Soap Suds Explosion

States of Matter (Finale)

Materials:

• Liquid Nitrogen
• Big stainless steel dewar
• 2-3 Tbsp liquid dishsoap, diluted with water
• Food coloring, if desired
• 2 gallon plastic bucket with 2-3 liters of warm water
• 10’ x 12’ plastic tarp
• Blue cryo gloves
• Safety glasses

Key Points:

• The water in the bucket is much warmer than the LN₂.
• When the LN₂ is dumped into the bucket, it warms up.
• When you heat a liquid, it turns into a gas.
• Gases take up much more space than liquids.
• When a lot of Nitrogen is warmed up really fast, it suddenly takes up a lot of space.
• The soap lets us see the Nitrogen gas because it makes bubbles.
• The more Nitrogen or soap we use, the more bubbles we get.

Warnings:

• This is the MOST DANGEROUS DEMO that the Physics Van does. Do NOT do this demo if you have not practiced it thoroughly.
• This demo is messy. If you’re indoors, use a tarp to catch most of the soap. You will probably still have to mop some up.
• After the explosion, the floor will be very SLIPPERY. Be careful not to fall.
• If you're using food coloring to make the soap look less diluted, use only 2-3 drops. More than this will stain people's clothing.
• Avoid any skin contact with Liquid Nitrogen.
• NEVER reach into the LN₂ with the cryo gloves. They are NOT waterproof, so the LN₂ soaks into the glove. This traps it close to your skin and freezes your hand. Be careful when you're holding the dewar to avoid touching the LN₂ with the gloves.
• Don't drop the dewar. Dewars are expensive, and we don't want to have to replace ours because of dents.
• Hold the dewar by its edges, not by the handle. The handle can separate from the dewar, making you drop it.
• Even if you are not touching the LN₂, the inside surface of the dewar is cold. Don't touch it with your bare hands.
• Remove watches and stuff from your hands and wrists when dealing with LN₂. That way, it can't get trapped close to your skin.
• Always wear safety glasses in addition to the cryo gloves for LN₂ demos. The LN₂ can splash up out of the bucket. Although these small drops will evaporate right off your skin, they could cause serious damage to your eyes.
• Nitrogen travels farther than water when spilled because it doesn't adhere to warm surfaces. If it reaches the floor, it WILL reach the kids, and they will try to touch it. Make sure that someone is holding up the edges of the tarp to catch any stray Nitrogen (or soap).
• Liquid Nitrogen freezes stuff. Examples are: people's hands or feet, expensive gymnasium floors. The tarp will help to protect the floor, but it’s just a safegaurd. The simple summary here is don't miss the bucket. Pay close attention to where the dewar is as you pour it. And if you do miss the bucket, don't hit your feet. Your shoes and socks will soak up LN₂ just as well as the cryo gloves do.
• Do not do this demo until you are comfortable with the audience and experienced enough that the explanation is second nature. You should be able to focus your attention entirely on what you're doing, without the distractions of being nervous or unfamiliar with the explanation.

Things to talk about:

• This demo goes all the way back to the beginning of the show when we were talking about solids, liquids and gases.
• There've been a lot of demos since we last used LN₂, so you should review the states of matter and how the LN₂ expands as it changes into a gas.
• In the dewar, the LN₂ is a liquid, but when it hits the warm water, it changes into a gas.
• Since you're using a lot of LN₂, it will take up a lot of space when it warms up.
• Adding some soap to the bucket will let us see how much space the gas takes up, since it will make bubbles.
• Just use a little bit of the soap, since more soap means more bubbles, and we don't want to make too much of a mess.
• While you're talking, other Van members can be adding more soap to the bucket. To be funny, they can use the entire container of soap. (If this is the case, the soap should be diluted. More soap really does mean more mess, and 2-3 Tbsp. of soap is messy enough.) Remember, MESSY = FUNNY.
• Hold the dewar overhand, with one hand under the back edge of the dewar and one curled over the front lip. Don't hold it by the handle, since the handle can separate from the rest of the container.
• Get a countdown from the audience and dump all of the LN₂ into the bucket. The more force that you dump it with, the bigger of an explosion you'll get.
• If you don't want to be totally covered with soap, move out of the way quickly. The soapier you get, the funnier it is, but you may not want to be wet for the rest of the day. (Remember: the floor is slippery, so be careful.)
• Once you've practiced with the demo and can get all of the Nitrogen into the bucket consistently, try to pull the bucket out of the way as soon as you've dumped the LN₂. With the bucket out of the way, the soap bubbles can go up higher.

Click here for the Adobe PDF version of this demo (text only), for printing purposes.