Sponsored by Hawai`i Space Grant Consortium, Hawai`i Institute of Geophysics and Planetology, University of Hawaii, 2001

CAT'S EYES
Created by Dr. Ronald Takata
Hawai'i Space Grant Consortium Associate Director, Honolulu Community College,
Future Flight Hawai'i instructor.

picture of shells

Background

Cat's eyes are lens-shaped seashells found along coral reef beaches. They are flat and circular one side and convex on the other. Some have convex surfaces which are colored. One in particular has a large green pattern, which gives a resemblance to an eye, hence the name.

The shells are the trapdoor or operculum of turban shells of the genus Turbor. They protect the animal when it recedes into the shell. Some have diameters in excess of 4 cm.

Coral and seashells, including cat's eyes, and the ocean sand that they produce are composed principally of calcium carbonate, CaCO3. Incidentally, limestone and marble are geological products of sand on the floors of ancient oceans, which have been transformed by time, heat and pressure. The chemical in vinegar which produces its sour taste and pungent smell is acetic acid, CH3CO2H. Acids typically react with carbonates to produce carbon dioxide gas, water, and a generic salt. The salt resulting from the reaction of acetic acids with calcium carbonate is calcium acetate, Ca(CH3CO2)2. It is a crystalline solid.

Beach Dynamics

You can find cat's eyes if you study beach dynamics and the understand the fluid dynamic behavior of these shells. First of all, the beaches much be coral sand beaches. This type of sand is composed of pulverized coral and seashells. Second, the waters must be a habitat for the turban shells. Third, beach waves sort out sand and gravel according to particle size. A careful inspection of a beach reveals gravel in the shore break, and a graduation of particle sizes from small to large as you move up the beach. A line of larger stones and pebbles is commonly found above the high water mark, where storm surf has deposited them.

The shapes of particles can also influence their location on the beach. Cat's eyes are usually found with the convex surface facing up. As a wave washes over the cat's eye, the moving water produces lift in the shell due to the Bernoulli effect. Hence they move further onshore than their size would dictate. Furthermore, if the beach is crescent-shaped and if there is a current parallel to the shore, the cat's eyes tend to be on the down-current end of the beach due to their enhanced mobility. This makes them very easy to find, if you know where to look.

Observing Cat's Eyes (grades 6 - 12)

Objectives

1. Observe the behavior of cat's eyes when vinegar is poured over them.
2. Record observations.
3. Explain observations.
4.. Design an experiment to test your hypothesis.
5. Formulate conclusions about the behavior.

Supplies:

Cat's eyes, vinegar, flat-bottom dish, goggles, gloves.

Procedure:

Caution: Vinegar is acidic. Wear gloves and goggles.

1. Label convex side of shells with a pencil.
2. Measure the diameter of the shells. Determine their mass if possible.
3. Place the shells in the flat-bottom dish and cover with vinegar.
4. Observe behavior of the shells.
5. Extension: leave the dish until the vinegar evaporates.

Cleanup:

The vinegar can be poured down the drain. Rinse dish with water. If the dish is left until the vinegar evaporates, rinse the dish with water.

Questions:

1. What causes the shells to move?
2. Is there any difference in the motions of the shells? Can you make any generalizations about motion versus size of the shells?
3. What are cat's eyes made out of?
4. What is vinegar made out of of?
5. What are the bubbles made of?
6. Explain the differences in movement of the different shells.
7. What chemical are seashells composed of? What is its formula?
8. What is the acidic chemical in vinegar? What is its formula?
9. What is the name and formula of the gas?
10. Write a balanced chemical equation for the reaction between vinegar and the shells.

Observing Cat's Eyes (grades K - 5)

Objectives:

1. Observe what happens when vinegar is poured over cat's eyes in a shallow dish.
2. Record observations.
3. Create a hypothesis to explain what happens.
4. Design experiments to test the hypothesis.

Supplies:

Cat's eyes, vinegar, shallow dish, goggles, gloves.

Procedure:

Caution: Vinegar is an acid. Wear goggles and gloves.

1. Measure cat's eyes.
2. Label the convex side with a pencil.
3. Place the cat's eyes in a shallow dish and cover with vinegar.
4. Observe and record your observations.
5. Explain your observations.
6. Extension: leave the dish for a few days until the vinegar evaporates.

Cleanup:

The vinegar can be poured down the drain. Rinse dish with water. If the dish is left until the vinegar evaporates, rinse the dish with water.

Questions:

1. What causes the shells to move?
2. Is there any difference in the motions of the shells? Can you make any generalizations about motion versus size of the shells?
3. What are cat's eyes made out of?
4. What is vinegar made out of of?
5. What are the bubbles made of?
6. Explain the differences in movement of the different shells.


Index of Activities.

http://www.soest.hawaii.edu/spacegrant/ScienceDemos/
Communications: Hawaii Space Grant Office
This activity is featured in Future Flight Hawaii, a K-12 Education Project of Hawaii Space Grant Consortium.
FEB 27 2001.