While preparing to give a presentation in Las Vegas, I was playing with liquid nitrogen to develop some new experiments that might work on stage. First I made a liquid nitrogen geyser (watch it here) and realized that the pressure change as liquid nitrogen turns to a gas could possibly be used to make an air horn. I ran to the dollar store and picked up as many noise makers as I could find and started retrofitting them to the top of containers. This is what came from that experimentation.
The Science of Liquid Nitrogen and the Liquid Nitrogen Airhorn
How Does a Liquid Nitrogen Airhorn Work?
Liquid nitrogen is -320 deg F (-196 deg C) which is incredibly cold compared to the air around it. Because the temperature difference is so great, the liquid nitrogen immediately 'boils' when it is exposed to air and turns to a vapor.
The gas laws of chemistry explain that a liquid takes up less space (less volume) than a gas. Think about it, the molecules in a liquid are much closer together than a gas which is why it acts like a liquid. As the liquid nitrogen turns from liquid to gas the amount of space it takes up increases. If we keep the volume of the container holding the liquid nitrogen the same then the pressure will increase inside the container. Having a hole in the container allows this pressure to escape and the smaller the hole the greater the pressure within the container and the greater the velocity of the gas escaping. You can clearly see this in the Liquid Nitrogen Geyser video as the gas builds and pushes the liquid down into the tube and out the top.
This is why the airhorns make noise when attached to the top of the flask. The nitrogen gas is being expelled from the opening in the flask and the rush of air makes the horns sing! Believe me, the sound was even more annoying in real life than it sounds on the video.
So, did it get the attention of the people in Las Vegas? Absolutely! It was a hit!
Ideal Gas Law and Liquid Nitrogen in a Container
The Ideal Gas Law explains the build up of pressure inside the container.
PV = nRT
P - Pressure
V - Volume
n - number of moles
R - gas constant
T - Temperature
Let's simplify this. Looking at the right hand part of the equation (nRT) we can see that "n" (the number of moles; or, more simply, the amount of liquid that has turned to gas) will increase because the liquid nitrogen is quickly turning to a gas. "R" is the gas constant and will not change (just in case you care, R = 0.0821 L atm/mol/K). "T" is increasing because the gas is warming as it moves closer to room temperature.
So, if we multiply nRT together as the liquid nitrogen is warming inside the container we would see this number continually increase as more liquid turns to gas. And, if the product on the right side is increasing, and it is equal to the left side (Pressure and Volume) those too must be increasing. This, in turn, causes the gas to be pushed out the top in a rush of air and we get the delightfully annoying sound of a liquid nitrogen airhorn!
How to Build a Liquid Nitrogen Airhorn
Supplies
Check local welding and medical gas suppliers
Plastic Soda Bottles
Before starting, be sure the noise maker party horn fits snugly in the opening of the soda bottle but that the opening is not obstructed - this could cause the bottle to explode! Fill the bottle 1/3 full of liquid nitrogen. Press the party horn into the top and allow the liquid nitrogen to turn to a gas. You may speed up this process by setting the bottom of the bottle into a container of water as this will increase the phase change from liquid to gas. Be careful that the noise maker does not freeze and obstruct the opening - this could cause the soda bottle to explode!
Keep on Learning!
~Craig
