Introductory Lesson:

Concepts conveyed | Materials | Procedure | Benefits | Resource

Concepts conveyed:

The purpose of this classroom demonstration is to get students introduced to and interested in chemistry. Indicators, acids and bases may also be conveyed in this demonstration.

Materials:

(for one group of students)

Part 1: Southern Sunset

• 1 crystallization dish (6 in. Diameter)
• 45 g of sodium thiosulfate
• 15 mL concentrated HCl
• 1000 mL of water
• 1 overhead projector
• 1 piece of cardboard with a 3 in. hole to cover projector surface
• 2, 500 mL volumetric flasks

Part 2: Go USA!

• 2.7 g of ferric chloride
• 0.4 g of potassium thiocyanate
• 0.8 g of potassium ferrocyanide
• 600 mL of water
• 3, 200 mL volumetric flasks
• 3 heavy sheets of filter paper (~10 x 10 in.)
• 2, 1 in. wide sponge paint brushes
• 3 plastic spray pumps and bottles
• 1 roll of scotch tape
• 1 package of Saran Wrap
• 1 pair of scissors

Procedure:

Part 1: Southern Sunset

Place the piece of cardboard with the three inch hole on an overhead projector. Prepare the following two solutions as follows:

1. Put 45 g of sodium thiosulfate into a 500 mL volumetric flask, and fill the flask to the 500 mL mark with water.
2. Put 15 mL of concentrated HCl into a 500 mL volumetric flask, and fill the flask to the 500 mL mark with water.

Pour equal amounts of the sodium thiosulfate solution and the dilute HCl into a crystallization dish. Place the dish on top of the hole in the cardboard. Turn on the overhead projector, and allow the light to pass through. What should occur is that the solution will turn from white to red to gray.

The white light of the projector is actually a mixture of colors. However, the light that we observe passing through the solution is red. This occurs when the other colors that make up white light (the shorter wavelengths, such as blues, violets, greens, etc.) are scattered away from the original direction of the light by the particles in the solution. Thus, only the red light continues along the same path and that is what we see. This is a similar phenomenon as that which causes sunsets to be red, even though the white light from the sun is a mixture of colors. (Note that the midday sun is not red because the distance the light travels through the atmosphere on earth to the observer is much shorter, and thus, more of the other colors of the sunlight can still pass all the way through.)

Part 2: Go USA!

Prepare the following three solutions as follows:

1. Put 2.7 g of ferric chloride into a 200 mL volumetric flask, and fill the flask to the 200 mL mark with water (Indicator solution).
2. Put 0.4 g of potassium thiocyanate into a 200 mL volumetric flask, and fill the flask to the 200 mL mark with water (Red coloring).
3. Put 0.8 g of potassium ferrocyanide into a 200 mL volumetric flask, and fill the flask to the 200 mL mark with water (Blue coloring).

There are two ways to carry out this demonstration: with stencils and with brushes.

Stencils Method
Make two stencils out of heavy filter paper: one with cutouts in the pattern of the red stripes of the U.S.A. flag and the other with cutouts in the pattern of the blue field of the flag. This can be done by drawing out the flag on the piece of filter paper, and cutting out the spaces where the red stripes or blue field are located with a pair of scissors. Cover each stencil with saran wrap for protection, and cut out the saran wrap in the same regions as the stencil. (This is so the red coloring and blue coloring will not bleed through on the other areas that are not red and blue, respectively, when the coloring is sprayed on the flag).

Use a second piece of filter paper for the actual flag. Lay the piece of filter paper that is going to be the flag on a flat surface. Place the stencil for the red coloring on top of it. Lightly spray the red coloring through the exposed areas. Place the stencil for the blue field on top of the flag, and lightly spray the cut-out area with blue coloring. Allow the flag to dry completely. (Note: The flag should be essentially white at this point.) Using scotch tape, attach the flag to the dowel and set it in a 1 liter flask (or other holder) to keep the dowel from falling over. To bring out the colors, spray the flag with the indicator solution. Use enough spray to obtain full color. Note: The sprays are hazardous and must not be inhaled.

Brushes Method
An alternative to preparing the stencils is to use the brushes to directly paint the color for the red stripes and blue field onto the filter paper that will become the flag. After taping the flag to the dowel and standing it upright, the flag should then be sprayed with the indicator solution (as described above).

Benefits:

• This is a great way to catch students’ interests and get them excited about the class. Yeah!
• For slightly more advanced students, the instructor can use this demonstration to introduce the ideas of indicators, which show the presence of an acid or base.

Resource:

• Chemistry for Museums: A Manual of Chemistry Demonstrations; Center of Science and Industry: Columbus, OH, 1976; Show #1, Demonstration #7 and Show #10, Demonstration #8; Project Funded by a National Science Foundation Grant #7423549.