Cold and Clammy

After spending most of the morning studying the dynamics of sand along California's beaches,  I had about 30 minutes left to tie up some loose ends I had left on my last visit.  On that visit, I had promised that I could catastrophically crush an aluminum soda can using just heat and cold, but it didn't work.  As soon as I left the school that day, I realized what I had done wrong, but instead of just explaining what I did wrong, I planned some activities to build up to an explanation.  The first was measuring the dew point in the room. (Note: there are lots of dew-point activities written up on the web; I'm just linking to a random one here out of laziness.  In particular, I saved time compared to the activity in this link by starting with cool rather than warm water.)  The dew point was about 10 C, in a room with a temperature of about 20 C.  I also had them answer some questions related to dew point, such as: Which city would you rather travel to, one where the dew point is 50 F or one where it is 80 F?  Explain why, and suggest a plausible location for each city.

Then we related dew point to relative humidity. I wanted to make a graph of amount of water that air holds, vs temperature of the air.  At any temperature, there is a maximum amount it can hold, so I can sketch this maximum amount as a curve which changes with temperature.  I elicited from them how I should sketch it: the warmer the air, the more water it can hold. On that same graph, how would we represent the air in this room? We know it's 20 C, and we know the amount of water in the air is substantially less than the maximum---if it were close to the max we would have seen condensation very quickly as soon as we began to cool the glass.  So I made a mark indicating that conceptually.  As we cooled the glass, we lowered its temp, so I drew a line going leftward from that point.  When it hits the max curve, it condenses.

So the dew point is an indication of how much water is in the air, but what we feel as humidity is really how much water is in the air relative to the maximum it could hold at that temperature. This is called relative humidity.  For example, the dew point was about 10 C, or about 50 F, and in a 70 F room that doesn't feel humid.  But in a 52 F room, that would feel clammy as well as cool.  So I asked the kids to brainstorm how they could build a device to measure relative humidity.  To my surprise (because I was hearing some whining) someone came up really quickly with the idea of a wet thermometer.  I said "Brilliant!" and tried to elicit more details.  Why is being wet important?  Because then there will be evaporation.  OK, how will evaporation change your thermometer reading?  There was much discussion of this, with about half the class leaning toward warmer and half toward colder, but eventually I steered them toward thinking about getting out of a swimming pool and feeling cold as all those little water drops on your skin evaporate.  The thermometer will definitely read a colder temperature! So how does this help you determine humidity? Well, if the air is very humid already, there won't be much evaporation, so the wet thermometer won't read much colder than a dry thermometer.  If the air is very dry, there will be a lot of evaporation and the wet thermometer will read much colder than a dry thermometer.  So we did the experiment, and we found about 16 C (61 F) for the wet one and 20 C (70 F) for the dry one.  Then we find a table which tells us the relative humidity as a function of dry-bulb temp and the temperature difference between dry and wet bulbs.

Now for the grand finale. I reminded them how much a substance expands when going from liquid to gas.  Similarly, when a gas condenses to liquid, it occupies much less volume.  So I put a small amount of water into an empty aluminum soda can, heat the can with a torch so that the gas in the can is mostly water vapor, then plunge the can upside-down into an ice bath.  The water vapor in the can condenses quickly.  Suddenly, there's a lot of empty space in the can, and it collapses catastrophically because the pressure on the outside of the can (standard atmospheric pressure) is so much greater than the pressure on the outside of the can (very little because the gas is gone).  When I tried to do this demo previously, I was not cognizant of the key role of condensation and I put very little ice into a giant pail of water, virtually guaranteeing that I would not get condensation.  You can see a video of this kind of demo here.  It was a satisfying conclusion.  Three kids wanted to take crushed cans home for keepsakes.

Liquid nitrogen cannon

This relates to our recent themes of solid/liquid/gas and pressure, but mostly it was for fun.  I brought a steel tube just a bit larger than a tennis ball, a 16-oz plastic soda bottle, a tennis ball, and some liquid nitrogen.  After the usual LN2 demos, we went outside to make the cannon.  I found a large object to hold the tube upright, then I loaded the bottle with about 4 oz of LN2, screwed the cap on tightly, dropped the bottle into the tube, and dropped the tennis ball on top.  Then we waited for the liquid nitrogen to boil into gas, which would make the pressure inside the bottle roughly 100 times atmospheric pressure.  It took about 5 minutes, and then the bottle exploded, propelling the ball hundreds of feet into the air and out into the neighboring field.  Even the ripped-apart bottle shot up in the air, about 50 feet.  It was awesome!

I had a second bottle and ball, but I was too greedy.  I put in about 8 oz of LN2 to get a bigger boom, but after a 7-8 minute wait we heard a hissing.  The bottle had a nonexplosive leak.  Since it was starting to rain as well, we went inside, figuring it would not blow.  But after another 7-8 minutes, we heard it blow.  Bottom line: it's awesome with just 4 oz.

Written in Fire

We split Friday morning into two unrelated activities: sound, and a
review of states of matter.

