Demos: 3E-01 Liquid Nitrogen Demos
A variety of demonstrations is performed, each illustrating the physical changes associated with ultra low temperatures. Examples include freezing flowers, driving a rubber spike into a piece of wood, shattering a cold handball and others that the demonstrator might dream up.
Directions: Because liquid nitrogen can cause painful burns on the skin, use caution in performing these demos. We recommend wearing insulating gloves and eye goggles. The equipment available is a Dewar filled with liquid nitrogen and (optional) a Styrofoam cooler. Begin by placing a fresh flower (you will need to provide this) or some plant material into the LN. Wait for the boiling to stop and then smash the plant on the lecture desk. A piece of rubber or plastic tubing may also be immersed and shattered. CAUTION: Pieces may fly off and hit someone, so do this with care. A lead bell is rung and one essentially hears no recognizable ringing sound because of the softness of the metal. But after immersion in the LN, the bell has a more noticeable ring due to the hardness. A piece of soft rubber, shaped like a wedge is held with tongs and immersed in the LN. The wedge is then placed point down on a piece of pine and a hammer drives the wedge into the wood. Place a handball in the LN and when it gets really hard, throw it against the side wall (or down onto the floor). It will smash into several pieces.
Optional: Pour the LN into the cooler. Place an inflated balloon in the cooler. After a brief time, remove the balloon. It will be shriveled and somewhat rigid. Place it on the desk and watch it expand as it heats up.
Suggestions for Presentation: You can make this as dramatic and entertaining as you like. First point out that the liquid nitrogen is very cold (-196°C or -321°F). Discuss how structures might change when subjected to these low temperatures. Do the demos listed above and/or others you might invent.
For the hams: This one is guaranteed to bring down the house. Ahead of time, freeze a sausage link. Just before lecture, place the link (glycerin helps here) in the index finger of a left-hand rubber glove (if you are right-handed). Trim the sausage so it just fills the finger of the glove. When you are about to extract something from the cooler, say, LN can cause severe burns, so Im going to put on this rubber glove before I put my hand into the cooler. Fold your index finger back into your palm as you put on the glove. Swish the finger around in the LN for a bit and then pretend that you suddenly got burned because of a leak in the glove. Say something like, Well, Ive lost that finger. Then pick up a hammer, place your hand on the table and start smashing the index finger. Pieces of meat will begin flying everywhere and you will definitely hear an audible gasp from the audience! (Pure entertainment!) You can complete the act by removing the glove and showing your finger missing!
Applications: Structures that are subject to extreme cold (including objects in space) undergo physical changes that must be compensated for.