Notes from a workshop held at the Dutch Neck School, West Windsor, NJ, May 18, 1998, by Steve Hoagland
LESSON COMMENTS
Lesson 1
Currently, more than 106 elements are known. The newest
elements were all made in very small quantities (a few atoms) in labs in three countries,
Germany, Russia, and the US. These elements are very unstable and only last a
fraction of a second, just long enough to register their presence in a detector.
Element 106 was recently given the name Seaborgium, after the Berkeley Nobel Laureate who
discovered it. This name was very controversial because Glenn Seaborg was still
living and no element had ever been named after a living scientist before. Seaborg
also discovered americium, berkelium, californium, and plutonium. Elements are often
named in the same manner as dinosaurs. They have been named after people, places
where they were found, and things that they look like. What names would your
students give to a new element if they discovered one?
Safety glasses provide protection from flying objects, safety goggles provide splash protection for the eyes.
Lesson 2
Typo in the note on page 35 of the teacher guide. Small cup 37 mL should read
(1¼ oz) not (¼ oz).
Lesson 3
Other properties that the students might measure or observe: bulk density (with
tie-in to Balancing and Weighing unit), hardness, and stickiness (how high can you pile
the unknown in relation to the same quantity of another unknown).
Why do some of the unknowns form a streak when rubbed on the black paper? Can this property be correlated with the (particle) size of the unknown? Try to find a microscope picture of paper (like those for the crystals at the end of Lesson 6).
Lesson 4
Alum functions in wound healing medicine as an astringent (skin tightener) and
styptic (to contract tissue such that bleeding is controlled).
Talc is chemically related to asbestos.
Cornstarch can be partially hydrolyzed (bonds between glucose units broken by reaction with water and a catalyst) to give dextrose (which is also called corn sugar and is just another name for glucose), maltose (a two unit sugar similar to sucrose and also named malt sugar because it was first identified from hydrolysis of malt), and dextrins (water soluble, gummy polysaccharides used in glue).
Lesson 5
Why does a little undissolved solid remain when alum and baking soda are dissolved
in water? There are two likely explanations: 1) the unknown samples are not
pure and/or 2) insoluble salts are formed to a small extent when the two chemicals react
with other salts already present in the water (hard water). See the supplementary extension for Lesson 6.
Suspensions can be characterized by how fast they settle or separate. This
suggests another experiment with suspensions of different compounds. Consider also
liquid/liquid suspensions. What factors might influence settling rate?
A suspension that never settles is called a colloid. Milk and blood are
examples of colloids. You can make a colloid from cornstarch or potato starch (see
the supplementary extension for Lesson 15). Add a
teaspoon of starch to 1 cup of water in a microwave-safe bowl or a small pan. Heat
the mixture while stirring (heat for 20 seconds, stir and repeat for microwaving) until it
comes to a boil. You will now have a gelatinous paste. Stir a teaspoon tip of
this paste into a fresh glass of water. It will look like it has dissolved but you
can do a test to show that it has not. Cut a small hole in a piece of cardstock or
other opaque material. Hold the piece of cardstock between a very bright light (150
watts or more) and the glass containing the colloid so that the beam of light shines
through the side of the glass. You will be able to see the light scattered by all of
the tiny starch particles suspended in the water. Try this experiment with a
solution of sugar in water. The light will pass through the solution and you will
barely be able to see it.
Lesson 6
Large crystals are grown for use in optical devices.
Lesson 7
Vinegar is a 5% water solution of acetic acid, chemical formula: CH3CO2H.
The reaction of vinegar and baking soda is:
Lesson 8
Why does starch turn color when iodine is added? In solution, the starch
molecules can twist themselves into helical structures similar to a bed spring or
stretched out SLINKY. If the starch twists around an iodine molecule, a new
"compound" is formed which has a purple color instead of the brown color which
iodine normally has in solution.
Lesson 9
What is the indicator substance in red cabbage? I have not been able to find
the answer to this question, yet.
BHT, which stands for butylated hydroxytoluene, and the related compound BHA (butylated hydroxyanisole), are synthetic antioxidants. Antioxidants, such as vitamin C, inhibit chemical reactions with oxygen. In the human body, antioxidants help prevent damage to cells. In food or plant materials, antioxidants prevent undesired color changes and rancidity.
Can you test colored solutions with red cabbage indicator? Most teachers find this difficult or impossible, and the students get confusing results that they interpret incorrectly. The presence of colored compounds in the test solution or colored impurities in the indicator solution will mask or confuse the standard color changes that occur when fresh indicator is used to determine the acid/base character of a colorless test solution or unknown. Try to contact a chemical lab, college or university near you to see if you can obtain some colorless indicator solutions to use with colored solutions.
Lesson 10
The heating station shown in Figure 10-2 and the student instruction illustration
seems too cluttered. A safer arrangement would be to remove all unknown jars,
measuring spoons, toothpicks, etc. and chairs to a separate table. The heating
station would have just the candle set-up, the match jar, and the team's foil-lined tray
for the hot baking cups. Teams could prepare the samples at a nearby table and when
done with the heat test they could record their results back at their desk or work tables.
Try spreading the solid unknown evenly over the bottom of the baking cup and slowly moving the cup in a circular motion over the flame to heat the bottom evenly. Compare this to placing the unknown in a pile in the center of the cup and holding the cup still while heating. What differences, if any, are observed?
Lesson 11
Chemical reaction for the "sinking gel" is probably:
Evaporate the solution that remains after removing the "sinking gel" by filtration. Compare the crystals to those you observed in Lesson 6.
Lesson 15
Most tap water samples will show as neutral when tested with red cabbage
juice. But some samples collected from natural bodies of water may test as acidic or
basic, depending upon what is dissolved in the sample. Basic water is called
"alkali" water. Rainwater is naturally acidic, owing to dissolved carbon
dioxide, but may become substantially more acidic from dissolved pollutants (acid
rain). I (Steve Hoagland) have some information on the
effects of water acidity on wildlife which could be used as a classroom activity to relate
this unit to living things.
Lesson 16
Red wine vinegar can be used as a "disguised" version of Liquid A instead
of coloring white vinegar.
Appendix D
Note that the liquid chemicals that are used in this unit are not pure; they are
solutions in water and/or alcohol. Vinegar, as stated above, is a 5% solution by
weight of acetic acid in water. Household ammonia is also probably about the same
strength (my bottle didn't indicate the concentration). Ammonia is a very corrosive
gas. The most that will dissolve in water at room temperature is about 25% by
weight. Ammonia will lose its strength if left open because the gas will escape into
the air (you could call this "going flat" because it is the same principle as
the disappearing soda fizz). Rubbing alcohol is typically 70% by weight in water;
90% solutions are available in some drug stores. Thus, when these solutions are
diluted to make the test solutions, they contain considerably less than 2% of the active
chemical, except rubbing alcohol, which is relatively pure compared to vinegar and
household ammonia.
Last edited September 20, 2000