For sound, I brought a lot of toys: tuning forks, a xylophone, etc.
The standard I wanted to cover was "sound is made by vibrating objects
and can be described by its pitch and volume" so I started with the
tuning forks and steered a discussion of pitch and volume (they
noticed right away that the tuning fork vibrated).  It was pretty
funny, as the kids focused entirely on pitch, and I could not get them
to guess, despite numerous hints, that VOLUME or LOUDNESS was a
difference between the two sounds I was playing, even when they were
the same pitch.  (It didn't help that when I really whacked the
xylophone hard, it did change its pitch somewhat as the whole thing
shook.)

Then I turned the kids loose to play with the tuning forks, the
xylophone, and a few other toys:

• bathtub flutes, which you can fill with water and then blow on while they drain.  The pitch corresponds (inversely) to the length of the wave which just fits in the air-filled part of the tube, so the pitch starts out high and then drops as the water level drops.

• plastic hoses flared on one end.  You whirl them around quickly, and they make an eerie whistling noise.  Same principle as the flute, only this time the length is fixed, and we make the air flow by whirling the tube rather than blowing on it.

• a "thunder stick" which is a long spring connected to a drum membrane stretched over one end of a tube (the other end is open).  Holding the tube and shaking it results in surprisingly loud boingy sounds.

Then we got back together as a group and talked about how our
observations are explained by sound being a wave.  To visualize this,
Teacher Pa and I stretched a very long spring (like the coiled wire
that ran from a telephone to its handset, in the old days before
wireless phones) across the room, and I bunched up my end and released
the bunch (still holding my end).  It was very clear that a pulse
traveled the length of the spring to Teacher Pa, and it bounced off
her and came back to me.  I related this to the behavior of the
thunder stick (what do you think happens if you hold the spring rather
than the tube?)  We also had two plastic cups linked by a string, and
Teacher Pa gave a chance for each kid to hear her voice carried along
by the string.

For the piece de resistance I brought a Rubens tube.  This is a long
tube with many small holes drilled in a line, connected to a propane
tank.  You turn the propane on and light the holes so it looks like a
row of 100 candles.  Now comes the cool part: there is a speaker
attached to one one.  Hooking it up so that music plays on the speaker
makes waves in the propane in the tube and pushes it out more in some
places than in others.  The fire dances to the music!  Music has lots
of tones mixed together though, so it's best to start with some pure
tones.  I brought a function generator to generate tones of any
desired frequency and amplitude, which is a really great
visualization.  Into and beyond break/snack time, kids and teachers
from all the rooms in the school were cycling through our room and
watching this.  The upper graders were transfixed.  They wanted to try
all their favorite songs.  We found that (to the dismay of some)
Gangnam Style was a really good one for making the flames dance.  We
eventually had to shut it down at close to 11:30, about an hour after
I first fired it up.  To see a Rubens tube in action yourself, check out this educational video.

From 11:30 to 12:15, Teacher Pa led us through some activities
reviewing the states of matter.  For example, she passed out images of
many different things and the kids had to paste them onto a poster in
the Solid column, the Liquid Column, or the Gas column.  This was
really useful for the kids, and for the teachers to assess how much
the kids got it.  In discussing this with the kids, we found a great
idea for next week: clarifying what we mean by "amount" or "size" of
something.  The difference between "size" (which most people would
take to mean a diameter, a distance across, or a height) and volume
came up in the context of gas expanding to fill its container, and it
was clear that we didn't have time to address it that day.  So we'll
do that next time.  It will play into one of the Investigation and
Experimentation standards about measuring length and volume, but I
want to keep it conceptually rich as I did last year.

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.

Liquid nitrogen ice cream

For the potluck on the last day of school, I made ice cream using liquid nitrogen.  Basic recipe: milk, cream, sugar.  For strawberry ice cream I added vanilla plus strawberry puree, and for ginger ice cream I added ginger syrup and ginger bits.  The LN2 cools it down real fast.  It was a big hit.

Liquid nitrogen

Last Friday (May 25) Vera visited Primaria and brought liquid nitrogen, which is as cold as Uranus or Neptune.  Pluto might warm up close to liquid nitrogen temperatures during its summer, but in its fall it gets a lot colder and nitrogen is thought to freeze out of its atmosphere onto its surface.  Fun stuff you can do with LN2 includes: (1) freezing a banana hard enough to use it as a hammer and pound a nail into a piece of wood; (2) make a balloon completely flaccid as the air inside cools...blow on it to warm it up and it pops back into its normal state; (3) freeze a racquetball and watch it shatter when thrown on the ground; (4) make ice cream instantly (Vera didn't do that one); (5) freeze a flower and see how it shatters when frozen; (6) freeze anything the kids suggest.  Vera also related it to the infrared camera I showed the kids earlier this year.  What color would LN2 appear on an infrared camera?

