Liquid Nitrogen Demos Overview
Owing to its extremely low temperature, liquid nitrogen can lower the temperature of objects that are exposed to it. Also, it has a boiling point that is much, much lower than the room temperature, which causes it to evaporate very quickly and expand. Below are the descriptions for several demonstrations that can illustrate the properties and effects of liquid nitrogen.
1) Liquid nitrogen in a kettle
When liquid nitrogen is added to a kettle it boils because of the exposure to room temperature. As it turns into a gas it escapes the teapot producing the whistling sound. This mechanism works similarly to how someone whistles while creating a small space between their lips to release vibrations. More vapor is produced as the temperature of the nitrogen rises, making a louder and higher sound.
2) Popping the lid of a Pringles Can
Because liquid nitrogen has a low boiling point, it turns into a gas very quickly when its temperature rises. The temperature of liquid nitrogen rises when it is placed into an ordinary can and covered with a loose lid. Fast evaporation of the liquid causes a significant amount of gas to build up inside the can. The walls of the can are under pressure from gas molecules. As enough pressure builds up, the gas will burst the lid and escape out of the can. For our demonstration, we will use only a small amount of liquid nitrogen, enough to pop the lid.
3) Shrinking of a balloon
When a balloon comes into contact with liquid nitrogen, it contracts. A reduction in temperature will cause the air’s volume inside the balloon to decrease for a fixed quantity of air at a constant pressure. The density rises as the gas molecules approach closer to one another, and thus, the balloon shrinks.
When the balloon is taken out of the liquid nitrogen, the temperature of the air rises, causing its volume to increase, and thus, it gets inflated again. However, it is essential to note that the amount of air inside the balloon remains constant throughout the demonstration. It is only due to the change in temperature that the balloon’s shrinking or inflation occurs.
For an ideal gas this is how the final and initial volume of a gas vary with temperature:
V initial / V final= T initial / T final
Instructor Notes
CAUTION!
Liquid nitrogen is a freezing-cold substance that can cause frostbite if it comes in contact with our skin. Thus, wearing safety goggles and protective gloves is highly recommended while performing any of the demonstrations involving liquid nitrogen at all times.
Also, when stored in a vessel other than a Dewar flask, the vessel should never be enclosed with a lid that completely seals it. If a cap covers up the vessel, it should have a small orifice to let the vaporized nitrogen escape. Sealing liquid nitrogen in a closed container can cause a dangerous explosion that could lead to severe injuries.
Liquid Nitrogen in a Kettle
- Firstly, place the kettle on a layer of foam or a flat surface. Ensure the kettle is empty, clean, and has no other substance. Keep the kettle open.
- Now, slowly pour the liquid Nitrogen from the Dewar flask into the kettle. Close the lid once the kettle is half full.
- As the boiling point of liquid nitrogen is very low, it starts vaporizing very quickly when exposed to an environment at a temperature higher than the boiling point. Vaporizing liquid nitrogen makes a whistling sound when it comes out of the kettle.
- More and more vapors of nitrogen accumulate in the kettle, developing pressure in the kettle. Observe that the intensity of the whistling sound from the kettle’s opening keeps getting higher and louder as the nitrogen gas escapes the opening with more speed.
This same phenomenon is also observed in steam engines.
Popping the lid of a Pringles Can
- Ensure that the lid of a Pringles can (or any other cardboard/plastic can) is lightweight, made of plastic, and easily removable.
- Slowly pour about a spoonful of liquid nitrogen from the Dewar flask into the can. Close the lid once there’s some liquid nitrogen in the can.
- Hold the can away from your body, having the top of the can facing upwards. The rising temperature of liquid nitrogen causes it to evaporate, expand, and exert pressure on the walls of the can. When enough pressure develops inside the can, the nitrogen gas causes the lid to be removed from the top with a force that causes the cover to fly up to some height and make a pop sound.
This demonstration is aimed to exemplify the effect of pressure exerted by a gas on the surfaces of the container they are kept in. Excessive pressure inside any container can lead to a dangerous explosion.
Shrinking of a balloon
- Blow air inside a balloon and tie it. Place a Pyrex crystallizing dish with a shallow depth and walls around it on a foam sheet layer.
- Pour some liquid nitrogen into the dish and bring a balloon in contact with it. Slowly start pressing the balloon toward the vessel as it starts getting compressed. It is recommended to use a pair of tongs for holding the balloon to perform this demonstration in order to avoid contact with liquid nitrogen.
- When the balloon has shrunk enough, remove it from the container and hold it upright to show it to the audience. The balloon will start inflating as soon as the temperature of the air inside the balloon begins to rise slowly.
- Repeat the above steps a few times to observe the same phenomena again.
Lowering the temperature of a fixed amount of gas leads to lowering the volume of air inside the balloon.