Class
notes by RE-SEED Leader - Alex
Vanderburgh
Introduction:
It starts, of course, with Ancient Greece. Our hero is Thales of Miletus,
who lived in 640 to 583 BC. He was a mathematician, astronomer, and philosopher.
And he wondered what everything was made of. His answer was "Water".
For him, Earth was a flat disk floating in an infinite sea of Water. Not
everyone thought water was enough. The other side argued there must be many
"elements". For Thales, however, "All things are water."
The "many elements" side won, but Thales gets credit for starting
it all.
Other Greeks increased the count to four: Water, Air, Earth, and Fire. This
was first proposed by Empedocles, but was accepted and modified by Aristotle
himself! Earth was first, as the center of the Universe. Water surrounded
Earth and was second. Then air surrounded them both. Fire came last, but
was even more heavenly - its natural place being way up there beyond the
sky. Aristotle added one more "element". He called it "aether"
- later known as "quintessence". It seemed to him that the stars
were so permanent and perfect that they must be made of something else.
[Quint means fifth. So Quint-Essence is the fifth element.] Down below,
everything is made up of the original four "elements". [Note that
"earth" is not dirt or rocks. Of course, they have "earth"
in them but could contain some water, fire, and air too.] Note that a block
of wood must be mainly "earth", since it is so solid. It must
also contain "fire" because it burns. And when it burns, smoke
and other vapors are formed, so it must have some "air" in it.
And finally, one of these vapors condenses into "water". Pretty
convincing stuff! Democritus is next. He introduced the word "atom"
to refer to the smallest possible piece of anything. ("Atom" comes
from a Greek word for indivisible.) Water was made of smooth spheres, (it
flows so easily). Earth is tiny cubes (it is so stable). Fire is very pointy.
(That's why it hurts!) I don't know what Air was like to him.
To Democritus, "atoms" were very very small. ("They can not
be smaller.") They are full and incompressible, having no pores. They
fill their space completely. [There was dispute over the possibility of
a completely empty place. Democritus said it had the same right to exist
as a full one.]
Atoms could have a different shape, position (e.g.up side down), and weight.
Everything is made from combinations. No atom can be destroyed, but a combination
can be changed into something else. It was this "something else"
that was interesting. [Gold is better than lead! There is some evidence
that they thought lead was a good starting place for making gold.]
The center of thinking moved to Alexandria, Egypt. The Egyptians had some
interesting practical procedures. They could extract metals from ore using
heat and charcoal. They could make glass from sand, and bricks from clay.
The Greeks called these processes for changing materials from one form to
another "Chemia". Then came the Arabs who added their word for
"the". (It was "al".) Thus was born "Alchemia"
which became "Alchemy" in English.
Alchemy came to have a bad name. One of its main goals was to make gold.
That is not bad in itself, many of their supporters were Kings. The trouble
was that there were a large number of cheats and fakers. Gold was found
in nature inside rocks, in its pure state. It was expensive to find it,
and to separate it from the rocks. One process is to crush tons of rock,
and wash the lighter stone dust away. Since gold is a heavy metal, it would
sink to the bottom - mostly as gold dust, but occasionally as a nuggett.
The dream of the alchemist was to "make" it from some cheap material,
such as lead. No doubt you recall that all things were made of Earth, Water,
Air, and Fire. Lead and gold must have had nearly the same proportions.
So they tried all sorts of schemes to change lead into gold. They never
succeeded, but they did create a large number of interesting
materials, among them chemicals we now know as "acids". The acids
were helpful in the search for "elements".
Nothing much happened during the dark age (300 - 1000). The Arabs were in
control of Alexandria, and the Roman Empire was fading away. After the Crusades,
some of the Alexandrian Greek and Arabian alchemy was discovered by the
Europeans. Europe began to take over as the center of Alchemy. The early
Greek thinkers were more Philosophers than Scientists. Scientists now insisted
that measurements and data should be gathered and that the theory be tested.
The old definition - that an element was indivisible - was preserved. The
new thought was that there must be more of them.
