The air we breath is about 78% nitrogen. Typically this is a gas but when brought down to an extremely cold temperature nitrogen becomes a liquid. Not only is liquid nitrogen fun to play with, but it gives you an opportunity to begin to explore low-temperature physics.
What You Need
- safety goggles
- tongs and thermal mitt
- dewar (specifically designed to contain liquid nitrogen)
- sample of liquid nitrogen in a secure container
- plastic beaker
- table or other surface that will not be harmed by very cold temperatures
- Pyrex bowl large enough to hold a small quantity of liquid nitrogen and immerse the other objects listed here
- 12 inches (approximately) of lead tin or other solder wire
- 20 g hooked mass
- metal tube with one end open and the other sealed
- medicine bottle with easy snap on/snap off lid or a 35mm film canister
- cork that loosely fits into the open end of the metal tube
- Liquid nitrogen must be handled safely. This means it must be stored in a specially designed container called a dewar, which is intended for this purpose. Do not put liquid nitrogen in a typical lunchbox thermos, which should never be used for liquid nitrogen.
- Liquid nitrogen is so cold, it can freeze human skin in a very short time. For all these activities, all participants should wear safety goggles and avoid any sustained contact with skin. Be especially carefully to avoid splashing liquid nitrogen, which could get trapped under clothing and cause freezing. Also, remember, objects that have been immersed in liquid nitrogen have themselves been brought to a very low temperature and should be handled appropriately.
Splitting a banana
- Carefully pour some liquid nitrogen into the Pyrex bowl.
- Using the tongs and thermal mitt, immerse the banana into the liquid nitrogen for about 15–30 seconds. Initial bubbling and vaporization diminish as the banana approaches equilibrium with the liquid nitrogen, as shown in Figure 92-1.
- Remove the banana and place it on the table.
- Take the hammer and strike the banana, as shown in Figures 92-2 and 92-3. (For contrast, you can strike an unchilled banana either before or after, showing the effect of the much-more-brittle frozen version.)
- Using the same bowl as the previous experiment, immerse some fresh flowers in the liquid nitrogen.
- Drop the bouquet on the floor or strike them on the table.
- Form the piece of solder into a coil roughly ¾ inch (1–2 centimeters) in diameter.
- Notice the lack of stiffness in the spring.
- Immerse the spring in the liquid nitrogen for roughly 15–30 seconds.
- Hang a 20 g (or so) hooked mass from the frozen spring and compare its stiffness with that of the room-temperature version.
Balloon—filled with air
- Blow up the balloon and tie a knot in the open end.
- Immerse the balloon in the liquid nitrogen.
- Observe what happens as the balloon is cooled.
- Remove the balloon from the liquid nitrogen and again observe what happens.
Balloon—filled with liquid nitrogen
- Pour a small amount (start with about 10 mL) of liquid nitrogen in a balloon.
- Tie a knot in the open end.
- Set the balloon on a table.
- Step back and make sure no one is near the (expanding) balloon and especially make sure no one's face is close to the balloon.
- Observe what happens as the balloon is exposed to the warmer air temperature.
Prescription container/Film canister
- Place the film canister (or a plastic prescription container with a snap-off lid) on a table top or on the floor. Do not use a prescription container with a screw-on or a child-proof lid that does not easily snap off with moderate force.
- Pour some of the liquid nitrogen into the plastic beaker.
- Pour some of the liquid nitrogen from the plastic beaker into the film canister. Fill the film canister about ¼ full with liquid nitrogen.
- Snap on the top.
- Stand back as pressure builds up in the container.
- Place the cork gun where it is aiming in a safe direction (and specifically not directed toward anyone's face).
- Pour about 50 mL of liquid nitrogen into the cylinder.
- Lightly place the cork in the open end of the cylinder. Do not jam the cork in so tightly that it cannot be pushed out by the pressure that will build up in the cylinder.
- Stand back. Pressure will build up as the liquid nitrogen evaporates.
The frozen banana and flowers will shatter. The solder will temporarily become much more spring-like. The air-filled balloon will shrink as the air inside contracts from the extreme cold, and then it will re-inflate as it warms up again. The liquid nitrogen-filled balloon will expand and possibly burst. The lids of the film canister/ prescription bottle will pop off. The cork will shoot out of the metal cylinder.
Why It Works
Objects become more brittle and contract from the extreme cold. As the liquid nitrogen evaporates, it occupies a much larger volume. For a given volume, the gas has a much larger pressure.
Other Things to Try
For many experimenters, liquid nitrogen may not be easily available on a daily basis. While you have a supply of liquid nitrogen available, you may want to consider doing the other projects that also require liquid nitrogen, such as Project 101 (effect of temperature on resistance) and Project 106 (superconductivity).
Liquid nitrogen provides an opportunity to explore low-temperature physics. This includes making normally elastic materials brittle. Materials cooled by liquid nitrogen contract. As liquid nitrogen evaporates, it expands.