021 Ghost Crystals

Expt 021 -- Ghost Crystals

A vial appears to contain only water and a small string noose hanging down into it. When the string is lifted out, a large shiny "crystal" is found to be tied up in the noose. When the string is lowered back into the water, the "crystal" again becomes invisible! In a similar variation, a secret message is revealed by pouring water in a petri dish of "crystals".

Chemical Concepts

Mostly as a result of the versatility of the carbon atom with its four covalent bonding sites, carbon-based polymers have an incredibly diverse range of applications: from car bodies to artificial organs to small water absorbing "crystals" that may be added to potting soil to keep house plants from drying out.
Light travels at different speeds through different substances. The reason we can see a clear colorless piece of glass submerged in a clear colorless liquid such as water is because the light gets slowed down and refracted (bent) as it passes from the water to the glass. Hydrated Soil Moistª "crystals" are mostly water with the same refractive index as water.

Safety

Use ordinary laboratory procedures.

Procedure

Place 2-3 mL of the small Soil Moist^® "crystals" into a large (approx. 1 L), clean glass jar and fill with deionized water to within about 3 cm of the top. Place the lid on and wait a day or two.

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When the "crystals" have grown to the point where they appear essentially invisible, pour out four or five of them into a clean petri dish. Although they appear like shards of glass they are actually quite rubbery like Jello.
Choose one of the "crystals" and, touching it as little as possible, tie a string around it snugly, but not so tight that it cleaves the "crystal" in half. Lift to test. This takes some practice!

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Cut a 0.5 cm length of plastic tubing from a plastic pipet tip. Place a bead of hot glue on the inside of a plastic vial cap. Firmly press the side of the tubing into the hot glue; cool. Tie the end of the string through the tube.

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Fill the vial with deionized water, pick the "crystal" up by the string, and lower it into the vial.

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Dislodge any bubbles which form on the surface of the "crystal" with a pin. If bubbles are a severe problem, chill in a refrigerator overnight, and allow to come to room temperature.

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Show your students the vial of water with the "empty" noose inside. Pass the vial around the room. Let them examine it closely, and postulate if it is even remotely possible that there might be something in the noose that we just cannot see.
Lift out the string, and show them the tied up "crystal". Show them how it disappears as it is placed back in the water.

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Since the students all want to touch the "crystal", you may want to pass around a petri dish containing a few extras -- along with, perhaps, some of the starting, pre-hydrated "crystals" to show the incredible size increase.

Variation A:

Soak some Soil Moist^® "crystal". Write a message or draw a picture on a piece of overhead acetate. Place a beaker of "crystals" on top of the acetate, and turn the projector on. The projected image is obscured. Pour water in the beaker to disclose the message. This part is quick and easy if the soaked "crystals" are on hand.

!!!Click here to See Movie.

Variation B:

I first learned about Soil Moist^® from Ed Escudero of Cincinnati, OH. He shows his students a jar full of what appears to be normal water -- they cannot see that it is packed full of the invisible hydrated "crystals". He tells them that the water coming out of the tap seems especially "hard" today, and then proceeds to pour out a cupful of water so hard that it falls out of the jar in chunks!

Questions

What is a polymer?
These "crystals" tend to absorb several hundred times their weight in water. List as many possible uses for these "crystals" as you can think of.
Why is the hydrated "crystal" essentially invisible in water? Why does it become visible when lifted out of the water?

Handout Makeup

Name ___________________________ Class ________

Teacher __________________________

BeckerDemos 021 Ghost Crystals

Watch the movies.

Describe the results.
Answer the questions.

Curriculum-

This demonstration may be used when discussing polymers, physical properties, light and optics, observational skills and short-comings, or in conjunction with solutions, colloids, and suspensions.

Activity-

Demonstration - Student or Teacher

This activity works best as a demonstration.

Time-

Teacher Preparation: 10 minutes (Start 2 days ahead. See Lab Hints.)

Class Time: 5-10 minutes

Materials-

15-20 Soil Moistª "crystals". Soil Moist^® is a product of JRM Chemical Division, 13900 Broadway Ave, Cleveland, OH 44125 (1-800-962-4010). It is reported to contain: "NONPLANT FOOD PRODUCT(S)-(25% Hydroxyethyl Methacrylate, methacrylic acid, acrylimide copolymer, crosslinked acrylic homopolymer for absorption and desorption of water, 35% silica to stabilize system.)" It is available at many garden centers. It is also available as "Ghost Crystals" from Flinn Scientific.
a large clean glass jar (approx. 1 L) with lid
1 L of deionized water
20 cm of string (1-2 mm in diam)
a petri dish
a clean, clear plastic vial (or 100-mL beaker)

Optional:

an overhead projector
an acetate sheet
a 600- or 1000-mL beaker

Disposal-

"Crystals" may be stored for reuse for many months. Dispose the "crystals" with solid trash.
Use care to keep the "crystals" out of your sinks where they might cause a clog.

Lab Hints-

Start the "crystals" soaking 2 days ahead. Prepare the vial and test the "crystal" for bubbles a day ahead.
Because oils from your skin can get absorbed onto the "crystal" and cloud up its surface, wash your hands thoroughly and, even then, handle the "crystal" as little as possible.
If bubbles form inside the "crystals", try placing the jar in the refrigerator for a few hours to dissolve the excess gas, then remove them, and as the water warms up, the dissolved gas tends to exit at the water's surface rather than back into the "crystals".
Over the course of several weeks, light, heat and water contaminants seem to cause a slight decay in the "crystal", leading to a very faint cloudiness. The "crystals", seem to last indefinitely, however, if stored in a sealed container in the refrigerator-- at least, they have lasted more than a year with no visible signs of decay (actually, they show no visible signs of anything!)
Use care to keep the "crystals" out of your sinks where they might cause a clog.

Observations-

Simply put, because the "crystal" is made up almost entirely of water, it essentially "looks" like water, and light passes from the surrounding water into the "crystal" without being refracted at all. (The "crystal" and the water are said to have the same index of refraction.) When the "crystal" is lifted out into the air, it becomes instantly visible: we can see its shape and size because, of course, air's index of refraction is very different than water's. It does raise an interesting possibility: if a substance were made that had an index of refraction equal to that of air, would it appear completely invisible?

Answers-

Q1. What is a polymer? A1. A polymer is a substance comprised of long strand molecules, usually carbon based, and usually derived from petroleum or from plant or animal products. Polymers include all plastics such as polyethylene and Teflon, as well as natural fibers such as wool, cotton and silk.
Q2. These "crystals" tend to absorb several hundred times their weight in water. List as many possible uses for these "crystals" as you can think of. A2. Many possible answers will come up, including: a soil additive to absorb excess water and then release the water when needed (this, of course, is the intended use of the product). an ice cube that when it thaws, does not dilute the drink it is in. a gas tank additive, to absorb unwanted water. an absorbent sand bag for avoiding flood damage!
Q3. Why is the hydrated "crystal" essentially invisible in water? Why does it become visible when lifted out of the water? A3. Because the "crystal" is almost 100% water, its index of refraction is almost the same as water's. If light rays are not bent as they pass from the water into the "crystal" and then back out into the water, then the eye taking in these light rays cannot discern that the "crystal" is there. Essentially, for us to be able to see an object, that object must react with light differently than the object's surroundings. When the "crystal" is lifted out of the water, it becomes surrounded by air, which does have a different index of refraction. That is why it becomes instantly visible.

Reference-

I first learned about Soil Moist^® from Ed Escudero of Cincinnati, OH.

Key Words 1-

light, diffraction, refractive index