An Arab named Jabir "discovered" two of them. For him, all metals
must contain a common element since they were so similar. He thought it
was Mercury. (a.k.a. "Quick-silver") Metals do not burn easily.
His other element is the common ingredient in all things that would burn.
He chose Sulphur since it burns so well. A Swiss alchemist called Paracelsus
added "salt" as an element to explain materials that were not
metals, but also would not burn. That brings the count up to eight. As of
about 1600 A.D., they are: Earth, Water, Air, Fire, Quintessence, Mercury,
Sulphur,and Salt. You might argue that fire can be dropped from the list,
since Sulphur seems to take its place. Note that we have added Mercury and
Sulphur - two "elements" that will stay on the list!
It is interesting to try to guess how they were thinking. After all, our
way of looking at things came down a long path from theirs! They had eight
building blocks, but they were not of equal importance. There was a difference
in supply, and probably in mystical value. For example, What was "Earth"
? Certainly not "dirt"! Did it have a color? Did they hope to
refine a pure sample? Everything was made somehow from these 4-8 building
blocks. Were all "atoms" the same size? [We know the shapes were
different.] What held them together?
It seems that they did little measurement or experimentation, but
after a few centuries of alchemy, this began to change. The alchemists had
prepared many interesting "chemicals" that needed to be understood.
They had names, but no symbols. Salt was just "salt" - it was
not called NaCl. We didn't know about Sodium or Chlorine. [That is not too
surprising. Both of them combine with other things - sometimes explosively.
Neither of them sit around waiting to be discovered.]
Once the chemists started to weigh and measure things, the "law"
of definite proportions was discovered and new names were used. e.g."
Salt" becomes NaCl, "water" becomes "H-two-O".
(The name reflects the
proportions.) This also had an affect on the list of "elements".
One thought comes to me. The Greeks and Romans knew about gold,
silver, copper, iron, tin, lead, and carbon. But only sulphur and
mercury made the list of "basic elements". The others were made
up
from the magic eight! It would have saved a lot of effort if someone
had proof that Gold, Silver, and Lead were "basic" too! Since
the
thinkers of ancient times were more philosophers than engineers, the
"basic" elements may have been more "ideal" or like
"Principles" than
like real things.
Sorry, but that's about as far as my historical research has gone. Electricity
added another tool for Chemists, and there were Optic devices that produced
spectrum analysis. Helium was named after the Sun since it was discovered
there before we could find it or make it here! And early chemists did not
see any reason to weigh things. It was in the 1800's that they found the
beginings of the Periodic table! (And it was a Russian chemist that gets
the credit.) (Mendeleev)
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Jargon
We have given "Scientific" meaning to certain plain English words
and have introduced a few others. Chemistry is going to add to this list
in a big way, but not in middle school. Look for element, compound, and
mixture. Think of others you can add. Afterall, we do want to use words
middle school students will be familiar with, and we may as well introduce
a few new words they will run into again. (Like "compound".)
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"Periodic Table of Elements"
Your school should have a wall chart something like Appendix 6. It
may show six more elements 104 thru 109. They are all synthetic and
very short lived. Elements past 109 may be "created", but we have
found all the low numbered stable ones. Isotopes exist for some
elements. [An isotope has the same atomic number, but with more
neutrons, and with different radioactivity. - Probably wont come up in
class, but if it does, especially if a student asks, we should talk
about it carefully, and in THEIR words.]
104 261.1087 Rutherfordium Rf
105 262.2238 Hahnium Ha
106 273.1182 Seaborgium Sg
107 262.1229 Nielsbohrium Ns
108 Hassium Hs
109 Meitnerium Mt
I feel that Atomic numbers and "Isotopes" are a bit advanced for
middle school. I think they may even do harm, for they will be seen as a
REAL thing rather than as a useful language. The "particles" will
be seen as little spheres, with physical attributes. How do you answer the
question "What is a Proton, or an Electron made out of? Is it the same
stuff? You can find numbers for their masses. How about their "radius"?
