Chemistry

Experiment A-8

Using Observed Changes to Seek what is Elementary

It is not obvious, based on what our senses tell us, that there are significant differences between how various materials are held together.  When a substance melts, it is clear that we are observing a change from the solid material to the less rigid liquid.  And likewise boiling suggests a change from the liquid to the gas which is formed.  To the ancient Greeks 2400 years ago, this suggested an elementary difference between solids, liquids, and gases.  Aristotle (of Stageira 384 to 322 B.C.) adopted the proposal by Empedocles (of Agrigentum 492 to about 400 B.C.) that the core elements of our material world are Earth (= solid), Water (= liquid), and Air (= gaseous), and the clearly related Fire (= energy).  Later the Alchemists adopted these Elements and still later added three Principles that seemed to control the properties of the Elements.  But it took 2000 years for the realization that there are hierarchies of combinations of the basic elements to be teased out of the maze of observations.  And entwined was the development of their notion of Fire to our current concept of Energy.  (Confused?  You have the company of over 20 Centuries of the world's greatest minds who were also confused!)

Now we believe that those FOUR ELEMENTS as proposed by Empedocles and Aristotle, founded on those key observations, were a false start.  One of the tasks of alchemists was, by using secret recipes, to transform one element to another.  For example, they had recipes for turning metals such as Copper into more valuable Gold.  But one of the leading alchemists, Abu 'Ali al Husain ibn Abdallah ibn Sena (980 to 1037), called Avicenna by Europeans, proposed that such transformations did not actually cause a material to change from one element to another.  Eventually Robert Boyle (1627 to 1691) criticized the FOUR ELEMENT THEORY and the THREE PRINCIPLES as not agreeing with the chemical facts.   Boyle requested chemists to review the experiments and devise a new theory.   Boyle proposed a new definition of element: A substance that cannot be decomposed into any simpler substance.  But how does one tell, using our senses, if say ice is more simple than liquid water?

The solution was derived from countless centuries of commerce and the alchemy drive to perfect transformation recipes.  For both, what we now call mass proved to be the best measure.  A century after Boyle, Antoine Lavoisier (Paris, 1743 to 1794, shown at right with wife and assistant, Marie) adopted Boyle's definition and demonstrated that mass could be used to determine which is simpler.  First Lavoisier did an experiment to demonstrate that if no material is allowed to enter or leave a container, the mass of contents remains unchanged no matter what transformations occur.  Having thus demonstrated that mass is a reliable measure, Laviosier claimed that if mass changes in a chemical transformation, that indicates a change in simplicity.  For example, when heat added to ice causes it to melt to liquid, the mass remains the same, showing that melting does not really take apart water into any simpler substance.  But when a piece of iron metal is rusted into a red powdery earth, it gains mass.  So the lighter iron is more elementary than the earth.  So the ancient Greeks who suggested the earthy materials were elementary, had it backwards!

But lest we get carried away thinking the ancient Greeks wrong, we should not discount the importance of their efforts and those of the alchemists.  Without their working assumptions and centuries of careful measurements and observations, there would be no information on which to base today's better understanding.  Today's science insists that our explanations be logically consistent with all relevant observations and measurements.  That said, one does not have to search far in today's society to find individuals and groups who see little value in explanations being founded upon measurements and observations.  Their conviction that truth is better founded on faith and philosophy can also be traced back to ancient Greeks such as Plato (~428 to 347 BC).

Experiments

Part I:  Changing Copper to Gold

Materials

• clean Copper (Cu) penny, preferably minted in 1983 or earlier.
• roughly equal amount of Zinc (Zn) granules: These can be trimmed from pure zinc used to protect roofs from moss, or filed from galvanized metal such as galvanized nails.
• roughly equal amount of sodium hydroxide, NaOH: Corrosive substance: wear splash protection eye goggles.  For spills on skin or clothing, DON'T PANIC, but immediately wash with water until no longer feels slippery.  For eye contact (sometimes stings!), DON'T PANIC, but immediately flush with comfortable temperature water for at least 15 minutes then get immediate medical attention.  (Hydroxides react with the oils in cell membranes to make a slippery substance called soap!  Eye damage can cause permanent blindness so don't delay first aid.)  Obtain this white substance, commonly called lye from drain cleaner powder such as Drano®.
• water (small amount for experiment, running water available for emergency use described above
• crucible, evaporating dish, or small Pyrex glass container that can be heated
• goggles (essential!), preferably plastic gloves and apron too.
• device such as pliers, tongs, or tweezers for removing penny from corrosive solution and holding in flame
• flame device such as propane torch: If body or clothing catch fire, DON'T PANIC, but STOP, DROP, and ROLL to extinguish fire.
• fire extinguisher for emergency protection

Procedure

1. Plan ahead to be prepared for possible emergency; You want to know what to do and be prepared before an emergency because emergencies don't allow time to sit and ponder what to do!  Each of us will likely encounter one or more emergencies in our lives.  While it is possible to be prepared, it is not possible to predict when an emergency will occur.  So at minimum, prepare when hazards are known.  Wear goggles, and preferably plastic gloves and apron.  Because of the use of fire, should step 5 be done outside away from flammable material?
   
   
2. Put Zinc, sodium hydroxide, in a small non-metal container with enough water to later cover a penny.  Stir, and bring to a boil.
   
   
3. Remove from heat and immediately immerse a clean, shiny penny in the solution so it is also in contact with remaining zinc.
   
