tested demonstrations A Simple Demonstration of Charles's Law Submmed by Nadine Szczepanski MacMurray College Jacksonville, IL 62650
A simple demonstration of Charles's Law can be done quickly and inexpensively. In just a few minutes, students can witness the expansion of gases that are heated and the compression of gases that are cooled. The opening of a large, round balloon is placed snugly over the mouth of a narrow mouthed 750-mL Erlenmeyer flask. The flaskis then heated in a strong flame. After just a few seconds, the balloon begins to inflate. Maximum inflation is accomplished in less than a minute. When a 750mL flask is used, the balloon inflates to a circumference of about 25 cm. Smaller flasks also may be used. I achieved the best results by holding the flask with crucible tongs, tiltmg it, and heating the bottom half of it in the flame of a Bunsen burner. You must take care to keep the balloon away from the flame. When the flask is supported on an iron ring stand, the risk of melting the balloon is too great. Best results also are achieved when a large balloon, inflatable to 9 in., is used. Smallerballoons have walls that are too thin; they easily break or melt. I t also is helpful to stretch the balloon a bit before putting it on the flask. After the flask has been heated and the balloon has been inflated, set the flask on a wire gauze. Lift a small portion of the balloon from the neck of the flask, squeeze the hot air from the balloon and replace it on the flask. Allow the flask to cool while you begin the discussion of Charles's Law. The students easily arrive a t the conclusion that the volume of a gas is proportional to temperature. I usually tell the first student to arrive a t that conclusion that if they had been born 200 years earlier, the law might have been named after her or him instead of Mr. Charles. The Chemicals Used in this Demonstration
Product Namea and Source
Uses
27%
Baqua Shock (hydrogen peroxide);swimming pool supplies store
Swimming pool clarifier
12%
Hydrogen peroxide; beauty salons
Gif scalp
3%
Granville, OH 43023
students also are able to predict correctly that the balloon should be sucked into the flask as the hot air in the flask cools hack to room temperature. The balloon does get sucked back into the flask, sometimes with a noticeable "swooosh!"
Hydrogen Peroxide Demo Curing the Brown Bottle-with-Black Cap Syndrome
Checked by David Speckhard Loras College Dubuque, IA52001
Concentration
edited bv GEORGE L. GILBERT Denison University
bleaching of hair
Hydrogen peroxide; drug store Antiseptic
Antiseptic 6.2-8.2 g/ Iodides tincture (ammonium iodide and potassium iodide in 100 mL 45% alcohol);"decolorized iodine";drug store The brand name Is used only to make it easier to find the pmduct.
submmed by Andy Sae Eastern New Mexico University Portales. NM 88130 Checked by L. Phillip Silverman Program Resources Inc Frederick. MD 21701 Chemistry oftan is viewed as a specialized subject only for the best of students, using chemicals that are somewhat mysterious and removed from the realm of ordinary consumers. The wmmon perception is that consumer products are safe; whereas, chemicals in a laboratory are dangerous substances found in brown bottles with black caps. The following demonstration may help emphasize that chemistry is used in the home, in the beauty salon, and in the swimming pool, among the many other places. Chemicals can come in familiar packaging and in different formulations. The same chemical can be safe or dangerous depending on its wncentration and its application. Materials Needed goggles gloves matches wood splints three 1-L glass cylinders or clear plastic tennis ball cans bubble bath liquid iodides tincture (an ammonium and potassium iodide solution) 3 medicinedroppers or pipets Baqua Shock (27% H20z) 12%HZOZ 3%HzOz
.Caution: Before starting t h ~ demonstration, s w e eye protection and gloves. Twenty-seven percent H,O, can cause serious chemical burns. Into each of three separate 1-L cylinders, place about 2 mL of bubble bath liquid. Into the first cylinder, place 30 mL of 3% Hz02:into the secondcylinder place 30 mL of 12% HzOz;and into the third cylinder place 30 mL 27% HzOz. When vou are readv the demonstration. add 2 " to ~erform . mL of Lodides tincture to each of thc three cylinders. Solid uutassium iodide also can he used. The uuantitv ofbuhhles produced can be seen as proportional td the concentration This demonstration was presented by the author at the 12th Biennial Conference on Chemical Education,,Davis, CA, August 26. 1992. Volume 71 Number 5
May 1994
433
of the H202. Perform the characteristic test of the decomposition of H20zby holding a glowing splint near the bubbles as they leave the cylinder; the oxygen will cause the glowing splint to re-ignite. The iodide catalyzes the decomposition of HzOz into water and oxygen. The mechanism for the I- catalyzed reaction is believed to be ( I ) : Step 1slow Step 2 fast
H20z(aq)+ I-(aq) 4 0 7 a q ) + H20(1) H202(aq)+ I07aq) +HzO(l) + Oz(g)+ I-(aq)
Some oxidation of I- occurs (2): H202(aq)+ 2Ht(aq) + 2r(aq)+ 2HzO(1)+ Iz(aq) The decomposition of hydrogen peroxide is an exothermic reaction (AH= -24 kcal). The expansion of the oxygen blows detergent bubbles that rise in a column in the cylinder. The amount of bubbles dramatically increases with
434
Journal of Chemical Education
the increasing concentration of H202illustrating the effect of concentration on reaction rate. One should point out to the students that while 3%Hz02 is safe for gargling and cleaning minor wounds, 12%HzOz is too strong t o be used for t h e same purposes, a n d 27% HzOzis dangerous a n d c a n cause burns o n cont a c t with skin. Hydrogen peroxide of these concentrations are readily available consumer products. (See the table.) Using them directly in the commonly available form calls the students'attention to the chemical properties and the proper use of chemical products on the market. Repeating the exact same demonstration using H202and potassium iodide solutions prepared in the chemistry stockroom, however, can show that the chemicals are the same, only the formulation differs. Literature Cited
