Teachers TryScience

Ask Students

• What is heat?
• What is cold?
• What is temperature?

If you hold ice cubes in your hand. What do you feel?

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)

Short Lecture

• Heat is the motion of atoms/molecules.
• Temperature is a measure of the average motion of atoms/molecules.
• Mention Ludwig Boltzmann and the Kinetic Theory of Gases

Review the safety instructions with the students!

Demo 1: Popcorn Skit

This demo 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!

• 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.

• Now we turn on the heat.

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.

Demo 2: Balloon in Jar

What happens when air (gas) is heated?

• Light a piece of paper and drop it into a large mouth jar.
• Place a water balloon on the mouth of the jar.
• The balloon will bounce up and down on the jar as the air inside heats up.
• The air in the jar expands and some of it leaks out the sides of the mouth of the jar. The density of the air left inside decreases; the air outside the jar is denser/heavier than the air in the jar.
• The pressure of the outside air pushes the balloon into the jar, because of the difference in density. (Notice that the balloon is pushed – not pulled into the jar!)

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.

Demo 3: LIN and Biological Materials

Banana. Take room temperature banana and hit the nail with it. This should damage the banana. After 3 minutes or so in N₂, hammer the nail with the banana.

Carnation/Rose/Any Flower. Freeze and shatter on table.

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.

Demo 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.

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?

Solids become hard and brittle when frozen.

Expanding Balloon

• 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 695X its volume!

Plastic Spoon. Nothing happens to spoon, plastic properties are similar.

Normal Orange Ping Pong Balls (Halex, is the major brand of ping pong balls)

• Ball will still bounce; it does not collapse like balloon.
• Color

Sulfur undergoes crystalline phase changes as you go from room temp to -321° F.

Dye in ball contains sulfur.

Depending on brand, it will noticeably change from orange to yellow.

Ping Pong Ball with Hole

· Use a dissection needle to poke a tangential hole in the ping pong ball. Use a black marker to draw a line on the ball.

• When completely submerged, the air in the ball will collapse to ¼ its normal volume, sucking in Liquid N₂ (actually it is getting pushed in by the greater pressure outside the ball).
• When the ball is brought out of the liquid N₂, it heats up. The boiling N₂ gas escapes through the hole and the ball spins (the black marker line on the ball makes it easier to see the spinning motion of the ball).

Demo 5: Liquid N₂ Expands!

LIN expands to 600 times its volume when it is changed into gas.

Dishwashing liquid. Pour liquid N₂ into dishwashing liquid. Huge bubbles will form demonstrating that the large volume of gas escaping. Bubbles can be picked up with tongs

Coffee Can. Pour some liquid N₂ into a coffee can. Put on plastic lid. After a few seconds the lid will pop off! It is the high pressure that forces the lid off as the LIN boils and the gas takes up more space.

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.

Questions to encourage teaching points

• What is heat?
• What is cold?
• What is temperature?