Chemistry: Gas Laws Smorgasborg

Chemistry: Gas Laws Smorgasborg

Chemistry Gas Laws Smorgasbord

The activities that follow represent the exploratory phase of the learning cycle approach. These memorable classroom demos done with simple equipment introduce students to the behavior of gases in different situations so that they may draw their own conclusions before being given formal instruction in gas laws.

One of the challenges of teaching chemistry is making the invisible world seem real and relevant to our students. Labs present the best opportunity to demonstrate this, but too often we, out of necessity, begin each lab with a litany of "don'ts" (don't eat food in the lab, don't touch the acids, don't look at the bright light being given off, etc) and it is rather rare that the material we study in chemistry lends itself to an experiential approach. I put this lab together because I was so inspired by Chris' "smorg" concept. I wanted to see if I could generate the same kind of enthusiasm and elicit the "ah-ha" moments from my chemistry kids as he routinely does with his physics students.

The active engagement with the phenomena in this lab is important in helping students confront their own preconceptions and or misconceptions and allows them to test their personal theories. The hardest part is resisting the temptation to give students the "answers" because they are so excited and get deeply involved in developing such interesting (read wacky) explanations.

The fun of watching the kids jump up, shouting a Seinfeldian "Get out!" when someone actually breaks a meter stick in two, listening to the surprised shrieks of students who successfully propel a potato slug across the room and watching happy students munch popcorn while they try to figure out why the kernels pop are all worth the extra set-up time you'll need to devote to this lab. Give a few kids some mole money (extra credit) to stay after school and help you clean up. Have fun!

Station 1: The Cartesian Diver

Key Concept: When the pressure on a gas is increased, its volume will decrease.

Set up a Cartesian diver in a soda bottle. Fill the bottle with water all the way to the top. Fill the diver with just enough water so that it barely floats on the surface. When the bottle is squeezed, the pressure increases on the air trapped in the diver. When the density of the air (gas) changes (increases), the diver sinks to the bottom (whether the bottle is sealed or not). Releasing the bottle releases the pressure.

The "diver" is the little tube half-filled with water inside the large plastic bottle. Note the position of the diver inside the bottle as the bottle sits on the table.

Now, squeeze the plastic bottle. What happens to the diver?

Now, take your hands off the bottle. What does the diver do now?

What do you think causes the diver to behave this way?

Arbor Scientific Single Cartesian Diver

Station 2: What makes popcorn pop?

Key Concept: When the temperature of a gas is increased, its volume will increase.

Students will be microwaving small amounts of popcorn in a clear bowl. Popcorn pops when the moisture inside boils and expands, bursting the kernel open.

Place about two tablespoons of popcorn in the clear plastic bowl. Put the top on the bowl and place in the microwave. Close the microwave door and turn the microwave on high. Watch as closely as you can as the popcorn kernels begin to pop, but of course DO NOT OPEN THE DOOR! As soon as the vigorous popping stops, turn off the microwave.

Describe what you see (yeah, I know, but try).

What do you think makes the popcorn pop?

Put your popcorn in a paper bag and then squirt in some butter if you wish.

Microwave Popcorn

Station 3: Balloon and the Flask (or Hot Air Ballooning)

Key Concept: When the temperature of a gas is increased, its volume will increase.

Place 10 mLs. or so of water in an Erlenmeyer flask. Stretch an un-inflated balloon over the mouth of the flask (250 mL. flask).

Place the flask next to a hot plate with a thermal (oven) glove so that students can move the flask easily from the hot plate to the ice water. Students will see how an increase in temperature can cause in increase in the volume of a gas.

Place flask on hot plate and let water boil.

What happens to the balloon? Why?

Now, put the flask in a beaker of ice and let it cool.

Now what does the balloon do? Why?

Balloon and the Flask (or Hot Air Ballooning)

Arbor Scientific Erlenmeyer Flask 250ml, 12 Pack
Arbor Scientific Round Balloons 12/bag

Station 4: Life in a Vacuum!

Key Concept: When the pressure on a gas decreases, its volume will increase.

This station can use a large vacuum chamber or the small vacuum chamber and Vacuum Pumper shown. Provide a balloon with a small amount of air tied inside. The balloon needs to be small enough that it won't seal against the sides of the chamber. Students will observe an increase in volume when they decrease the pressure in the chamber.

Observe as a partially inflated balloon is placed inside a Vacuum Chamber, in which the air is slowly evacuated. To do this, simply drop the balloon into the chamber. Place the lid and hand pump on the top and pump the air out. Repeat with a marshmallow and shaving cream. IMPORTANT: clean up your mess!!!!!

What happens to all of these objects?

Why do you suppose this sort of gas behavior occurs?

Arbor Scientific Vacuum Pumper and Chamber

Station 5: Iron Man (a.k.a. Magdeburg Hemispheres)

Key Concept: The earth's atmosphere exerts pressure on objects.

