It's a Gas

Last Friday I discussed solids, liquids and gases with the 1-2
graders.  I brought in samples of each to provide a basis for
discussion.  In addition to the obvious (a wood block, a glass of
water, a balloon filled with air), I brought some things designed to
stimulate their thinking: a rubber band, a cloth, a balloon filled
with water, and sand.  We took about 35 minutes to discuss how we
could define solid, liquid, and gas.  It's not as obvious as you might
think at first; for example, if liquids and gases flow unlike solids,
why can you pour sand?  Does that mean sand is a liquid?  I wish I had
time to document our discussion here!  I'll just document that at the
end it is important to note that substances can change from one state
to another and back depending on the temperature.  Water is the best
example: we talked about glaciers, lakes and rivers, and rain, which
they have already studied this year.  But it's worth mentioning other
examples lest they think this is peculiar to water.  The metal parts
of their desks were once liquid, which was poured into a mold.

In the hour after snack break, we did a more extensive experiment.  I
handed out cups with (small amounts of) vinegar, and they wrote down
observations: it smells funny, it's liquid, etc.  They did the same
with cups of (small amounts of) baking soda.  They also weighed both
cups on the scale together.  I found it easier to use a kitchen scale
which read grams rather than a scientific scale which reads to a
hundredth of a gram, so that I didn't have to explain decimal points.
Then they mixed the two and observed the reaction.  They drew it and
wrote down their observations.  Then we observed what was left: it
smelled different, and it weighed less (typically by a few grams out
of about 100 to start with).  We figured out together where the
missing grams went: when the bubbles popped, the gas escaped (in the
earlier session we had talked about this with respect to balloons).

They did all of the above in small groups (individuals, actually,
because we had three adults and four kids!), but we spent a few
minutes at the end summarizing what we learned.  One child wrote on
his worksheet "V [vinegar] +B [baking soda] = air" so that was a great
place to start discussing.  Did we know that gas was air?  What else
was produced?  They seemed to not recognize that what was left in the
cup was also a result of the reaction and should go on the right hand
side of the equation.  Is the stuff left in the cup just leftover
vinegar and baking soda?  No, because it smelled different.  The
equation written by the child was a great insight, but by the end we
produced a more accurate equation with more words.

Finally, it is important to note that this is a chemical reaction: we produced
some new kinds of substances! This is very much unlike water going
from liquid to gas, which they might have in mind as a model
transformation of a substance.  Because some of them like explosions,
I related it to the chemical reactions in explosions.  Explosions (ok,
most explosions) are chemical reactions too; they just happen faster
and give off more heat.

Then the kids spent 5-10 minutes drawing a scene with as many
different solids, liquids, and gases as they could think of, labeling
each.  At the very end, I rewarded them with a show of Diet Coke and
Mentos fountains.  It was raining, so I bought 1-liter bottles whose
fountains could be contained in the sink.  This turned out to be a
great capstone for the morning, as carbonation seemed to be a new idea
to many of the kids.  We tasted the Diet Coke before and after
defizzing to see the effect of gas on our taste buds.  We also
discussed how the gas was in the liquid and normally comes out slowly
(you can see small bubbles coming out when the cap is off) but comes
out quickly with the help of Mentos.

For those who want more: Here is an entertaining video of Diet Coke and
Mentos reactions.  If you want to go beyond entertainment and learn more
about why it happens, you should watch the Mythbusters episode on Diet
Coke and Mentos.  They do experiments with different ingredients to figure
out what is most responsible for the reaction.