High School Biology-Chemistry SMILE Meeting
08 October 2002
Notes Prepared by Porter Johnson and Ken Schug

Ed Scanlon [Morgan Park HS, Biology]     Capture-Recapture Sampling Method [Handout]
This method is used for estimating the size of animal populations. This exercise presents a popular method useful for estimating the population size of a single species of highly mobile animals, such as most vertebrates. Some literature refers to this method as the Lincoln-Peterson method.

N:  This is the total number of individuals in a population
M: This is the number of individuals in the first sample-- you must mark this, then return them back into the environment.
n:    This is the number of individuals in the second sample.
R:   This is the number of marked individuals in the second sample.
We may now estimate the size of the population (N) using the following formula:
N = M n / R
Standard Error: There will always be some error when you do any type of sampling. The formula to find the standard error for this method is:
SE = ( M2(n+1)(n-R) / (R + 1)2(R + 2)

Marking: After catching, mark and release the individuals as soon as possible. Use a method to mark the individuals that will not come off or adversely hurt them.

Ed got us all actively involved, indoors and out, in the Capture-Recapture Sampling Method [for additional details of the method see A Practical Study of the Capture/Recapture Method of Estimating Population Size: http://www.rsscse.org.uk/ts/bts/dudley2/text.html] used by naturalists/conservation professionals to estimate populations of animal species in the wild, using toothpicks to represent the species of interest. We all went outside to a lawn near the Life Sciences building where Ed had previously strewn an undisclosed number of toothpicks and we conducted a five minute "search" that netted 196 toothpicks, Each toothpick was marked unobtrusively with a small black dot from a felt tipped pen and (as we turned our backs), Ed redistributed these in the same area. In actual animal studies a harmless marker is used, e.g. putting a small distinctive nick in the fin of a fish (not likely to occur naturally). We then conducted another search collecting 214 toothpicks (our searching skills seemed to have improved!) of which 140 were marked. Ed presented the formula n = (a ´ b) / c where a = 196 the total collected the first time, b = 214 the total collected the second time, and c = 141 the number of marked toothpicks recovered in the second search. When Pat used her calculator and announced that n = 299.6, Ed's face fell and he said "are you sure?" Pat rechecked with the same result and Ed said "I can't believe it!" Most of us thought the answer must not have agreed with reality until Ed pulled out a paper from his pocket and dramatically held it up: 300, we read! We burst into applause because Ed had previously warned us that "errors" of 20 % are common in this kind of population estimate. Ed handed out another version that can be done indoors, e.g. on a rainy day.

Great Job, Ed! and all for the price a a box of flat toothpicks!

Therese Donatello [St Edwards Middle School]     The Properties and Phases of Matter [http://www.science.uwaterloo.ca/~cchieh/cact/c123/phases.html]
introduced this phenomenological activity by reminding us that the universe is composed of matter and energy, both of which exist in different forms, but cannot be created from "nothing". Thus the "laws" of Conservation of Energy and of Matter, though we know matter and energy can also be inter-converted (E = mc2). She applied these ideas to phase changes --- inter-conversions between the solid, liquid, and gas forms of a pure substance --- which involve energy. This explains why we feel cold when emerging from a shower, because the liquid water uses energy to evaporate as a gas and takes that energy from our skin, thereby lowering its temperature. A handout sheet showed a typical time vs temperature plot for starting with a solid and adding heat until it becomes a gas. Even if heat is being added at a constant rate, the temperature increase is not steady, but contains two plateaus corresponding to the (normal) melting and boiling points where  the added energy causes a phase change with no increase in temperature. Terry distributed sets of three thermometers and small pieces of absorbent cotton. We wrapped the cotton around each thermometer bulb - left one dry, soaked the second one with water, and the third one with rubbing alcohol (dyed green for identification). We then took temperature readings and discovered that the two thermometers with "wet" bulbs gave lower temperatures than the dry one, with the alcohol slightly cooler than the water. At Terry's suggestion we then fanned the thermometers, and found a further decrease in temperature for the two wet bulbs. A typical set of readings was (in the order dry, water, alcohol in degrees Celsius) 24, 20, 19 without fanning and 23, 15, 11 with fanning. Again the cooling is due to the liquid removing heat from its surroundings --- which includes the thermometer bulb  --- to change from a liquid into a gas. A technique based on measuring the difference in temperature between "wet" and "dry" bulb thermometers is the traditional way to determine relative humidity of the atmosphere. Thanks, Terry, for an exciting and educational experience! This also shows us why alcohol rubs are so cooling. [The temperature difference between alcohol and water occurs because the alcohol evaporates faster, even though the energy per gram needed for evaporation is actually larger for water.]

Teri Roland [Joliet West HS] Why Leaves Change Color     [ http://www.na.fs.fed.us/spfo/pubs/misc/leaves/leaves.htm]
Teri first asked us for an explanation, and after rejecting several suggestions --- such as that the trees were blushing because they were losing their "clothes" --- she explained that the fall colors result from chemical compounds that have been in the leaves all summer,  but have been "drowned out" by a much higher concentration of green chlorophyll. When winter approaches, it is a signal (not fully understood but probably triggered by fewer hours of sunlight) to the (deciduous) tree to shut down chlorophyll production and prepare for its winter "nap". Teri started by putting spinach leaves in a blender, adding some acetone (nail polish remover) and "pureeing" to get a dark green solution. We then put a small drop on a long strip of chromatography paper (other paper can also be used, e.g. coffee filters, regular filter paper, certain paper towels, ... ) and inserted it into test tubes containing a solvent (90% petroleum ether and 10 % acetone) which Teri had developed  by experimenting with different mixtures. In a short time the colored pigments started moving up the paper (by capillary action) and we clearly saw a separation of colors with green moving faster and yellow following up behind. The separation occurs because the molecules making up the various pigments have different attractions for the paper; the ones more strongly attracted moving more slowly. By comparing the behavior of a sample with known compounds, it is possible to rule out certain compounds because of different rates of movement. Teri also showed us a faster way to prepare the chromatographic strips, which would be especially useful for comparing leaves from several different trees . The leaf is placed on the paper, and the edge of a coin is pressed against the outer surface firmly enough to make a "grass stain" on the paper. Then proceed as before. What a good fall project with a lot science in it for further investigation! [Many plants will show more than one green pigment, indicating that there are different types of chlorophyll present.] 

Thanks, Teri!! See you next time!

Notes taken by Ken Schug.