxide in the exhaled breath to form limestone. Limestone is a white powder that is not soluble in water. As the jar stands for a while, the limestone will begin to fall to the bottom of the jar.

The reaction scheme is as follows:
CaO (lime water) + CO₂ (breath) = CaCO₃ (solid calcium carbonate)

B. SLIME: Rapid Change in Viscosity
Ask for two volunteers to help in the demonstration. Instruct the two students to work together and pour the cup containing 50 mL of PVA solution into the cup containing 5 mL of sodium tetraborate solution and to mix the two ingredients with a stirring rod to make slime. Add a couple of drops of food coloring to make it more fun. Discuss the change in viscosity as slime is produced from the mixture of the two liquids. Why is this change in viscosity an indication that a chemical change has taken place? Allow the students to pass around the slime and move on to the next part of the demonstration. The viscosity increases because the PVA, which consists of long strands of molecules, is crosslinked by the tetraborate, forming a mesh. This “mesh” has much more resistance to motion than the individual strands.

C. Baking Soda and Vinegar: Formation of a Gaseous Product
Ask for one volunteer to add 1 tablespoon of baking soda to 1 cup of vinegar. Discuss how the formation of the gaseous carbon dioxide can be an indication that a chemical change has taken place. Write out the chemical formulas for each species involved to help the students begin thinking about chemical compounds.

Baking Soda                NaHCO₃
Vinegar                          HC₂H₃O₂
Carbon Dioxide        CO₂

This overall acid-base reaction is:

NaHCO₃ + HC₂H₃O₂ = CO₂ + H₂O + NaC₂H₃O₂

D. Discoloration of fruit: Occurrence of a Color Change
One hour before class, cut the unpeeled apple in half and peel the banana. Allow them to remain uncovered until class time. For the demonstration, cut the second unpeeled apple in half and peel the second banana and let students observe the color differences. Discuss with the students what has occurred. Apples, bananas, and other fruit discolor when peeled and exposed to air. This browning occurs because the cells in the fruit are being broken down. The chemicals released by the damaged cells react with oxygen resulting in rapid color and taste changes in the fruit. Discuss how this change in color can be a third indication that a chemical change has taken place.

Part 2: Physical Changes

Ask students to bring a mixture of any sort to the next scheduled class meeting. Do not answer any questions. The assignment is really quite easy, but naturally students will think it is harder than it really is. Use the examples brought by the students to discuss the characteristics of a physical change. Examples of physical change include changes of state of matter (gas to liquid, liquid to solid, and visa-versa), formation of some solutions (sugar in water), breaking up a larger piece of a substance into smaller pieces. (It is important to be wary of chemical changes that might also be present in these mixtures.)

Benefits:

• Students will see how changes in common products are relevant to their life, and, in turn, learn the concepts more effectively.
• Instructor can use the mixtures that students bring to class as means to award extra credit or give attendance points. An activity like this also allows the instructor to assess the creativity of his/her students (one can never predict what the students will bring to class with them).

Resource:

• 1. VanCleave, J. P. In Chemistry for Every Kid; Sobel, D.,
  Ed.; John Wiley & Sons: New York, 1989; p 70-73, 80.
  
• Coleman, M.R., Personal Communication, Department of Chemical Engineering, University of Arkansas, Fayetteville, 1996.