Liquid nitrogen is very cold, has a very low boiling point, and expands rapidly at room temperature as it becomes a gas. These properties can be illustrated in demos using everyday items like a teapot, pringles can, balloon, and flowers. Additionally, with a high-temperature superconductor cooled by liquid nitrogen, the Meissner effect can be demonstrated.
Activity/Demo Instructor Notes:
- Many of the safety precautions taken in handling liquid nitrogen can help to explain its properties and can be mentioned during the demonstrations.
- The boiling point of nitrogen is -320 °F, meaning nitrogen has to be colder than this in order to be a liquid. Temperatures this cold are unfamiliar to us in everyday life; we are used to interacting with nitrogen as a gas (up to 78% of air is nitrogen) and liquid nitrogen has to be produced by humans. Liquid nitrogen is so cold that it is dangerous to touch with your bare hands. The presenter who is pouring liquid nitrogen should wear gloves and participants should be warned not to touch it directly. Goggles are also a good idea.
- Gaseous nitrogen takes up 696 times more space than liquid nitrogen, and at room temperature, liquid nitrogen is always boiling. This means that it is important to store the liquid in a container that is not completely sealed. For example, the stopper on our container has a hole in it. If the newly created nitrogen gas is kept in a small volume, the pressure will increase quickly. This is illustrated by the demo of a top popping off of a pringles can, and this effect can cause an explosion in a container that is better sealed.
- If too much liquid nitrogen leaks in a small, poorly ventilated space, then there is a danger of asphyxiation. Due to the additional nitrogen gas, the oxygen levels in the air may become too low, which could cause unconsciousness or death
- Pour some liquid nitrogen into the teapot. Ask participants what they notice. The steam from the pot looks like water boiling on the stove, but here there is no heat source. What does that mean about the boiling point of nitrogen? What does that mean about the temperature of liquid nitrogen?
- Pringles Can
- Ask participants if they think liquid or gaseous nitrogen takes up more space. What do they think will happen when liquid nitrogen is poured into a pringles can and the top is placed back on? When you do this, point the top away from people. Even a small amount of liquid nitrogen should make it fly off. This should work multiple times, as long as there is still some liquid nitrogen in the container.
- Blow up a balloon and pour liquid nitrogen in the crystallizing dish. Ask participants what they think will happen when the balloon is placed in the nitrogen. Exploding is a common guess, but it should start to shrink instead. Depending on the audience, you could use the ideal gas law to explain this process, or just say that as temperature decreases, the air in the balloon will take up less space.
- Putting flowers in liquid nitrogen instantly freezes them. Participants can then break the flowers with a hammer. To control the mess, there is a plastic sheet to break them on. You may let the participants compare breaking frozen flowers to unfrozen flowers. This demo can also be done with marshmallows.
- Maglev track
- Introduce the magnet track and high-temperature superconductor to the participants. You can show that at room temperature, the material does not have superconducting properties, it doesn’t attract or repel the magnetic track. Then put the high-temperature superconductor in the liquid nitrogen. You can mention that freezing a material — from flowers to superconductors — changes its properties. Freezing flowers was relatively intuitive because we all have experience with frozen objects, we know that food becomes more brittle in the freezer. However, freezing the superconductor changes it in an unfamiliar way. The liquid nitrogen should make the material’s temperature lower than its critical temperature. Due to the Meissner effect, the superconductor should repel magnetic fields and float along the track.
Hannah Woodward, 2022-23 Wonders of Physics Outreach Fellow