   
4. After penny turns gray (usually less than a minute), remove penny from solution, rinse corrosive solution away with water, and dry penny for inspection.
   
   
5. Hold the gray penny in a blue flame, heating uniformly just enough until it turns a uniform gold color.  Don't overheat.  (The inner blue flame contains unburnt (propane) gas that reduces any black CuO that forms back to Copper.)
   
   
6. Dip hot penny in cold water to cool, then dry for display.
   
   
7. Zinc and solution may be used for several pennies before discarding solution down sewage drain followed by additional water.  Rinse all equipment with lots of water.  (Recall one of uses for sodium hydroxide is as a drain cleaner!)

According to the alchemy, the ADDITION of zinc and fire made the more elementary penny into the more valuable gold.

Modern chemistry suggests that before the first reaction the Zinc reacts with the corrosive sodium hydroxide solution to form ZnO₂^-2 ions.  Later when the copper is placed in contact with remaining Zinc metal in the solution, Zinc metal forms Zn^-2 ions while ZnO₂^-2 electroplates out as a coating of silver colored Zinc metal on the surface of the penny.  When heated the Zinc melts at 420°C and forms a bronze alloy (frozen metal solution) with the copper underneath.  If the ratio is about 1:1, the color will be pale yellow.  More Copper in the alloy results in a redder color and more Zinc results in a gray color.  Unlike earlier U.S. pennies, those made after 1983 are a sandwich of a Zinc core with only a thin coating of Copper.  Because overheating will melt the inner Zinc, if you use a newer penny heat very gently.

Part II:  Detecting a Change in Ice: Sublimation

Melting of ice to liquid water and boiling liquid water to steam are common occurrences often observed.  But it is not uncommon for solids to directly sublimate to a gas, even when the temperature is BELOW their melting points.  Those who live in cold climates might notice that in a cold dry spell following a snowfall, the amount of snow gradually diminishes.  This is not just a matter of compaction.  If it remains cold and dry long enough, the snow could entirely disappear, even though it was never warm enough for any of it to melt or seep into the ground.

In the days before refrigerated trucks, produce needing to be kept cold during shipment was often accompanied by dry ice (frozen carbon dioxide, CO₂).  Dry ice under one atmosphere pressure doesn't melt but sublimates to its invisible gaseous form when it warms to -78°C.  (Thus, because it doesn't melt to a liquid, it gets it's name.  Liquid CO₂ can exist under high pressure such as inside a CO₂ fire extinguisher.)  Sometimes dry ice is placed in punch bowls to make the punch carbonated.  It is also occasionally placed in warm water where greater energy transfer increases the sublimation to -78°C gaseous CO₂, forming cold bubbles which rise to the water surface and pop.  The splash of the warm water creates a region of high humidity over the water.  The now warming CO₂ gas cools that water vapor, which condenses a visible misty cloud.

In this investigation, people who do not live in cold climates can investigate sublimation using ice cubes in a freezer or the freezing compartment of a refrigerator.

Materials

• freezer
• couple ice cubes
• way to measure cube size

Procedure

1. For those with ice makers this may only require finding an ice cube made some time ago and comparing with a newly made ice cube.  For others it may require making an ice cube and measuring it.  Mass would be preferable, but its circumference or length could do.
   
   
2. Place the measured cube in a location where it won't be disturbed for some time. Check occasionally to see if you can detect any size change.
   
   
3. What accounts for the change?  How would an alchemist explain it?  How would a modern chemist explain it?  If Lavoisier is correct and the water is still the same substance as ice and water vapor (both are H₂O), what would you expect to see is the difference if you had a way to see atoms and molecules?

Part III:  Determining Which is more Elementary

In this investigation a measured amount of a substance is chemically transformed.  By comparing the amount after with the initial amount, it should be possible to determine whether the product or reactant is more elementary.  If the substance gains mass, then something has combined with the initial substance.  If the substance loses mass, then something was removed from the initial material leaving the product more elementary.

Materials

• A measurable amount of baking soda, NaHCO₃
• measuring device (balance, a home made one will do!)
• one or two equal sized sheets of Aluminum foil to hold and transfer the baking soda
• baking oven

Procedure

1. Weigh some baking soda on a sheet of Aluminum foil.  If you don't have an accurate balance to weigh this, construct a simple equal arm balance.  Use the balance to obtain two equal amounts of the baking soda, each on equal sized sheets of foil.  Being very careful to not lose any in transfer, use one of the amounts for the next step while keeping the other unchanged to provide a later comparison to determine if the mass decreased, remained unchanged, or increased.
   
   
2. Being careful to lose none in transfer, broil the baking soda for 1/2 hour as warm as possible (>520°F).
   
   
3. Allow the soda to cook to room temperature then weigh again.
   
   
4. Did the mass change?  Is the original baking soda or the product more elementary according to the definition of Boyle and Lavoisier?
   
   
5. If the formula for the baking soda is NaHCO₃, speculate what the formula might be after cooking.  (Hint: Try rubbing a little between wet fingers to confirm or refute one possibility; wash well afterwards.)

If you need course credit, use your observations recorded in your journal to construct a formal report.

This might not seem a very significant series of investigations, but by doing a large number of similar experiments on as many kinds of substances as possible, chemists of the century following Lavoisier determined a list of which substances are truly elementary and which are compounds of multiple elements.  The end result has radically improved the understanding of our material world, led to much increased standards of living, and is significantly increasing all our life expectancies.

References