edited by: RON DELORENZO Middle Georgia College Cochran. Georgia 31014
Where
id That Number Come From?
Ronald DeLorenzo Middle Georgia College Cochran. GA 31014
After showing problem solutions to students, a common question that teachers receive is, "Where did that number come from?" Teachers can reduce the frequency of this question by slightly modifying the typical way in which numbers with units are written. Measured numbers need more than single units such as grams, liters, and meters. Units should he expanded to indicate both specific items (e.g., gramsof NaC1) and action (e.g., "grams of sodium chloride that react"). Consider the following three examples. Example 1 A student ran at a constant speed of 12 kmh. How far does she run if she runs for 2.0 h? Solution
The modified use of units, as discussed ahove, is
Reading from left to rieht. .. . the above solution tells us that the running time is 2.0 h, that a diitance of 12 km is travrled , that this results in a distance for every 1.0 h of r u n n i n-~and of 24 km traveled. ~~
~
Example 2
How many liters of earhon dioxide form at STP when 6.0 g of carbon are burned in excess oxygen? Solution
The balanced chemical equation representing this reaction and the modified use of units, as discussed ahove, are
6.0 g C react.
22.4 L CO, form = 11 L CO, form 12 g C react
Reading from left to right, the above solution tells us that 6 g of carbon react, that 22.4 1 of carbon dioxide form for every 12 g of carbon that react, and that this results in the formation of 11L of carbon dioxide.
Common Chromatography Misconception Ronald Starkey University of Wisconsin. Green Bay Green Bay, WI 54302
Students often have erroneous concepts concerning the separation process on a gas chromatography column. A frequent misconception is the notion that the separation of compounds involves differing lengths of time that each compound spends in the mobile (gas) phase. Of course, this is not the phase in which the difference in residence time occurs. I t is the differing times spent in the stationary (liquid) phase 514
Journal of Chemical Education
that is responsible for the separation. In fact, all the components that reach the detector spend the same length of time in the mobile phase, since they all have the same distance t o traverse and are transported by the carrier eas (which has a constant flow rate). I have found an escalat'ranalogy useful inclarifying the partitioningphenomenm in~asrhromatugraphy. I t is also suitable for most other ch;omatographic separations if the descriptions of the mobile and stationarv phases are changed. For example, column chromatographi generally employs a solid stationary phase and a liquid mobile phase. The Escalator Analogy
A large group of people is shopping in an eight-floor department store that has escalators as the only means of upward transport between the floors. All shoppers enter the building on the first floor a t the same time, and they will eventually reach the eighth-floor cafeteria. The shoppers arrive a t the cafeteria a t different times because they each spend different lengths of time shopping on the seven lower floors of the store. The differing amounts of time spent shopping on the various floors (the stationary phase) accounts for the different arrival times a t the eighth floor since they each spent exactly the same amount of time on the escalators (the mobile phase) to reach the eighth floor.
A Simple Illustration of Zone Refining Alvan D. Whlte Stockport College of Technology Stockport S K I 2UQ, U.K.
Noone can have any doubt about the importance of silicon and germanium in today's society. These two elements are used in the manufacture of a wide range of electronic components, and for this purpose extraordinarily pure materials are required. The process hy which these ultra-pure suhstances are obtained was developed originally by Pfann' and is known as zone refining or zone melting. In Pfann's original experiments, a zone of molten material was caused t o travel alonga rod of germanium by passing the rod through a short furnace. He discovered that the impurities were carried forward and, by repeating the process a number of times, a high degree of purification could he obtained. In some experiments, the concentrations of impurity were reduced to one part in 109. Zone refining resembles other important laboratory techniques, such as chromatoma~hvand solvent extraction. in that i t depends on the diitribuiion of a substance hetween two phases. In zone refining the impurity is distributed between the solid and liquid host, whereas in chromatography a substance is distributed between the stationary and mobile phases and in solvent extraction the distribution is between two immiscible liquid phases. The purification of a substance by zone refining makes use of the fact that when a homogeneous liquid mixture is cooled to the point where solidification takes place, the composition of the crystalline
' Pfann, W. G. Sci. Amer. 1967,62
solid is usually different from that of the liquid, the impurity being a t a higher concentration in the liquid main component than in the solid main component. Thus, as the molten zone moves along the sample, the material that resolidifies behind the heated region contains less of the impurity than the liquid mixture from which it crystallized. Therefore, if themolten zone is well mixed, the impurity tends toaccumulate in the liquid and is carried along to the end of the sample. A simple example of the phenomenon of an uneven distribution of solute between solid and liquid phases is afforded
by watching a child with a Popsicle (ice-lolly or "drink-on-astick"). This consists essentially of a colored aqueous fruit drink (e.g., orange, raspberry, black currant) that has been rapidly frozen around a wooden/plastic handle to give asolid Popsicle of uniform color. When it is being consumed, some of the solid is allowed to melt and the resulting liquid is sucked away. If this suckingprocessis carried out vigorously, it often results in the formation of a completely colorless zone of ice. This is because the coloring matter (the impurity) has distributed itself preferentially in the liquid phase (water) leaving an almost colorless (i.e., impurity-free) solid phase (ice).
Volume 63 Number 6 June 1986
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