Students will observe the strength of atmospheric pressure in this memorable demonstration.

Take the two black rubber cups and slap them together quickly with a lot of force. Now, try to pull them apart using the handles. Ask a friend to pull one end and you pull the other.

Can you pull them apart? Why or why not?

Now, "burp" the cups, i.e., allow air to come in the center by peeling the sealed cups from each other.

Now what happens? Why?

Iron Man (a.k.a. Magdeburg Hemispheres)

Station 6: This Sucks! I'm under so much pressure! (Impossible…Science CAN'T suck!)

Key Concept: The earth's atmosphere exerts pressure on objects.

This station has a few steps. It's important that you assure that students follow the progression of tasks as laid out. This helps build the acquired knowledge to make the final conclusions. Hopefully, students won't spill any water, but it might be a good idea to set up this station over a sink. Also make sure each student uses a dry card.

Pour some tap water into one of the cups provided. Obtain a straw and sip some water.

Why does this work?

Figure 1.
Figure 2.

Now, suck up some water and place your index finger over the top of the straw. Lift the straw out of the cup.

What happens? What causes the water to remain in the straw?

Fill a Styrofoam cup brim full (overflowing) and place an index cards securely on top. Make sure there is good contact between the card and lip of cup. Now, gently turn the cup sideways.

What happens to the water?

Now, gently turn the cup upside down and carefully let go of the card.

What happens to the water now? How is this possible?

Station 7: Mass a Gas

Key Concept: Air is a form of matter and has mass that can be measured.

Along with understanding atmospheric pressure, students can discover that air has mass. As shown in the photo, a liquid crystal temperature strip can be added to show the rise in temperature when the bottle is pressurized.

Attach a Pressure Pumper to an empty two-liter pop bottle. Measure the mass of the bottle to at least the nearest 0.1 gram. Record the mass below.

Pump the pressure pumper 200 times and record the mass of the bottle again.

What happened?

Why did the mass change?

Remove the Pressure Pumper and record the mass of the bottle again. Explain what happens.

Arbor Scientific Pressure Pumper Kit

Station 8: Ruler of the World!

Key Concept: The earth's atmosphere exerts pressure on objects.

This is another demonstration of the strength of atmospheric pressure. Students won't believe that the air is stronger than they are! Note: The meter stick will break. Breaking a meter stick makes this activity really memorable, but you might want to use another thin piece of wood.

Take a meter stick and place it on a desk so that it extends a bit over the desk. Place two full sheets of newspaper over the section of the meter stick that remains on the desk. Smooth the newspaper out several times so it lies on the table as flat as possible.

Now, try to karate chop the meter stick. What happens?

Why were you able to do that?

Why is the newspaper important?

Arbor Scientific Meter Stick 6 pack

Students can get overly exuberant when performing this demo with possible harm to themselves and those around them. I would suggest that a warning or perhaps that students be supervised when carrying out this activity. Safety Glasses are a must.

Station 9: Super Duster & Office Buster

Key Concept: When the volume of a gas increases, its temperature will decrease.

Obtain a can of compressed "air," such as those used to clean electronic equipment. As you depress the nozzle, the liquid inside (typically an HFC) responds to the reduced pressure by "boiling" or rapidly turning into a gas.

This is an endothermic process so the can gets extremely cold (can even cause frost-bite if you hold it too long.) I like the students to relate this to the phenomenon of water boiling at lower temperatures at high altitudes due to the lower pressure. (Lots of campers know this very well.)

The classic Drinking Bird uses a similar concept and makes a good companion to this station. Simply challenge the students to explain the bird's motion in terms of pressure and fluids.

Arbor Scientific Drinking Bird

Wrap your hand around one of the duster cans. Make sure your palm is in complete contact with the can. Now, depress the nozzle.

What do you feel? Why?

Shake the can. What do you notice?

Try to explain what happens when you depress the nozzle.

Station 10: Nice Shot Spud!

Key Concept: Pressure and volume are inversely related. The volume of a gas decreases as the pressure that the gas exerts increases.

The potato launcher is great for demonstrating this concept because there is a section of trapped gas between the two potato chunks that gets increasingly compressed as the dowel pushes one chunk nearer the other. Eventually the pressure being exerted on the plug end chunk becomes so great that it is fired out of the "launcher". If a good seal is not made when loading the potato pieces the launcher won't work. Caution** Be sure to demonstrate this station to students first and warn against pointing the launcher at anyone! (under penalty of bodily torture to the Perp!)

"Stamp" a plug of potato with each end of the Potato Launcher tube. Use the plunger to firmly push one plug into the tube. Try to aim the other plug at the target. Do NOT aim the launcher at a person!

The Potato Launcher Boyle's Law

What happened?

Why did/didn't it work?

Push the plunger all the way through to empty the launcher for the next group.

Slip a large washer on to the dowel and use the duct tape to secure. This forms a good hand protector when pushing the plunger in.

April 01, 2003 Collin Wassilak

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