This is also a great demo for elementary (and older) kids, although we haven't done it there yet.   Keep in mind, it is a demo, not a hands-on activity, although you can definitely involve the kids in thinking of what to freeze next and making predictions for what will happen,

The Sift Hits the Sand

Today was my first day with the 4-6 year olds.  I generally try to
think of an activity which builds on or is related to what the kids
are doing the rest of the week, so that my visits are not put in a
pigeonhole marked "Science" which has nothing to do with the rest of
their lives or studies.  (A big point I want to get across is that
Everything is Connected.  At the university level this might mean
emphasizing the unity of knowledge---students tend to see different
chapters of a textbook or different lectures as unrelated pigeonholes,
and must be prodded to think about the connections, which are actually
the important part!  But for these kids, it's enough to make
connections between science and their everyday lives.)

But this being the start of the school year, the emphasis so far has
been on community-building, and there wasn't an obvious hook into
physical science.  Teacher Jessica said that the kids had been
fascinated with some aspects of sand, so I thought of a way to build
on that.  I had them separate big, medium, and small particles from
the sandpile, and used that to discuss solids, liquids, and molecules,
as well as engineering.

Simple materials.

Before class, I built seven sets (seven is the maximum number of kids
per group) of coarse and fine sifters at low cost as follows.  I took
a 4" diameter PVC pipe and sliced it into short segments to form the
frames of the sifters.  For the mesh, I bought screen material.  I
wanted a variety of mesh sizes, but this was difficult at the hardware
store.  I ended up using what is basically window screen material.  I
also had on hand a much coarser wire mesh designed to form a skeleton
for papier mache constructions.  So I had two sizes, although I would
have liked even more and I will keep my eye out for different
materials in the future.  (A baker's sifter has a finer mesh, but mine
had no walls so it was too easy to spill the sane rather than sift the
sand.)  I cut the meshes into circles and duct-taped them onto the PVC
frames.  I also brought some small cardboard boxes, some paper coffee
filters, and 21 (3 for each child) 44-oz plastic cups, which happen to
have mouths which fit well with the 4" PVC pipe.  I wanted to bring
tweezers as well, but I forgot it.

I showed each group that I had been able (before class) to obtain one
cup of big stones and woodchips, one of medium stones, and one of fine
sand, and I gave them 10 minutes or so to experiment with any and all
of these tools to see if they could do it.  They all pretty much got
it, usually with some guidance (as much to keep them focused as to
show them how to do it), and no one found it so easy as to be boring.
One of the girls found an advanced way to do it: stack the coarse
filter on top of the fine filter on top of a cup, load the top with
sand, and shake the whole thing to do it all at once.  Like an oil
refinery, but with the heavy stuff staying on top!  This is why I
mentioned engineering: although I often emphasize the cognitive value
in being able to understand or accomplish something in more than one
way, there is often great practical value in finding the most
efficient way!

The oil-refinery configuration with the finer mesh in the middle. If we had more types of mesh, we could separate into many different sizes all at once.  Chaining together many separation devices to derive an ultrapure sample is a principle used in a variety of contexts I did not discuss with these kids, such as uranium enrichment. They might be able to see that a coin-sorting machine might be built this way, though.

We then talked about alternative ways to do the separation.  Some had
wanted tweezers to separate the particles one by one; I forgot to
bring tweezers, but that's a valid---even if very
time-consuming!---way to do it.  No one thought of using the box, but
when I asked how they would use the box about one kid in each group
guessed that if I just shake a box full of this mixture, the bigger
pieces come to the top.  I even brought a cereal box to make the
connection to every kid's experience of the small pieces of cereal
always being on the bottom.  This is because only the small particles
are able to fall into the small gaps which open up when the box is
shaken, very much like a sifter.

Pub mix after a light shake: the trend from small things at the bottom to big things at the top is pretty clear.

Next, I asked them if we could figure out a way to separate the fine
sand into even finer particles.  We tried a coffee filter, but the
holes in the coffee filter were too small to let any sand through.
Here I made the connection to the atomic theory of matter: water does
go through the holes in the coffee filter, and so must be made up of
very small particles, too small to see.  The same with air; air is able to push things because it is made up of small particles, even though we can't see them.

Finally, we talked about solids vs liquids.  I can pour sand from a
cup, so is it a liquid?  Most didn't want to say it's a liquid but
couldn't say why.  Again, it's useful to point out the progression of
sizes.  The bigger stones could be poured out of a cup but look
nothing like the flow of a liquid.  The finest sand flows more like a
liquid, but not quite.  The liquid has invisibly small particles, so
flows perfectly smoothly as far as we can see.  You can pour sand and
make a pile, but you cannot pour water and make a pile of water!

At the start of each group, I promised them that we would experiment
with quicksand if they made good choices during the main experiment.
The night before, I whipped up a batch of water-soaked sand, which,
with some imagination, could be quicksand. (Quicksand is water-soaked
sand, but apparently not quite the kind of sand we have in our
sandbox!)  This mixture of a liquid plus small solid particles has
interesting properties which are between those of a solid and those of
a liquid.  They had fun with this, but I plan to someday make better
quicksand, perhaps with corn starch.

All in all, I think this 20-minute activity worked very well for the
4-6 year-olds, and I think it will be something they will continue to
experiment with even after my visit.  I limited it to 20 minutes
because we had to get four groups through, but a longer time would be
fine too because many kids wanted to do more sifting.