GAS LAW DEMONSTRATIONS Init 3/19/2009 by Daniel R. Barnes

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

Gas Law Variables PV = nRT P = pressure V = volume n = number of particles T = temperature

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

Crushing an empty, plastic water bottle 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?

Cartesian Diver The 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 cork Baking Soda Vinegar Don’t shoot anyone’s eyes out. Why does the “cork” shoot out of the bottle?

Baking Soda & Vinegar in 2L Bottle w/Balloon Use funnel to put c. 1” of vinegar in bottom of bottle. Attempt to inflate balloon inside of bottle – won’t get bigger than a handball or so. (Explain.) Use straw as air escape passage  bigger inflation of balloon. Inflate big enough to press up against inside wall of bottle and flatten. (Explain.) Use other funnel to put baking soda on top of balloon. Put cap on bottle Shake bottle. Note behavior of balloon. (Explain.) 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” Marshmallow Soda Syringe assembly Unboiled water (must still have dissolved air in it)

( ) KELVIN INTERLUDE

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? 0oC 21oC 57oC 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. Okay. This is a little bit underwhelming, but it has expanded to 6% bigger than it was in the beginning.

The balloon expands as it gets hot, but not by much. Maybe we need to REALLY heat things up. Let’s say the sun heats the balloon up to the boiling point of water. 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. 0oC 21oC 57oC 100oC 260oC 427oC Let’s heat it up to the hottest temperature setting of many ovens, 500oF. What is that in Celsius? 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.

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. 0oC 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.” 2.5 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. 2.0 Volume / L 1.5 Gases expand when you heat them because molecules speed up as they get hotter, causing them to bang around harder and push outward. 1.0 0.5 0.0 100 200 300 400 Temperature / oC

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. 2.5 Zero Celsius is not the coldest possible temperature. 2.0 Volume / L 1.5 Even at 0oC, gas molecules are still zipping around pretty fast, banging into stuff pretty hard. 1.0 0.5 How cold would a gas have to get in order to shrink down to zero volume? 0.0 100 200 300 400 Temperature / oC

EXTRAPOLATE To determine this, we need to . . . 2.5 2.0 Volume / L 1.5 1.0 0.5 0.0 100 200 300 400 Temperature / oC

EXTRAPOLATE 2.5 If you draw a line through the experimental data points, it hits the x-axis at about -273oC. 2.0 Volume / L 1.5 1.0 0.5 0.0 -400 -300 -200 -100 100 200 300 400 Temperature / oC

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.

oC = K - 273 To convert Kelvins to Celsius . . . subtract 273.