Dry Ice /Liquid Nitrogen Demonstration
Lesson 2 of 5
Objective: Students will be able to observe uncommon states of matter, such as dry ice sublimating (solid to gas) and liquid nitrogen boiling at room temperature (liquid to gas).
PE: MS-PS1-4 – Develop a model that predicts and describes changes in particle motion, temperature, and state of a pure substance when thermal energy is added or removed.
DCI: PS1.A: Structure and Properties of Matter: The change of state that occur with variations in temperature or pressure can be described or predicted using these models of matter.
S&E Practice: Obtaining, evaluating, and communicating information
CCC: Cause and Effect
The first step for this demonstration is to obtain dry ice (typically sold in grocery stores) and liquid nitrogen (sold at gas and welding supply stores).
TIP: Dry ice is fairly simple to obtain. I typically purchase 1.5 lbs, which sells for $8.27/lb, the night before and store it in a Styrofoam portable cooler packed with ice. Liquid nitrogen is a little more difficult in that you first need to locate a place to purchase it and have the correct container (called a dewar).
A dewar is basically a souped-up thermos, designed to maintain the temperature of liquid nitrogen for several hours. I purchase it the day before and store it overnight in my classroom. It is commonly referred to as LN2 and sells for about $20 for 10 liters (includes a teacher discount). If your school does not currently own a dewar check with your local high school.
TIP: Before I owned a dewar, I found out that the same place that sells LN2 also has a dewar that they hardly ever use. I approached the salesman and asked if I could rent his dewar for a day in order to use it for a science classroom activity. He agreed to rent it to me without any fee as long as I promised to return it ASAP. When I returned the dewar I included a student signed thank-you note and several photos of the day. Now I purchase LN2 from him at a discount every year. Dewars retail for about $450.
Material needed: dry ice ($12.32/1.49lbs), Styrofoam portable cooler (dry ice storage) liquid nitrogen ($19.66/10 liters), dewar (LN2 storage)
Demonstration material, small hammer, several small bowls, hot plate or access to warm water, liquid soap, large coin (quarter), small fish tank, blowing bubbles solution, rags, water dispenser type water bottle, 4 feet of tubing (diameter of tubing should fit snugly over mouth of water bottle, metal spoon, fresh carnation flowers, racket balls, cherry tomatoes, plums, bananas, construction nails, block of wood, large beaker, tongs.
The students will be treated to several demonstrations; it is a very fun day and is typically the highlight of the chemistry unit. I always demonstrate with proper eye protection and a lab coat. During the course of the demonstrations the kids are to be taking notes on what is occurring at each station. Their assignment is to create two drawings that represent what occurred with the dry ice and the liquid nitrogen.
I encourage my students to film any of these demonstrations; the kids typically post hundreds of photos to their social media pages.
TIP: I have nine tables in my room and perform nine demonstrations in one class period. My big room is that everyone must stay in their seats. I promise everyone will have a front row seat to at least one demonstration. If a student leaves their seat to crowd another table, their table misses a demonstration. Peer-pressure keeps students in line. I typically start out with simple demonstrations and save the best for last, this assures kids farthest away from the first demonstration still have an exciting view later in the class period.
Safety Warning – Dry ice can burn. It is at -78.5° C. I start out the day wearing chemical gloves to protect my fingers. I have found that if you juggle pieces of dry ice between your fingers you’ll be fine. Gripping the dry ice in a closed fist is asking for trouble. I allow my students to play with very small pieces (sand grain size) of dry ice. Sometimes they may burn their fingers, but no real harm occurs.
Demonstrations 1 & 2
Demo 1 – dry ice in a beaker of warm water
This is the first demonstration of the period. I make sure I have a supply of warm water available (warm water makes a better effect than cold water). Break a small piece of dry ice (size of a golf ball) off with a small hammer and place it in a bowl of warm water. The dry ice will bubble, along with fog emerging on the surface. Leave the dry ice in the bowl to continue sublimating (solid to gas), skipping the liquid phase. Point out to the students that sublimation is occurring.
Tip: At this point students in the back of the room may try to crowd this table for a better look. Remind them that every table will get a turn and if students get up prematurely they may loose a demonstration for their table.
