Charles Malerich, New York NY
Work hard, be fair, and have fun.
A soda bottle and hook setup for studying conservation of mass
co-authored with Patricia K. Ruff, Warren NJ
reprinted from May 98
Introduction
Combustion and many other chemical reactions produce
invisible gases and seem to make solids and liquids disappear.
Here we describe a simple setup for demonstrating conservation
of mass in gas-forming reactions and for correcting this common
misunderstanding. The setup uses a hook to separate reactants
in a closed soda bottle and can be made in less than ten
minutes for virtually no cost. Also, the setup does not require
the buoyancy correction needed when a balloon is used to close
the system1 nor a high precision balance to show that a
significant mass of gas has been formed.2 Multiple runs can be
carried out with a single setup to show the reproducibility and
generality of conservation of mass for these reactions. An
alternative to this setup is the soda bottle and test tube setup
described by Duffy et al.2
Assembly of setup
The materials needed are: a one-, two- or three-liter soda bottle
with a cap that has an easily removed soft plastic seal (a wide
mouth bottle is best); needle and thread; and a paperclip. The
hook setup is sketched in Fig. 1 and is made by removing the
plastic seal from the cap, passing the thread up and down
through the seal, and then tying the ends of the thread in a knot
to form a loop about 3-5 cm (1-1/2 to 2 inches) long. A hook is
made by bending the outer arm of the paper clip out. The setup
is completed by putting the seal back in the cap and hanging the
paper clip from the loop.
Demonstration procedure
To carry out a reaction under closed conditions, the liquid
component of the reaction mixture is poured into the bottle, a
package (the package can be made from a piece of tissue and
tape) containing solid state reactants and/or catalysts is
suspended from the hook and inserted into the bottle, the
system is closed by turning the cap tight, and the reaction is
initiated by inverting the bottle. The mass of the reaction system
is monitored by massing the entire setup before and after
inverting the bottle. To show that a significant quantity of gas
has been produced, open the bottle and mass the system after
opening. Some examples of reactions that can be used to
demonstrate conservation of mass are given in Table 1. All of
these reactions form invisible nontoxic gases and are amenable
to down-the-drain and circular file disposal.
8 Chem 13 News/March 2009
Fig. 1. Sketch of cap and hook setup.
Safety
Running a gas producing reaction in a capped soda bottle
requires that the pressure increase from the reaction not burst
the bottle. The pressure increase is controlled by the limiting
Reagent. Table 1 identifies and gives quantities of limiting
reagents that yield a pressure increase of one atm (15 pounds
per square inch) for a one-liter bottle at a temperature of 25oC (a
warm room). With this pressure increase, a mass difference of
one gram or more has been observed between the opened and
closed bottle. To check the margin of safety, a soda bottle was
fitted with Delfiner's valve cap3 and the pressure in the soda
bottle was increased by pumping to 6.5 atm (100 pounds per
square inch) without bursting the bottle. This result indicates
that the margin of safety for a one atm pressure increase is a
minimum of a factor of six and that the capped soda bottle setup
can be used safely to show that gas forming reactions conserve
mass.
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Table 1. Quantities of limiting reagents for producing
one liter of gas at one atm and 25oC
------------------------------------------------------------------------------------Reaction and chemical equation
Quantity of limiting reagent
b
Decomposition of hydrogen
peroxide by yeast catalyst
2H2O2 → 2H2O + O2
a
100 ml of 3% hydrogen peroxide
Baking soda or vinegar may be used as the limiting reagent
for this reaction. The calculation assumes vinegar
containing 5% acetic acid. The other reactant can be used
in greater quantity safely.
Citric acid should be the limiting reagent for these reactions.
The quantity of washing soda or baking soda is the minimum
that should be used with the given quantity of citric acid.
Also, 100-200 mL of water was used as solvent in testing
these reactions.
Baking soda and vinegar
NaHCO3 + CH3COOH
→ CO2 + H2O + NaCH3COO
50 mL of vinegar or
3.4 grams of baking sodaa
→ 3CO2 + 3H2O + 2Na3(C6H5O7)
1. David Martin, D. Randy, and Nicholas C. Thomas, Journal of
Chemical Education, November 1995, page 925.
Washing soda and citric acid
3Na2CO3 + 2H3C6H5O7
References
5.2 grams of citric acid or
4.4 grams of washing sodab
2. Daniel Q. Duffy, Stephanie A. Shaw, William D. Bare and Kenneth
A. Goldsby, Journal of Chemical Education, August 1995, pages
734-736.
Alka-seltzer or baking soda and
citric acid
3. Al Delfiner, Newsletter of The Chemistry Teacher Club of New York,
3 Alka-seltzer tablets or
3NaHCO3 + H3C6H5O7
→ Na3(C6H5O7) + 3H2O + 3CO2
November, 1995.
∎
2.62 grams of citric acid and
3.5 grams of baking sodab
Killer treats
Chemical party video
In 1831, Dr. O’Shaughnessy researched the candy being sold
on the streets of London. Bee Wilson writes about his findings
in her 2008 book called Swindled: The Dark History of Food
Fraud, from Poisoned Candy to Counterfeit Coffee. Wilson
writes
Want to see a fun chemistry video. Go to the website called
Marie Curie Actions — a program for research funding that is
awarded by the European Commission. Go to its webpage
called “Are you Marie Curious” and select “Marie Curie Video”
or just use the following address. http://ec.europa.eu/
research/mariecurieactions/mariecurious/index.htm .
He found that parents who bought these treats for their
darling children were dicing with death. Of the samples
he collected, the red ones were often coloured with
lead or mercury; the green sweets with copper-based
dyes; and the yellow, with gamboges, a purgative
resin-based dye from the Far East now used to colour
Buddhist robes, or more perilously, yellow chromate or
chrome yellow of lead.
You can also find the video on YouTube.com. Look for
“Chemical Party” posted by eutube. It is a little outrageous
which makes it memorable for students. We recommend you
watch it first, just in case it does not suit your class.
On writing well
Clutter is the disease of American writing. We are a society
strangling in unnecessary words, circular constructions,
pompous frills, and meaningless jargon… But the secret of good
writing is to strip every sentence to its cleanest components.
Every word that serves no function, every long word that could
be a short word, every adverb which carries the same meaning
that is already in the verb, every passive construction that
leaves the reader unsure of who is doing what — these are the
thousands and one adulterants that weaken the strength of a
sentence. And they usually occur, ironically, in proportion to
education and rank.
— William Zinsser
Taken from The ACS Style Guide, A Manual for Authors and
Editors, 2nd Edition, Janet S. Dodd Editor, 1997; page 8.
Cartoon by Dietmar Kennepohl
March 2009/Chem 13 News 9