คำบรรยาย (139)
0:01Gas! It's all around you.
It's in space. It's on Mars.
0:06It's dissolved in your blood, and in your
soda.
0:11And it's easy to forget that we're submerged in an ocean of gas, but it's there all the time.
0:15You can feel it if you wave your arms around.
0:17Can't look cool while you're feeling it but
you can feel it. It's there.
0:21Those little molecules and atoms bumping around
against your hands as you wave them around.
0:25Feel it? Are you doing this?
0:26I've got good news and bad news about gases.
0:28First the good news, when they're behaving
themselves
0:31it is extremely easy to describe their behavior
theoretically, experimentally and mathematically.
0:36The bad news is, they almost never behave
themselves.
0:50The first mathematical description of a behavior
of a gas was a link between pressure and volume.
0:56In a closed system like the inside of this
balloon,
0:59when we decrease the volume of the balloon
the pressure inside goes up.
1:03And if we could somehow expand the balloon,
then the pressure inside the balloon would go down.
1:09If I keep pushing on it the pressure inside
might go so high that it'll break.
1:12I hope...I can't do it.
It's a very strong balloon!
1:16The relationship here is a simple one.
1:18When you multiply the pressure and volume
together, you get a constant.
1:21As long as the temperature and amount of gas
stays the same, so does that constant.
1:26It's called Boyle's Law, and it was a pretty
big deal back in the 1600s.
1:30It's also, one of the greatest scientific
mis-attributions of all time.
1:35Robert Boyle was a super rich Englishman,
raised in Ireland.
1:40His father was so rich that he paid another
family to raise his children.
1:44I guess because he was too busy administering
lands or something.
1:47Boyle had lots of great ideas about science
and chemistry.
1:50His most important one, and this is arguably
even more important than Boyle's Law,
1:54being that chemists should publish papers
not on what they feel is correct,
1:58but rather on theories that have been backed
up by experimentation.
2:01Richard Towneley a wealthy, but considerably less wealthy, Englishman struck up a relationship with Boyle.
2:07Telling him about some of his work that would disprove one of those "But it feels right" kind of chemists.
2:12Boyle published the paper mentioning that
work, which he called Towneley's Hypothesis.
2:17But which ended up, because of Boyle's superior scientific standing and possibly his wealth, being called Boyle's Law.
2:23But here's the really messed up thing:
2:24the experiments that led to the creation of
this theory were actually done largely by
2:28Towneley's family friend and physician, Henry Power, who's not a member of the aristocracy at all.
2:34He was just a working class scientist.
2:36Power was working on a publication that would
have snared him the position as discover of
2:41the relationship between the pressure and
volume of a gas.
2:43But Boyle, having discussed the ideas with
Towneley privately, published his first,
2:48attributing Towneley as the sole researcher, ensuring that Power's contributions were all but lost to history.
2:54Henry Power's Wikipedia page didn't even mention
Boyle's Law until a few weeks ago,
2:59when I personally added a paragraph about
it, with proper citations of course.
3:04But no matter who thought it up or who it
got named after, Boyle's Law is pretty cool.
3:08For a given amount of gas at a constant temperature, pressure times volume always equals the same number.
3:14But where is that constant coming from,
3:16and why is it different for different amounts
of gas at different temperatures?
3:20Well it was more than a hundred years before
we'd figure out the answer to those questions,
3:24with the help of a Frenchman Jacques Charles and our old, Italian house-elf friend Amedeo Avogadro.
3:30Charles and Avogadro created equations much
like Boyle's law
3:33with two features of a gas being linked directly
together by constants.
3:37Charles discovered that volume divided by
temperature equals a constant,
3:40as long as the pressure remains the same.
3:42And then Avogadro figured out that volume
divided by the number of moles in the container
3:46at a constant pressure and temperature gave
yet another constant.
3:49But here's the crazy cool thing:
3:51all of these scientists were basically dealing
with a different form of the same equation.
3:56An equation that we must never forget, and is gonna be stuck in my head until I die, and here's how it works:
4:02Pressure times volume is equal to the number of moles of substances times a constant times temperature.
4:07P V equals n R T: The Ideal Gas Law, which works for all gases as long as they behave themselves.
4:14Now here's the cool part,
4:15using this equation we can show how all of these chemists were dealing with the same relationship.
4:20They were just clumping various variables
together in different orders.
4:23All of the chemists we just mentioned: Charles, Avogadro and Boyle (or more properly Towneley and Power),
4:28figured out their contribution to the Ideal
Gas Law experimentally.
4:31But more interesting to me, is that it can
be understood theoretically.
4:35First, we have to understand what each of
these variables actually mean.
4:38In that same way the atoms and molecules that
make up gases
4:41are bouncing against things, applying pressure
to them.