I am more comfortable with the "law of definite proportion", and
that the elements in any column seem to have similiar properties. (I wonder
about the rows.) In general, I prefer to leave some mysteries and I try
not to state "facts" that must be memorized without any idea where
they came from. An especially weird form of knowledge is "facts"
about theoretical models - e.g. the mass of an "electron". [These
"facts" are true by definition!]
Another "interesting" observation is that the elements have pecular
abbreviations. Silver is Ag, Mercury is Hg, Gold is Au etc. At least, Carbon
is C, and Oxygen is O. The "reason" is historical, and there are
no doubt stories to tell for many of them. Early known
elements are named in Greek and Latin. Some appear to be English. New elements
are named in honor of famous scientists.
The story of the table itself is probably not over. For many years there
were gaps - especially in the "rare earths" - but they were found.
No doubt the inferences from the "gap" helped the searchers by
predicting the atomic weight and some characteristics. There is still room
for philosophic discussion too. Along with the table there is talk of "electrons,
protons, and neutrons". None of these is weighed or measured, but they
surely seem to have been "counted". For some people they seem
as "real" as anything else! They have been useful, and progress
does demand that we go beyond our own senses, but it seems worthwhile to
be aware that we have done so.
Mixtures and Compounds
A mixture often produces a physical change, a compound comes from
chemical change. Which is which in the list below?
Burning a candle
Melting a candle
Tearing paper
Burning paper
Soaking a newspaper
Boiling water
Dissolving salt in water
Rusting a tool
Frost on your window
Souring milk
Autumn leaves
Mixing Vinegar and Baking Soda
The "best" distinction is called the "law of exact proportions".
Chemical compounds have exactly the same proportions of ingredients. Mixtures
can have a wide range. e.g. A mixture of sand and water will vary from wet
sand to sandy water. Water is made by mixing Hydrogen and water at a high
temperature. You get an explosion and some water, but always two Hydrogens
and one Oxygen combine! The result is not even similar to either of them!
You can often tell "dissolving" from "reacting" by physical
observation. The "reaction" is usually more spectacular. It bubbles.
Or a solid powder is formed (a "precipitate"). The color may change
abruptly. Some of the new compounds have a sharp odor! Significant heat
is produced. (Like burning, or explosion!)
Mixing water and alcohol does not produce anything new, but there is one
surprise. Heat is produced, and the mixture has less volume than the sum
of the water and alcohol! But you can vary the proportions.
Incidentally, what is vinegar - a mixture or a compound?
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Demonstration: Some "chemicals" are invisible. (Like Air or
Carbon Dioxide. Some students wont know that "C0-2" is
the Fizz in "carbonated" drinks.)
1. Light a short candle in a beaker so that the flame is inside.
2. Put 2 tablespoons of baking soda in a balloon.
3. Put 1 cup of vinegar in a bottle.
4. stretch the balloon over the bottle top.
5. Pick up the balloon so that the baking soda falls in. The vinegar should
bubble briskly and blow up the balloon. It should stop after a while. (One
of the ingredients will be used up.) If we do this on a balance, we should
see no change in mass. [Do we have more "air"?]
6. Now take the balloon off and very slowly "pour" the carbon
dioxide into the beaker. It will snuff out the candle! [It will look like
magic since the carbon dioxide is invisible!] Note: Use a funnel. Carbon
Dioxide is heavier than air, but the natural flow of air around the candle
will blow the
carbon dioxide away!
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Many chemical reactions can be stimulated by electricity. (More about
this when we get to electricity.) When you put electrodes into
water, you will see bubbles form on both electrodes - one gets twice
as much! They are the hydrogen and oxygen that water is made of! It doesn't
take too much power. We can do this with a dry cell! [Practice at
home. Pure water is no good, add a little salt or Sodium Hydroxide.
Also some electrodes work better than others. Many schools will have
special equipment.
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Errata:
Page 94 Line 14 Change ferrocyanide to borate.
Page 98 Line 18 Change third second test tube to third test
tube.
Page 99 Line 20 Change burning split to burning splint.
Sources:
The Search for the Elements - Isaac Asimov 1962 - Not science fiction at
all, but entertaining and easy to read. I find it makes me want to find
out more. |