Demo 2 – dancing coin, screaming spoon
Break another piece of dry ice off and place it in the center of the desk. Take a warm coin (warm it between your palms and use it to cut into the dry ice. The temperature difference will make a hissing sound (call for quiet in the room – good dramatic effect). Once the coin is halfway submerged let go of the coin. If it is positions correctly (practice makes perfect) the temperature fluctuation between the coin and the dry ice will cause carbon dioxide gas to sublimate into the air and cause the coin to quiver in the dry ice. Next warm the spoon in the palm of you hand and place the bottom of the curves ladle directly on the dry ice. It will emit a squeal as the dry ice, once again sublimates carbon dioxide; the warmer the spoon the louder the sound.
Demonstrations 3 & 4
Demo 3- large soap bubbles
I use the same bowl with dry ice that was used in demo 1. Soak a wet rag with water and dish soap. Dip your finger in the liquid soap and run it across the lip of the bowl, coating it with a fine amount of soap. Next carefully drag the wet rag across the top of the bowl, with the goal of producing a soap film bubble over the top of the bowl (practice makes perfect). Once the soap bubble is it place it will expand up fill the bowl with white carbon dioxide fog. You can pop the bubble to cause a sensations amount of carbon dioxide fog to spill over the top of the bowl.
Demo 4 – popping dry ice bubbles
Using that same bowl, add a few drops of liquid soap directly to the water and vigorously stir it with a finger for a few seconds. The agitated soap will produce many small bubble filled with white carbon dioxide fog. I scoop a small handful and smear it on the tabletop and let the kids pop bubbles.
TIP: To keep the other table excited I also smear some carbon dioxide bubbles on several quiet tables, bringing them into the demonstration. I sometimes do this to tables that have already seen a demonstration, letting them know that staying in their seats after a demonstration also has advantages.
Demonstrations 5 & 6
Demo 5 – levitating bubbles
This is one of the hardest demonstration, but well worth it if you can pull it off. I place a sample of dry ice in a beaker of warm water and place the beaker in an empty fish tank. The tank will now fill up with carbon dioxide. One of the properties of dry ice is that as a gas it is heaver than air, meaning it will sink to the lowest point in a sealed container. This property was used to invisibly extinguish a candle in: ‘Chemical and Physical Changes Lab’. The key is to get plenty of invisible carbon dioxide to settle in the bottom of the fish tank.
Using the bubble solution I carefully blow a bubble and try to get it to land in the fish tank. Don’t blow into the tank or you will blow all the carbon dioxide out. Instead blow across the top of the tank. I can only get it to work occasionally, but when it does work the bubble will magically float in the middle of the tank, floating on a layer of invisible carbon dioxide.
As background information I tell them that back in the old west when settlers were digging water wells on the prairie, they would often suffocate on their own carbon dioxide exhaled while they were digging. Since carbon dioxide was heavy it would stay at the bottom of the hole. To protect themselves (not knowing the science) they would often fashion small bombs using gunpowder tied in a piece of fabric with a fuse. The resulting explosion would push the carbon dioxide out of the hole. You can find this in Little House on the Prairie by Laura Ingles Wilder.
Demo 6 - bubble maker
This is the most complicated rig for these demonstrations. I use a sparklets type water bottle and an old hose from an abandoned shop vac. The diameter of the hose fits perfectly on the mouth of the sparklets water bottle. Pour warm water into the spaklets bottle and add several small pieces of dry ice (the more you add the longer this demonstration will last. Fit the hose onto the mouth of the sparklets water bottle. White carbon dioxide fog should pour out of the hose. Use the soapy bowl from demo 4, dip the end of the hose into the soapy bowl until a film of soap covers the hose. The billowing white carbon dioxide fog will produce a cascade of soap bubble falling out of the hose. I lift the hose up high and allow several soap bubbles to land on the desk and pop with a dramatic carbon dioxide explosion. This photo typically makes it into the yearbook.
Liquid nitrogen is much colder than dry ice (-196° C) Respect the LN2, wear gloves and remove rings and bracelets. If LN2 happens to splash on your bearskin, then warmth of your skin will momentarily deflect the LN2. However, rings will freeze, without knowing it the damage will be inflicted. Always wear eye protection. I point out that they will observe boiling liquid nitrogen at room temperatures, because LN2 has a very cold boiling point (-195.8° C). Boiling does not mean hot, as most student think, but is a change of state from liquid to gas and can occur at any temperature.
Demonstrations 7, 8, & 9
Demo 7 – flash freeze flower, fruit
Wearing gloves I fill a large beaker with about 700mL of LN2. It will immediately begin to boil into a gas. I explain that LN2 has a boiling point of -195.8° C (slightly warmer that it is in the dewar) it will immediately begin to boil once it has been exposed to the warmer air. I dip a carnation into the boiling LN2 and remove it, showing the kids a frozen flower. For a dramatic effect I crumble the petals of the flowers. I then place small pieces of fruit into the beaker and allow them to flash freeze. Wait about a minute and carefully remove with tongs. I lay the frozen piece of fruit on the table for the students to examine. While they are not looking I smash the frozen fruit with a hammer, resulting in a splintering of frozen fruit (think Gallagher) that shoots everywhere with squeals of shock from the students. The longer the fruit is allowed to freeze the better the cracking sound will be.
Demo 8 – flash freeze ping-pong ball
This is similar to demo 7, but instead of freezing fruit, you will substitute a ping-pong ball. For best results, leave the ping-pong ball in for (insert time). Add more LN2 if necessary. Fish the now partially deflated ping-pong ball on a table and again smash with a hammer. If the ball is sufficiently frozen it will shatter like glass.
Demo 9 – flash freeze banana, hammer nail
Again add a banana to the beaker full of LN2 (at this point the beaker should be frosted over) and allow it to freeze for (insert time). Remove the banana with tongs, use a nail with a wide head (not finishing nail) and use it like a hammer to drive a nail in a block of wood.
As I am demonstrating the dry ice and LN2 my students are recording in their notebooks what they are observing. I stay away from the words "taking notes'" as this has been an overused phrase and often means busy work in the eyes of students.
In their science notebooks, students divide their paper up into nine equal sections. I tell them to draw a giant hashtag that covers their entire page. Each section should be numbered and labeled to correspond with each demonstration. At this point the kids are to document "what" they see and save the "why" for later. It is important to emphasize that they can use diagrams as well as words (or both) to describe the event. Their documentation must be thorough enough that it will still remind them of the details three days later. If they think they would forget something in three days, they must include it in their documentation process.
If they include a picture I tell them that it is only a rough drawing (doesn't need to be in color) and it must include labels to describe the event in total.
As a culminating activity on the next day, the students create a detailed drawing of two events and describe why the event occurred they way it did. They will divide their paper in half and use their notes from the previous day. The pictures must be neat (final product), include labels, a description of the event, highlight the temperatures involved, and use a minimum of three colors.
After the last demonstration I check very carefully to see if any stray pieces of dry ice may be floating around the room. I make sure to collect the remaining dry ice and put it away. Many students ask where they can get dry ice and LN2 and my typical response is "at the science store".
I also ask the class what sublimation is and if boiling means "hot". They learned in "Determining Freezing Point Lab" that candle wax freezes into a solid when it cools down. These demonstrations serve as a good bookend to that lab.
If there is any leftover LN2, I have the kids raise their feet off the ground and I splash the remaining LN2 across the floor, under their feet. The LN2 freezes any dirt into clumps and those clumps roll across the floor.
If you wish to review the different changes of state, I have included two PowerPoints
1) Changes of State - Graphically reviews physical changes of state
2) Changes of State 2 - A mnemonic sentence to help the students the names of the physical changes of state
Steve Spangler has a commercial kit with directions to build your own rig that will blow soap bubble and fill them with white carbon dioxide fog and/or helium. Bubbles filled with helium will float to the ceiling and pop, raining white fog below.
Steve Spangler put LN2 in a sealed 2-liter bottle and put it under an inverted plastic 10-gallon trashcan. Check the 6:53 mark in the video. The exploding 2-liter bottle sends the trashcan almost 100 feet into the air. Use this example at your own risk as you are demonstrating a dangerous bomb to your students. I would love to put a nose cone and fins on the trashcan, effectively turning it into a giant rocket - just don't have the courage yet!