Andrew's Science Log

Science notes – with experiments!

Weather demos

leave a comment »

drinking bird

drinking bird

A summary of several demos I shared with five classes (in a row! harder than I thought) of sixth graders at my son’s school. They were learning about the water cycle, so I did some things about water vapor and air pressure. Water vapor itself is invisible, we can observe it only indirectly by condensation and so on.

Important: Please be careful, especially here with eye protection and fire. Losing eyes is bad. Fire can get away from you quickly, and an alcohol flame can be invisible. Proceed at your own risk.

1. Rain In A Jar (the water cycle)

Despite my innate desire that everything work just right, I have learned that kids really love hands-on stuff.  The trick is to find things that are interesting and safe and usually work even with, uh, offbeat techniques.

This little project that I lifted from somewhere-on-the-web illustrates the water cycle.  It is quick and cheap and works with kindergartners as well as sixth grade or older kids.  Just make the questions more difficult for older kids (vapor pressure is a good topic).

The students in pairs add about 200 mls. hot water to wide mouth GLASS jars they brought from home. I used boiling water to get things going, which in retrospect I think was too hot because the ice melted too quickly.   Be wary of scalding.  Plastic containers like peanut butter tubs can’t withstand heat, but kids will bring them anyway. 🙂

The students cover their jars with aluminum foil, pushing the center down far enough to hold several ice cubes. Don’t use too much ice or the ice melt will spill and make a mess.

Within a few minutes, water condenses on the foil and starts to drip. Voila, rain.

The evaporation and condensation will continue until the water cools and the ice melts, perhaps 30 minutes. The students can prepare the jars first thing and watch them during class. The instructor could do a large version with a 3-liter beaker or so.

A nice wrinkle is that the experiment presents all three phases of water. The use of ice isn’t essential, you could use dry ice, liquid nitrogen, knock yourself out. But it does look neat.

2. Drinking bird

Set up the classic “drinking bird” and explain that it is a heat engine, that is, it converts heat into motion. It is essentially a glass tube connecting two bulbs, the “tail” filled with a volatile liquid. I have a 20 year-old one filled with the now-banned Freon, and a new one uses methylene chloride, nasty for other reasons. They can be purchased for $5.

The head is cooled by water evaporation, much as sweat cools our heads. The cooling condenses the Freon or whatever within the bird’s head, such that the greater vapor pressure in the tail forces liquid up the neck. Eventually the top-heavy bird topples, the liquid drains back into the tail, and the cycle starts over.

Note that you don’t get something for nothing here — no perpetual motion. The bird removes heat from the surroundings, which is “lost” into the vaporization of water (and it takes a lot of heat to evaporate water). It is not a perpetual motion machine, even if you do replenish the water, because it needs that constant supply of heat to keep evaporating water.

Here are some further experiments:

  • Blow air gently over the bird’s head.  What does the bird do and why? Use an alternative “drink.” Denatured alcohol (95% ethanol) from a hardware store works great. Dilute it with water to reduce flammability. By the way, vodka — another form of diluted ethanol, ~40% for 80 proof — works, too (use the cheap stuff! adults only). Compare the dip rate of two birds simultaneously.
  • Look up the boiling point and vapor pressure of alcohol.  Alcohol evaporates more readily than water (higher vapor pressure), taking heat with it. Dab alcohol and water to different parts of your hand and blow on them to illustrate the cooling effect. (The colored liquid inside the bird evaporates more readily than water, but it is toxic — don’t break open the bird.)

Note that denatured alcohol is normally 95% ethanol plus toxic methanol and other impurities. This renders it undrinkable, to avoid liquor taxes but at great risk to alcoholics.  Methanol can cause blindness.

  • Measure humidity, that is, make a hygrometer. Count pecks per minute to calibrate against a working hygrometer. More pecks means less humidity. Is that absolute humidity or relative?
  • Enclose the bird in a large clear bag (or for the classy touch, a bell jar). The bird slows thens stops. Why? Rising humidity “dampens” evaporative cooling that drives this heat engine — the way sweating doesn’t help you on a sticky day — but the change happens so quickly that I think reduced convection or something else is in play. I’d like to try heating or cooling the bell jar with all else constant to see what happens, as the vapor pressure limit increases with temperature.

A sling psychrometer measures humidity on the evaporation principle. One thermometer is covered by a wick, the second not. When air is forced over the thermometers, the “wet” thermometer cools. The difference between the wet and dry bulb temperatures and a psychrometric table (below) or chart provides the humidity.