4:44This balloon is inflated because the molecules
are bouncing around inside of it,
4:48bumping into the inside of the balloon harder than the molecules bouncing off the outside of the balloon.
4:53Scientists generally measure pressure with
the S.I. unit of force:
4:56Newtons per area, meters squared, which is
shortened to pascals.
5:00But since pascals are so tiny we either use
kilopascals
5:03or we use the pressure here on earth at sea
level, that we call one atmosphere or one atm.
5:09Completely by chance, one atmosphere is equal
to 101325 pascals,
5:14but that's so close that we often just say that one atmosphere is 100,000 pascals or 100 kilopascals.
5:21Volume is the amount of space particles have
to exist inside of.
5:25So yeah, that makes sense, when the volume
goes down, the pressure goes up,
5:28because there are more particles in a smaller
space, and they'll each hit the walls more often.
5:32N is simply the amount of gas, the number
of moles in the system.
5:35Here I am decreasing the amount of gas in the system and in response the volume is decreasing. Obviously.
5:41But so is the pressure inside the balloon.
5:43R in the Ideal Gas Law is called the Universal
Gas Constant.
5:46Even though, as we will see in a coming episode,
it is neither universal or constant.
5:50It's 8.3145 liters kilopascal per kelvin mole.
5:54Temperature, is experienced by you and me as hot or cold but at the atomic level it's kinetic energy.
5:59Literally, how fast or slow the average particle
is moving.
6:03So if temperature goes up, so will the pressure
as the particles are moving faster
6:07and thus will run into the sides of the container
more often.
6:10So now we know about all of the little bits of the Ideal Gas Law, so let's take a look at it in action.
6:15We here at Crash Course generally like to be very safe. This is a little bit of overkill here.
6:20I put a little bit of water into this soda
can and now I'm boiling it.
6:23So instead of atmosphere gas in this can right
now there's water vapor,
6:27and it's hot and all the molecules are zipping
around like crazy.
6:29We pick it up and we plop it down inside of
that -- ooh! -- and it crushes itself.
6:36So what just happened there?
6:38Well, let's see what the gas law can tell
us.
6:40So which of these things are changing?
6:42Starting on the right hand side: R, is constant,
so that can't change.
6:46The temperature of the gas though, that definitely
changed;
6:48it drops like mad when it's exposed to the
ice water.
6:51N is decreasing too as water vapor is condensing into liquid water, it thus disappears from the gas phase.
6:56So the next result on the right hand side
is a decrease,
6:59and that means that the left hand side has
to have a decrease too.
7:02So on to the left hand side.
7:04The pressure does indeed drop because the lower temperature makes the molecules move more slowly,
7:09thus bumping into the sides of the can a lot
less than before.
7:12And volume drops too, but not quite for the
reason you might think.
7:15It's really that the pressure inside the can
goes so low, that the pressure outside the can,
7:20the atmospheric pressure, literally crushes
the can.
7:22Now I understand that you probably don't think
this is as cool as I do,
7:26but understanding the physical reality of
atoms and molecules smacking into things is
7:29a special kind of beautiful for me.
7:31It's also pretty cool that if you know any 3 things about a gas, you can figure out the fourth using the ideal gas law.
7:37Of course, not all gases behave ideally,
7:39and all gases deviate from the law at low
temperatures or high pressures.
7:43But we'll save that discussion for a later
episode.
7:47STP means standard temperature and pressure,
7:50which according to the lords of chemistry is 0 degrees Celsius and 100,000 pascals or 100 kilopascals.
7:56One mole of any ideal gas takes up 22.4 liters of space at STP, which is a fact that can simplify a lot of calculations.
8:04Absolute zero is the temperature at which
all movement of all particles stops.
8:08It is zero kelvins or -273.15 degrees Celsius.
8:12And that's all for this episode. Thank you
for watching Crash Course Chemistry.
8:15If you were paying attention you learned about
8:17how the work of some amazing thinkers combined
to produce the Ideal Gas Law;
8:21how none of those people were Robert Boyle,
8:24and how the Ideal Gas Equation allows you to find out pressure, volume, temperature or number of moles,
8:29as long as you know three of those four things.
8:31And you learned a few jargon-y phrases to help you sound like you know what you're talking about.
8:35This episode of Crash Course Chemistry was
written by me.
8:37The script was edited by Blake de Pastino
8:39and our chemistry consultants were Dr. Heiko
Langner and Edi Gonzalez.
8:43It was filmed, edited and directed by Nicholas
Jenkins.
8:46Our script supervisor was Caitlin Hofmeister
and our sound designer is Michael Aranda.
8:50Our graphics team is Thought Cafe.
