Intro chemistry: Vinegar and baking soda

Grades 1-3 wouldn't have been able to handle the molecule-size
estimation lab I did with grades 4-6 today, so I did the classic
vinegar-and-baking-soda experiment.

I started by eliciting what they already knew about atoms and
molecules, and trying to organize the barrage of facts they gave me.
Some knew very advanced things like a carbon atom binds with four
other atoms, but they didn't necessarily have a clear idea of how to
define atom or molecule, or who thought of the idea of atoms and why.
(Note to self: bringing in the ancient Greeks doesn't help because
they don't know who the ancient Greeks are.)  Most simply, if you take
a substance like gold and divide it into smaller and smaller pieces,
an atom is the the smallest possible unit which still behaves like
gold: you might be able to split that atom, but then the split pieces
will not behave like gold.

For other substances, like water, we call the smallest unit a
molecule.  Why?  It turns out that we can make water by uniting two
other substances, hydrogen and oxygen, and we can get the hydrogen and
oxygen back by splitting the water.  We can't make hydrogen or oxygen
by any analogous process, so the smallest unit of water does not have
the same status as the smallest unit of hydrogen, oxygen, or gold.
Hydrogen, oxygen, and gold are therefore called elements whose
smallest unit is an atom, and water is called a compound whose
smallest unit is a molecule, which in turn is composed of more than
one atom, usually atoms of different types.  (We also reviewed states
of matter---solid, liquid, gas---to avoid any confusion there.  Just
about any substance can be put into any of these states with enough
heat.)

Making water from hydrogen and oxygen is a bit dangerous because it
releases a lot of energy, and splitting water requires electricity,
which doesn't sound like a good lab for 1-3 graders.  So we looked at
a reaction between vinegar and baking soda.  First, I had them combine
the two and just observe what happens: it bubbles violently for a time
and then settles down, and there seems to be less baking soda at the
end of the process.  This is a chemical reaction: the baking soda and
the vinegar each donated some atoms to make a new compound which
escaped because it's a gas at room temperature.  Chemical reactions
are naturally explained by the atomic nature of matter, whereas if we
didn't believe in atoms we would have to stretch to explain all these
reactions.

Next, I had them measure how much mass was lost to the atmosphere in
the reaction.  They redid the reaction, this time carefully weighing
the plastic cups, the reagents, and the products.  At the simplest, we
wanted to compare the total mass of "one cup with some vinegar and one
cup with some baking soda" with "the cup with the product of the
reaction plus the empty cup."  Groups found a difference of 1-3 grams,
easily measurable.  I had given each about 1.5 teaspoons of baking
soda and a similar shot of vinegar, for a total of about 40 grams of
reagents.  The plastic cups were about 6 grams each.

With a bit of time left, I told them that the gas given off was carbon
dioxide.  Could they think of any reactions which take things from the
atmosphere rather than give things to the atmosphere?  I steered the
discussion toward something they knew about: plants. Plants take in
lots of carbon from the atmosphere; that's basically most of their
mass.  When you grow a plant in a pot, it takes some trace elements
from the soil but it basically doesn't use up the soil even as it gets
pretty big.  It gets most of its bulk from the atmosphere.  It's
especially important to be aware of that because the reactions we
cause when we drive a car or indirectly cause when we turn on the
lights put carbon in the atmosphere, which causes global warming.
Planting trees can help take some carbon back out, and of course we
should use less to begin with.


Note: The kids needed more time than expected with the arithmetic
(just subtracting the initial mass from the final mass). This is more
a reflection on me than on the kids; they are 1-3 graders after all.