  • Can you use a sling psychrometer to measure humidity below freezing? (Yes, but how?)

Note: This table has drawn so many search engine visitors that I’m adding the raw data in CSV format (change the extension to .csv and import into a spreadsheet such as OpenOffice), two classic psychrometric graphs, and another graph with slightly different details. (Sorry these references are all in Farenheit!)

psychrometric table

Psychrometric table data from the classic Bulletin of the U.S. Weather Bureau No. 1071 (summarized here).  Assumes sea level pressure; for variations see NWS humidity tables.

  • Convert to solar: Convert the bird to a radiation-driven “sunbird” by removing the felt and painting the head white or silver (white is a better emitter of heat, silver is the better reflector of radiation — how are these qualities important? What’s the difference between radiation and heat? (There is no such thing as heat radiation!) How would you test?) How would you test it?); perhaps just cover it with foil and adjust the balance at the pivot. Paint the tail black. Expose to the sun or a very bright light.

See also on building a simple psychrometer.

3. Make a cloud.

I’ve seen this experiment done with 2-liter soda bottles, but I happen to have a 12-liter boiling flask (hey, who doesn’t?).

Put some warm water into the vessel. Wait a bit and swish it around so some can evaporate. Then place your mouth over the opening of the vessel and blow hard to pressurize it, hold, and release. Nothing happens. (The children may enjoy your apparent failure.)

Next light a couple of kitchen matches, blow them out, and let the smoke into the chamber. I seem to get more smoke when it blow the match out before the head finishes burning. Pressurize again and release. Ta da, a faint cloud. Pressurize and it disappears. Release, and it reappears.

The large flask helps with the effect, maybe someone could contrive something like this with an empty aquarium? How about a well-sealed house?

The science goes something like: The warm water increases the humidity inside. Blowing (pressurizing) raises the temperature, allowing more water to evaporate, and introduce the humidity of your breath. The loss of pressure cools the air, and suddenly the air is supersaturated. But a cloud forms only if there are condensation nuclei, provided by the smoke.

Let a student try it, too. Someone with good breath.

4. Vacuum water fountain

This is a fun experiment, dramatic, and mildly dangerous. It illustrates that water vapor exists and takes up space, plus the atmospheric pressure around us that we don’t feel. Standard sea level atmospheric pressure of 14.7 psi/100 kPa is equivalent to what you feel diving under 10 meters or so of water. (Mind you, that would total 2 atmospheres, counting the one you started with.)

First, wear safety glasses.  Possible flying glass.  Back the kids away from the table.

Beforehand draw the tip of a 5 mm. flint glass tube of about a half meter into a nozzle half the diameter. Insert it nozzle first into a stoppered 1-liter boiling flask containing a small amount of water, leaving a space between the tip and the bottom of the flask. The stopper has to be very tight; use some water to get it seated well. I put a ball valve in the second hole in the stopper to increase my comfort level that the flask wouldn’t explode. (Did I mention safety glasses?) Also set out a beaker filled with at least as much cold water as the volume of the flask.

Hold up the flask and ask what’s in it. Water, obviously, and air, or oxygen, nitrgoen, carbon dioxide, water vapor, etc., all adding up to atmospheric pressure.

Next, heat the water in the boiling flask on a lab stand to, well, boiling. Let it get good and hot. Point out that the white stuff exiting the tube isn’t steam/vapor, rather condensation as the vapor expands and cools. After all, where does the mist go when it disappears a few inches away? A flame held at or near the tube’s tip will eliminate the mist.

Remove the heat. Be careful, water touching hot glass will cause a sudden burst of steam and maybe crack the glass. I blew the stopper out once. Swish the water around while it’s still heating to avoid overheated glass.

Ask what’s in the flask. Claim there’s almost no air, just water vapor. This may sound odd, given the small amount of water that boils off, but water vapor occupies over 1000 times the volume of its liquid. Ask what would happen if the vapor suddenly condensed into liquid water.

The water in the flask will not only be turbulent, it may also, as one student exclaimed, boil, especially around the stopper. I am very curious but skeptical whether the pressure inside is indeed low enough to cause boiling — it is possible to boil room temperature water with enough of a vacuum — or it’s just air leaking in. But leakage can’t account for much. When the experiment is over there will be almost no air in the flask. (Be careful the stand doesn’t tip over!)

At this point the flask is cool and can be safely handled.



Written by Andrew Wells Douglass

3 April 2008 at 09:02

Posted in Physics

Tagged with

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: