Getting Connected: What do the Students Know?
Today, we are going to explore the exciting world of Material Science. Material Science is the study of the behavior of materials under stress. It is an exciting field of research that assures that the products we use are safe.
We will be exploring this field by putting different materials under the stress of extreme temperatures.
Bounce a racket ball. Ask students: What makes this ball special and fun? (It bounces)
The bounciness of a racket ball is due to the physical property call elasticity. Elasticity is the ability of a material to deform and return to its original shape and size. What do you think of when I say the word elastic? (Rubber bands)
A rubber band when stretched is in an unstable state. It has a lot of potential energy in this state, and when released returns to its more stable, unstressed state.
Racket balls are made of rubber. Rubber molecules are highly elastic due to their disulfide bridges which basically act as tiny molecular springs. When the racket ball impacts on the floor, it shape is deformed. Rather than the balls bottom being round, it is flattened resulting in more potential energy being transferred to the ball. This is an unstable state for the rubber, which is why it quickly returns to its more spherical shape by the potential energy be transferred into kinetic energy of motion, i.e. a bounce!
Would you want to make a ball out of Play Doe? Probably not, because the clay in Play Doe is not elastic. It is plastic. A material that is plastic does not return to its original shape when it is deformed. This is why Play Does doesn’t bounce!
Now that we have some observations under our belt, let’s see if we can form a hypothesis or predict what would happen if the rubber ball is put into a very, very cold substance.
Pour Liquid Nitrogen into a large steel bowl. Tell students: “This substance is very, very, very, very, very, very, very, very, very cold!”
Ask, what is the coldest substance you know about? (Most students say Ice. Point out the temperature of ice is 32F or 0C. Some students will say Dry Ice. Point out the temperature of Dry Ice is -109.3°F or -78.5°C)
Tell students: In our workshop, we are using Liquid Nitrogen. It is a liquid at -321F. It is so cold that it boils at room temperature (72F)!
Which begs the question:
Today's outside temperature (ask kids)
A really hot day? (ask kids)
A really cold day? (ask kids)
Water boils (ask kids: 212° F)
Water freezes (ask kids: 32° F)
Sharing the Wealth of Knowledge
Explain to students: The notion of difference between heat and temperature was first explored by Albert Einstein’s teacher: Ludwig von Boltzman. Boltzman realized that the only way to derive thermodynamics from mechanics was to visualize gas as made of discrete atoms. His work was based on the concept of atoms—without atoms it was impossible for him to establish statistical mechanics.
Let me ask you, which contains more heat – an ice berg or a candle flame? (Most students at this point will say candle flame because it is hotter)
What Ludwig von Boltzman helped us to understand is that heat is the sum of the total motion of ALL the atoms/molecules in a system. On the other hand, temperature is the AVERAGE motion of the atoms/molecules. We can also call this motion, Kinetic Energy. Using Boltzman’s definition, the ice berg has more heat because it has more stuff.
A good metaphor for understanding this concept: If we added up the energy of everyone in this classroom this would represent heat. If we choose students to come the front and do jumping jacks for a minute the energy of each of the individuals is much higher than the other students who are sitting still. That energy would be measured as temperature. So while all the students combined have more heat, the two students doing jumping jacks have a higher temperature.
Ask students: Which has a higher temperature? A candle flame or an oven? (Most students will answer an oven)
Explain to students: The highest temperature our home oven can reach is around 600F. A candle flame’s temperature is 2500F! Copper pennies will melt in a candle flame. This is why at baking temperatures you can use parchment paper in your oven. Papers flame point is 451F which is higher than the average baking temperature of 350F
Review the safety instructions with the students!
Demonstration 1: Popcorn Skit
Show students the bottle of popcorn (be sure to take off the cap).
Ask students: What is the smallest piece of matter you can have? (Atoms)
Collections of atoms join together to form molecules.
We are going to imagine that the popcorn kernels are molecules of water
Slowly vibrate the corn kernels. Say, “This represents water in its solid state called ice. The molecules have only enough energy to vibrate slowly.”
Ask, how do you melt ice? (Add energy in the form of heat)
Swirl the bottle so the kernels are rolling around each other. Say, this represents a liquid. The molecules are freer to move around each other. This allows the water to take the shape of its container.
Ask students: what happens if I keep adding heat? (The water will boil creating a gas called steam)
Shake the bottle so vigorously that the kernels start flying out of the bottle. Say, this represents a gas…notice the molecules now fill their container and beyond. When you see the bubbles in the boiling water you are actually observing large numbers of water molecules escaping into the air!
So the process of going from a solid to a liquid is called melting; going from a liquid to a gas is called boiling. We associate this process with high temperatures because that is our most common experience. That is ice to steam. (32F to 212F transition in temperature).
Each chemical has a temperature threshold into which they pass from one state into another.
Let’s think about another chemical. Steel melts at 2750F and boils at 3000F.
What is air made of? (Nitrogen 78%, Oxygen 21%, and 1% other stuff: Water, CO2
The nitrogen we experience is always in the gaseous. In fact, it boils at -321F! This means that to the liquid nitrogen in my bowl this room we are in is very, very hot.
Take a spoonful of liquid nitrogen out of the bowl and pour into the air.
This activity may need to be limited to a small group or fraction of the class, although it would be a great learning activity! Students will get excited while doing the skit. Remind them not to bounce off each other … even though this is a more accurate representation of molecular motion!
Boltzman’s theory will also help us to understand the different states or phases of matter. Do you know what they are? (Students will usually say there are three states of matter. Point out that scientists have discovered 21 phases of matter, but have theorized there are up to 500! Foams, Gels, Powders, Non-Newtonian Fluids, Amorphous Solids, Soft Solids, Bose-Higgs condensates are all examples of these other states of matter)
We start with a solid.
The kids put their hands on their neighbor’s shoulders to simulate bonds in a crystal lattice. You are very cold—as cold as you can be—at absolute zero. The atoms are frozen solid with no motion at all. (-273C or 460F)
This will mean that the kids start wiggling, but the bonds are not broken, so we still have a solid. Because you are hot, you are radiating heat energy. This heat energy is called infrared since it has less energy than red and we can’t see it. More heat makes you glow red, then orange, then yellow, and finally you are white hot. We can demonstrate this by slowing cranking up the voltage on a clear light bulb.
With still more heat, the bonds are partly broken so that now you can move around if you stay close to your neighbor atoms. Now you are a liquid.
Now we add even more heat, so the bonds with your neighbor atoms are completely broken and you are free to roam all over. You go in all directions, bouncing off the other atoms and the walls – now you are a gas.
Alternative Activity: Use hand motions instead of students moving around the room.
What is Air?
21% O2 / 79% N2
If you cool a gas enough, it becomes liquid
Liquid N2 is basically liquid air
Daniel Rutherford discovered Nitrogen in 1772.
Nitrogen is found in the solar system. On Triton (one of Neptune’s moons) geysers shooting out of its volcanoes are streams of liquid N2. There is solid nitrogen on the surface of Pluto.
Demonstration 2: LIN and Biological Materials
Banana Hammers
Tell Students: Let’s see if we can apply the idea of the motion of molecules to creating new materials. Tell Students: “I can never find my hammer at home. But I can always find a banana. So, this gives me an idea. Let me know if you think this is a good idea. What if I could make a banana hammer!? (Students will usually laugh).
Ask Students: “Do you think is a good idea? No?”
Take the banana and hit it with the metal spoon. Ask students: What does that sound like? (Like a thud)
Now take the metal spoon and hit the side of the metal bowl. Ask students: What does that sound like? (Ringing)
That ringing sound is characteristic of a metal. Other characteristics or properties of metals are shininess and energy conductivity. The banana at room temperature is not a metal. Its molecules are huge. In long chains called polymers. Polymers typically do not conduct energy, such as sound well. We call these materials insulators. Polymers also exhibit plastic behavior, that is, they will permanently deform when stressed. If you step on a banana it will remain squished!
Plastics owe their resilience to ductility—the ability of the plastic’s long, chain-like molecules to stretch, sometimes to several times their original length. Individually, the stretching molecules absorb energy; collectively they dissipate stress from the point of impact, preventing breakage.
This communitarian approach, however, only works when molecules are free to slip past, around, or through one another (imagine a bowl of just-cooked spaghetti coated with olive oil). If the motion is restricted in some way, the molecules can’t stretch and the stress remains concentrated in a small area. And if the concentration gets too great, the material will fail, creating a crack that can propagate into a fracture. This ability to slip without letting go is the key to ductility, and to avoiding brittle fracture in plastics.
A key factor in the molecules’ ability to slip and slide is temperature. Specifically, there is something called the “glass transition temperature” (Tg), which is the point below which an amorphous solid (such as glass, polymers, tire rubber, or cotton candy) goes from being ductile to brittle. For most common materials this temperature is so high or so low that it is not easily observed - the Tg of window glass is 564 degrees C, and that of tire rubber is -72 degrees C.
Take the room temperature banana and hit the nail with it. This should damage the banana. “Hmmm, this wasn’t a good idea, after all!”
“What if I could make the banana harder? What science principle did we learn about today that could help us?”
“Yes, we could slow down the molecules by cooling them! The Kinetic Theory of Gases tells me that as I cool a substance the molecules that make up that substance slow down and compact together.
Put the banana into the LIN. Be sure to ask them to listen carefully…SHHH…you can hear the LIN boiling! Yes, the banana is really hot compared to the LIN. It is like putting a very hot stone into water.
WHILE WAITING FOR THE BANANA TO HARDEN GO TO Carnation Activity below.
Glass Carnations
Ask Students: “How many of you have allergies? I have allergies to flowers….In fact, I really hate flowers because they make me sneeze. I think we should have a National I Hate Flowers Day”. Imagine going to your friend’s house with these carnations. Show the flowers to the students. Be sure to let some volunteers confirm they are real flowers.
Freeze the flowers, and then holding them up and waving the “smoking” flowers, you will sing this little song (sung to the tune of the Barney Melody):
I hate flowers
You hate flowers
Cuz they make us sneeze for hours
Then dramatically shatter the flowers on the table
Explain: Biological materials become brittle when frozen. Since water expands when it is frozen, these materials are permanently damaged. If thawed, the flowers and banana will be limp because their cells will have popped and lost their water.
Which brings me back to the banana! Take the banana out of the liquid nitrogen. Tap it with the hammer. The students should notice that the banana has a ringing sound similar to the metal bowl. Tell the students: The banana’s molecular structure has changed to resemble that of a metal. The water has crystalized! Hammer the nail with the frozen banana. Tell students: I have made a banana split…hahaha!
SAFETY MANAGEMENT TIP: REMIND STUDENTS NOT TO PICK UP THE FROZEN BANANA PIECES.
· Notice, that the banana and the flowers became both hard and brittle when frozen. The words "hard", "strong", "stiff", and "brittle" all mean rather different things. Diamond is the "hardest" material known because a sharp diamond point will make a scratch in a smooth surface of any other material. This hardness does not mean that diamond is a strong material for all applications, because it is brittle -- diamond will crack along well-defined crystal planes when struck appropriately, as with a jeweler's tools. When forces are applied to a diamond that are not exactly set up to make it crack, then diamond is very very strong -- it is used in very high-pressure apparatus called diamond-anvil cells.
Demonstration 4: Elasticity of Solids
Marshmallows. Freeze two marshmallows in the LIN. Use the hammer to shatter one of the marshmallows. Let the other marshmallow thaw. It will regain its elasticity.
Penny. Freeze and shatter on table.
Ask Students: Could the failure of Napoleon’s 1812 western European campaign be explained by something as tiny as a button? When exposed to very low temperatures, tin starts to crumble to a powder. In Napoleon’s regiments, everything from the officer’s greatcoats to the foot soldiers trousers was fastened with buttons made of tin. Were the soldiers of the Grand Army fatally weakened by the brutal Russian winter because their uniforms fell apart?
Titanic – was the iron of the ship more brittle due to the cold temperature?
Explain: Solids become hard and brittle when frozen.
3. Expanding Balloon
Explain: With the exception of water, most materials contract/shrink when they are cooled.
Cool balloon in N2, will shrink to 1/4 original volume
(PV=nRT, T=77° K for Liquid N2, T=298° K at room temperature)
77/298 is about 1/4
Small balloon works best because it shrinks fastest
As the liquid N2 boils it expands to over 600X its volume!
4. Racket Ball
Remember the Racket Ball? Let’s take it out of the LIN and see how the rubber has changed.
Carefully remove the ball with the large spoon. Using your gloved hand, try to bounce the ball. If it is a blue, orange, or yellow ball it will shatter. The sound is like an imploding light bulb! This is because the air inside the ball has liquefied leaving a vacuum. The ball implodes. If it is the green ball it will not bounce, but it will also not shatter.
Explain: Sulfur undergoes crystalline phase changes as you go from room temp to -321° F. The rubber in the ball becomes more brittle and hard.
Demonstration 5: Liquid N2 Expands!
Explain: LIN expands to 600 times its volume when it is changed into gas.
2. Liquid N2 Dance. Carefully spill a small amount of the LIN onto a table and watched tiny drops of it dance around. Ask the students, “Why does it do that?” The nitrogen evaporates at the surface of the table, which provides a cushion of air for the drop to sit on. The cushion of air thermally insulates the drop to minimize further evaporation. As a result, you see the drops dance around without boiling away and without interacting with the table and getting slowed down or smeared out. This is called the Leidenfrost Effect!
Questions to encourage teaching points
What is heat?
What is cold?
What is temperature?
