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Name: Leslie B.
Status: educator
Age: 30s
Location: N/A
Country: N/A
Date: 1999-2001


Question:
Can you please let me know what demonstrations I can do for my sixth grade science class with dry ice and liquid nitrogen? My colleagues are purchasing these materials tomorrow.


Replies:
If you put the dry ice in some water it will produce the fog effect, if it rests on a table the fog will spill over the container then when it reaches the edge of the table it will roll off like a water fall, more of a special effect than anything else. As for the liquid nitrogen get a flower and a bouncy (hollow) rubber ball, bounce the ball around then stick it in the nitrogen for a couple of seconds and "bounce" the ball again, throw it to the ground and let it smash to pieces. you can do the same with the flower, when its frozen it too will shatter into pieces.

I do not really know what you'd like to demonstrate. Just make sure to wear the correct safety stuff when playing with both dry ice and liquid nitrogen. Another one may be to use a one liter plastic soda container, put a little water in it then put a piece of dry ice in it put the cap on it and run, make sure everyone is a safe distance, as the dry ice dissolves and goes to its gas phase it will produce enough pressure to burst the container and usually with a pretty loud pop. I would recommend doing this outside!!

Michael Baldwin


Liq. N2: Shatter a piece of rubber tubing, or a flower. Pour some l-N2 on the floor and watch it scatter across the floor on the cushion of N2 gas that flash evaporates from the liquid. Pour l-N2 on the back of your hand for a few seconds. It will not burn because the gas forms an insulating layer between your hand and the l-N2. Do not do it for too long though. And do not try it with dry ice. Show that either l-N2 or CO2 gas puts out a flame. Configure the demo depending upon what you have available. You can "pour" the cold CO2 or l-N2, both of which are much heavier than air at room temperature, on a burning straw or candle for example. Pack one end [about 4 inches] of a length, about 18-24 in. of thin wall rubber tubing, with crushed dry ice. Tie off both ends before they become brittle. You now have a CO2 "snake" that will grow as the dry ice evaporates, and if all goes well, will explode when the pressure gets high enough. Of course do not let anyone get too close.

Vince Calder


First of all, remember that these materials are very cold and cause burns. When placing materials in either liquid nitrogen (hereafter LN2) or dry ice, always use tools such as hooks, tongs, and tweezers. It's best to wear gloves when handling either material. You can handle dry ice in your bare hands for short periods (< 1 second), and if LN2 contacts your skin, it's not a problem as long as it IMMEDIATELY runs off.

There are a lot of fun demonstrations, and some even teach science. One of my favorites from a teaching point of view is to inflate a balloon with air and then to slowly pour LN2 over it. The balloon will shrink to a much smaller volume, on account of the air inside cooling and contracting. Then when you stop pouring the LN2 over the balloon, it rapidly expands back to its former size.

Another favorite to demonstrate the effect of low temperatures is to illustrate the "glass temperature" of rubber. Both dry ice and LN2 are colder than the glass temperature of most rubbers. The glass temperature is the temperature below which the polymer is rigid and glass-like. Find a nice big strip of rubber (a big rubber band will do) and place it in with some dry ice (it takes a while to cool down - periodically tap it with a rod to see when it becomes hard) or liquid nitrogen (much faster - just keep it there until the LN2 ceases to boil so furiously). Take it out, place it on a hard surface, and hit it with something heavy, like a hammer. It clinks and shatters like glass. When the shards warm up, they are just regular rubbery rubber again. Think of other rubber objects you can destroy this way. I can tell you from personal experience that if you choose to use a NERF ball, keep it contained in a plastic bag or something. The little pieces of resulting rubbery dust are very difficult to clean up.

Another fun demonstration with a chemistry component is it illustrate some of the chemical properties of carbon dioxide as well as being fun. This is the conversion of water-insoluble calcium carbonate, CaCO3, to soluble calcium bicarbonate, Ca(HCO3)2. The reaction is

CaCO3(s) + CO2 + H2O = Ca(HCO3)2.

What you do is begin with a dilute suspension of calcium carbonate in water. This will be a milky white liquid. Add a chunk of dry ice and stir until the dry ice evaporates away. (That is the fun part - makes lots of heavy white vapor that kids love.) If you get the amounts right, the aqueous mixture is now a clear, colorless solution. (If it's not, either you didn't use enough dry ice, or you don't have enough water to dissolve the calcium bicarbonate.)

If there is time, you can also demonstrate the reverse reaction, known as the "boiler-scale reaction." Boiling a solution of calcium bicarbonate drives off one molar equivalent of carbon dioxide, forming solid calcium carbonate.

Ca(HCO3)2 = CaCO3(s) + CO2(g) + H2O.

This is the reaction that puts a white crud inside hot-water pipes, coffee pots, and boilers when the source water is "hard," that is, contains calcium and magnesium carbonates.

There are lots more things you can do with dry ice and LN2, but these ideas ought to get you started.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois



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