Master Violet Ray Generator

From the realm of dubious remedies we have this item seen in a museum.

Flash Rocks

Certain quartz-containing rocks will emit a yellow flash when struck together. Generally speaking the best rocks to use are rounded white quartzite pebbles. When the rocks are struck together with a glancing blow, the tiny quartz crystals within the quartzite are compressed. These crystals apparently build up charges due to the piezoelectric effect. The charges recombine within the rock (sort of an internal sparking) producing yellowish flashes and sometimes a burnt sulfur smell.

ABOVE LEFT: Striking (in this case vigorously rubbing) two quartz-laden rocks together.

ABOVE RIGHT: A faint flash from these rocks. Photographing the flashes is extremely difficult; the flashes are best seen in person.

Electrorheological (ER) Fluid

Application of a high voltage across a slurry of cornstarch and corn oil reversibly increases the viscosity of the slurry.

ABOVE: OK, so the pictures of the electrodes did not turn out that well. But, the shadows of the electrodes on the blackboard are visible. At left are a pair of bare electrodes with no voltage applied; at right are the electrodes with the ER fluid suspended between them due to 2500 applied volts.

Oil spot

The detergents in motor oil spread out on the surface of water to make colorful oil spots.  The varying colors result from destructive interference of white light by the varying thickness of the organic layer. Sometimes this thickness is less than 100 nm, making it a nanoscale structure. The pattern of yellow to purple to blue moving from the outside inward into the pattern as the light-interfering layer gets thicker also shows up in other structures.

Solvent Swelling Demonstrations

When a solid is typically dissolved into a liquid, the solid breaks up into individual molecules or ions which disperse throughout the liquid as much as possible, forming a solution. But what if the molecules in the solid are large enough to be all tangled up with each other? The liquid disperses into the solid as much as it can, but the solid still remains largely intact. The solid does swell, however, as it absorbs liquid. This process is called solvent swelling. Perhaps the most common example of this phenomenon is pasta swelling in hot water as the water molecules slip in between the starch molecules of the pasta. Here are some other examples of solvent swelling:

ABOVE LEFT: The superabsorbent polymer powder from a disposable diaper is placed in a Petri dish that contains a shallow pool of water (with green food coloring added). ABOVE RIGHT: The powder solvent-swells as it absorbs water (elapsed time: 10 min). These photographs were taken of the image on an overhead projector screen.

ABOVE LEFT: Silicone caulk that has cured overnight on a glass plate. ABOVE RIGHT: The cured caulk solvent swells in contact with toluene (elapsed time: 1 hour).

ABOVE LEFT: Segments of rubber bands. The one on the left has been solvent swelled by soaking in toluene for 10 min. ABOVE RIGHT: Segments of rubber bands. The one on the left has been solvent swelled by soaking in cyclohexane for 10 min.

More information:

Campbell, Dean J.; Rupe, Sarah B. Solvent Swelling and Optical Rotation Demonstrated on the Overhead Projector J. Chem Educ. 2000 77 876.

Ross, Joseph H. Demonstration of solvent differences by visible polymer swelling J. Chem Educ. 1983 60 169.

Grape juice prints on clothes

Yesterday I was at the Wein and Saengerfest in New Braunfels, TX.  There was a tent there where people could stomp grapes and then place their grape-juiced footprints on T-shirts (50% cotton and 50% polyester).  The footprints were sprinkled with table salt and allowed to air dry. The people stepped into a bleach water solution afterward to remove the dye from their feet.  It was interesting to me how different feet seemed to absorb the color differently. The directions that came with the shirts say to soak them in vinegar for 10 minutes, then machine or hand-wash in cold water separately or with like colors.  It dawns on me that a science club could do this on T-shirts featuring anthocyanin molecules and/or witty sayings (e.g. Chemistry is a Juicy Subject).

"Like Dissolves Like" - A Children's Toy

A simple demonstration polar and nonpoloar solutes and solvents may be made using a clear plastic bottle, yellow cooking oil, water, and food coloring. The dye components in many food colorings are polar and would rather stay dissolved in water rather than in nonpolar cooking oil. Therefore one can make a bottle half-filled with oil and half-filled with water and food coloring. When the bottle is shaken the two layers temporarily mix, yielding a different color (ie. yellow and blue combine to make green - a good illustration of color mixing). The bottle cap can be superglued or epoxied to the bottle to keep the contents in the bottle. Bottles of this sort have been used at day care centers for years (special thanks to Karen Campbell for introducing this demonstration to me).

ABOVE LEFT: Bottle with separated layers: oil on top (ferrocene added to enhance the yellow color) and water and blue food coloring on the bottom.

ABOVE MIDDLE: The shaken bottle (note the mixed green color). The separation will return in a few minutes.

ABOVE RIGHT: My daughter playing with the sealed bottle.