GAS LAW DEMONSTRATIONS

Init 3/19/2009 by Daniel R. Barnes

. . . use kinetic theory to explain the gas laws.

Gas Law Variables

PV = nRT

P = pressure

V = volume

n = number of particles

T = temperature

Push two suction cups together.Try to take them apart.What’s resisting your efforts?

Suction Cups Kissing

Tighten the cap onto an empty, small, plastic water bottle.Try to crush the bottle. (Explain.)Remove the cap.Try to crush the bottle again.Much easier, isn’t it? (Explain.)

With its lid off, crush the bottle down as far as you can.Watch it spring back to a larger volume (size).Now, crush it again, but this time put the cap on before you let it spring back.The bottle will remain crushed and small. (Explain.)With the cap still tightly on, try to pull the bottle back to its former size.What’s resisting you?

Crushing an empty, plastic water bottle

Cartesian DiverThe setup in the picture uses an eyedropper with some air trapped in it. I prefer to use aluminum foil balls. They’re cheaper.

What happens when you squeeze the bottle? Why?

Baking Soda & Vinegar Cork Cannon

Sparkling Cider bottle w/plastic corkBaking SodaVinegarDon’t shoot anyone’s eyes out.

Why does the “cork” shoot out of the bottle?

Baking Soda & Vinegar in 2L Bottle w/Balloon

1. Use funnel to put c. 1” of vinegar in bottom of bottle.2. Attempt to inflate balloon inside of bottle – won’t get

bigger than a handball or so. (Explain.)3. Use straw as air escape passage bigger inflation of

balloon. Inflate big enough to press up against inside wall of bottle and flatten. (Explain.)

4. Use other funnel to put baking soda on top of balloon.5. Put cap on bottle6. Shake bottle. Note behavior of balloon. (Explain.)7. Hold bottle upside-down. Remove cap. It’s going to

spray, so move your gradebook away from the sink. If you’re lucky, the balloon comes out of the bottle partially or completely. (Explain.)

Vacuum pump victims:

Balloon“Peeps”MarshmallowSodaSyringe assemblyUnboiled water (must still have dissolved air in it)

0oC

Let’s say you’ve got a balloon, and it’s freezing cold out, but just barely freezing.

The sun comes out and warms up the balloon to about room temperature. What should happen to it?

21oC

Wow! Did you see how big it got? J/K. It only got about 2% wider, but it did expand a little.

Okay, now let’s pretend it gets REALLY hot, as hot as the highest recorded temperature for Death Valley.

57oC

Okay. This is a little bit underwhelming, but it has expanded to 6% bigger than it was in the beginning.

Let’s say the sun heats the balloon up to the boiling point of water.

0oC

The balloon expands as it gets hot, but not by much.Maybe we need to REALLY heat things up.

21oC 57oC 100oC

Darn. It’s only expanded to 10% wider by the time it’s at the boiling point of water. This is a real let-down.

Let’s heat it up to the hottest temperature setting of many ovens, 500oF. What is that in Celsius?

260oC

Golly. The balloon is as hot as an oven on full power and it’s only epanded 25% beyond its original width. I’m not impressed.

Let’s say we heat it up to the temperature on the sunny side of the planet Mercury.

427oC

0oC

Looking at how little the balloon seems to expand, it’s hard to imagine that the fiercest storms are caused by air expanding and rising. It doesn’t really seem to expand all that much, but it does, and it does have dramatic consequences.

21oC 57oC 100oC 260oC 427oC

WARNING: This thought experiment has some flaws in it, so don’t take it TOO seriously. For one thing, Charles’ Law is only valid in situations of constant pressure. In an expanding balloon, the rubber’s elastic force increases with stretching, adding to the pressure in the balloon. The pressure inside a limp garbage bag comes closer to equalling that of the surrounding atmosphere, but the pressure inside of an inflated balloon is always higher than the outside air pressure. (That’s why air always shoots out of a balloon when it’s allowed to escape.) This difference in pressure increases as the balloon inflates, since inflation requires the rubber to stretch more and more. I’m sure this makes the math more complicated somehow. I suppose I should have used a limp garbage bag in the example instead of a balloon, but garbage bags aren’t as pretty as balloons, so I used a balloon. So sue me.

Gases do expand when you heat them.This principle is known as “Charles’ Law.”

If you heat up a body of gas and measure its volume as you heat it up, you notice that it does expand with heat.

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Temperature / oC0 100 200 300 400

1.0

0.5

1.5

2.0

2.5

Gases expand when you heat them because molecules speed up as they get hotter, causing them to bang around harder and push outward.

0.0

Gases expand with heat, so if the volume of a gas is not zero, then it stands to reason that is does not have zero heat energy.

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Temperature / oC0 100 200 300 400

1.0

0.5

1.5

2.0

2.5

Even at 0oC, gas molecules are still zipping around pretty fast, banging into stuff pretty hard.

How cold would a gas have to get in order to shrink down to zero volume?

0.0

Zero Celsius is not the coldest possible temperature.

To determine this, we need to . . .

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Temperature / oC0 100 200 300 400

1.0

0.5

1.5

2.0

2.5EXTRAPOLATE

0.0

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Temperature / oC0 100 200 300 400

1.0

0.5

1.5

2.0

2.5EXTRAPOLATE

-100-200-300-400

If you draw a line through the experimental data points, it hits the x-axis at about -273oC.

0.0

Celsius is a great temperature scale if your goal is to have an easy time memorizing the freezing and boiling points of water.

However, it seems like it would be more appropriate if zero degrees really meant zero heat energy, which means zero molecular motion.

To reflect this molecular motion view of temperature, a third temperature scale was invented.

The scale is named after the scientist who first theorized the idea of a coldest possible temperature.

In the Kelvin temperature scale, zero really means zero.

The coldest possible temperature . . .

Is known as . . .

Please notice that a Kelvin is just as big as a degree Celsius.

Because of this, the Kelvin temperature is always . . .

273 higher than the Celsius temperature.

To convert Celsius to Kelvins . . . add 273.

To convert Kelvins to Celsius . . . subtract 273.

oC = K - 273