Lesson 12:

Concepts conveyed | Materials | Procedure | Benefits | Resource

Concepts conveyed:

The purpose of this classroom demonstration is to show the effects on physical properties and phases of substances by cooling them from room temperature to the temperature of liquid nitrogen. In addition, the phase transition of liquid to gaseous nitrogen is easily observed.

Materials:

• 4 L of liquid nitrogen (The temperature of liquid nitrogen is -320 degrees Fahrenheit or -195.8 degrees Celsius. Take caution in handling the liquid nitrogen to avoid frostbite).
• 3- carnations (or any other fluffy flowers)
• 1- ice cube tray
• 1- thin-walled rubber ball (paddle ball or racquet ball; ping-pong balls do not work well)
• 1-pair of insulating gloves

Procedure:

Part 1: Flower Power

Squeeze a carnation or another fluffy flower, or knock it onto a table top, to show its pliancy at room temperature. Immerse the flower in liquid nitrogen. Heavy bubbling occurs as the warm flower heats the liquid nitrogen into a gas phase. Wait until the heavy bubbling ceases and then remove the flower with tongs or insulating gloves. The flower is now hard and brittle. Squeeze the flower quickly or strike a table top with it. (If squeezing, squeeze and release quickly to avoid frostbite.) The flower will shatter into many pieces. Most of the weight of a flower is water. Placing the flower into the liquid nitrogen freezes the water into ice, making the flower brittle. Just as when an icicle breaks when it falls onto the ground, the flower shatters when struck against a hard object.

Part 2: Liquid Nitrogen icicles and popsickles

When making popsickles or icicles, a tray of fruit juice (which contains water) or pure water is allowed to cool in a freezer until the water turns to ice. In this demonstration, a different method for cooling is used: liquid nitrogen. Fill an ice cube tray with water so that the tray is only half full. Then, fill the upper half with liquid nitrogen. The water will freeze to ice. Where does the liquid nitrogen go? It evaporates when the heat in the water is transfered to cause the nitrogen to form a gaseous state and disappear in the air. (This is considered conservation of heat!) Use gloves to handle the ice cube tray to avoid frostbite.

Part 3: Follow the “bouncing” ball

Show that the rubber-walled ball has normal behavior by bouncing it on the floor. Then place the ball in the liquid nitrogen until the bubbling ceases. Remove the ball with tongs or with insulating gloves, and throw the ball onto a hard floor. (Do not hold the ball in your bare hands.) The ball will shatter. If the rubber wall is fairly thick, it might take a lot of force to get it to shatter. Often, fragments of the ball will fly in all directions when it breaks, so it is essential to place a transparent shield (made of an unbreakable, transparent and colorless plastic, like plexiglas) between the ball and the audience to prevent the students from getting hit with the fragments.

When a substance like rubber is cooled down, the motion of the molecules slows down and the substance stiffens. Materials processing industries make good use of this phenomenon. For example, when rubber rings for seals and gaskets are produced there is excess rubber that forms a fringe or seam where the two halves of the mold comes together. This fringe of excess rubber is usually much thinner than the rings and can be knocked off by first freezing the rubber rings, followed by shaking in a container with the other rings.

Benefits:

• Most students show a great interest in experimenting with liquid nitrogen, even though they may have seen the demonstrations before.
• The instructor can allow individual of groups of students to try one of these demonstrations, to increase participation.
• Additional participation can be encouraged by asking students to bring an object to class which they believe should show a change in physical properties when cooled by liquid nitrogen. They can predict the response and then test the response in class when their object is cooled in the liquid nitrogen.

Resource:

• Chemistry for Museums: A Manual of Chemistry Demonstrations; Center of Science and Industry: Columbus, OH, 1976; Show #5, Demonstrations #2,4, 8; Project Funded by a National Science Foundation Grant #7423549.