Physics: Phase Changes on a Molecular Level

Let's look at phase changes on a molecular level. Get your miniaturizing ray guns ready.

CoursesPhysics
LanguageEnglish Language

Transcript

00:47

liquid okay so maybe watching ice melt isn't your idea of magic

00:51

maybe reminds you of Mrs. Chimel Farbs grammar class but underneath it all on a

00:56

molecular level well some pretty crazy stuff is happening phase transitions

01:02

really are kind of magical by which I mean most people don't really understand [People sleeping in the audience]

01:07

what's happening and there are things going on that your eyes can't see... Take

01:11

this big block of ice for example it looks well a lot like ice cold solid

01:17

boring ice just sitting there not moving but inside the ice like way inside

01:23

everything is moving if we were able to look at the molecules in this ice cube [Molecules moving]

01:28

well we'd see a whole lot of buzzing going on the molecules would be packed

01:32

together in an orderly fashion kind of like fans at a football stadium but

01:36

excited fans who are all vibrating like Packer fans and if you give the ice cube

01:41

a little heat well all they might all do the wave....

01:46

and as the IceCube receives heat from the air around it well, the molecules [Molecules vibrate]

01:51

start to vibrate more and more it's like when the home team is making a

01:54

last-minute drive for the winning touchdown and you know what let's keep

01:59

this metaphor going once the frozen water has gotten enough heat and has hit

02:04

a specific temperature it's like the home team just caught a Hail Mary pass [Man catches football in end zone]

02:08

The bonds that we're keeping the molecules tightly bound to each other

02:12

are broken and the molecules are able to slip past each other and move around

02:16

more freely like fans storming the field the molecules are still contained in the [Molecules run onto football field]

02:22

stadium but they're moving past each other under each other just to go nuts

02:26

and here's where the whole fans at a football stadium game thing well it kind

02:30

of breaks down see when you heat up a solid it gets hotter meaning its

02:34

temperature gets higher makes sense right but when it starts to melt its

02:38

temperature stops increasing every substance has a melting point even the [Fire burning metal]

02:44

Grinch's heart... Once the temperature hits that magical melting point, things

02:48

start to get runny but that process doesn't happen all at once an ice cube

02:53

doesn't just turn into liquid water in an instant it melts gradually and the

02:58

part that still solid stays at the same temperature even though it continues to

03:03

take in heat all the heat that's taken in just makes things melt more quickly [Snow melting on the ground]

03:07

the amount of heat that it takes to turn an entire solid liquid without changing

03:13

its temperature is called the heat of fusion and we're not using fusion in the

03:18

nuclear sense here people which is good because if it was a nuclear reaction [Magician with ice cube and ice cube explodes]

03:22

every time an ice cube melted well we'd be in trouble you can look at the

03:27

ancient Latins for this one the original root word for fusion is Fundere...

03:32

which means to melt or to pour and we know how much science loves it's

03:38

dead languages so of course well they kick it old-school with the original

03:42

meaning scientists are the worst right? like why can't they be cool and hip like

03:47

magicians okay so now we've got ourselves a puddle [Magician with a puddle on stage]

03:52

a magic puddle that is but the show doesn't end there my friends oh no

03:56

there's plenty more magic where that came from because once again with just

03:59

the power of heat I can make this puddle disappear into thin air in fact I can

04:04

make it vaporize yeah vaporization evaporation

04:08

whatever you want to call it when a substance reaches its boiling point it [Water boiling in a pan]

04:11

stops being liquid and it becomes a gas now of course water doesn't need to get

04:15

up to 212 degrees Fahrenheit to evaporate the individual molecules are

04:20

able to get enough energy to turn into gas even at room temperature I prove

04:26

that every night with a magic trick I like to call pop-sweat yeah but adding

04:30

heat guarantees a gas state when we keep adding heat to our liquid water well,

04:35

the molecules keep moving faster and faster but they're still bonded together [Molecules moving]

04:39

they can't bounce themselves completely free at least not yet

04:43

which is why liquids maintain their volume if you take a gallon of water and

04:47

poured into a kiddy pool you're still gonna have a gallon of water and you're

04:52

also gonna have a really lame pool party well the water isn't going to expand and

04:57

somehow fill the pool liquids maintain their density and they also can't be

05:01

compressed that compression thing is important to imagine you have a tube of

05:06

water it's closed on one end and open on the other and you've got some sort of [Man with a tube of water]

05:11

plunger mechanism you know like how a syringe works try as you might you're

05:15

not gonna be able to press down on that plunger to make the water take up less

05:18

space which is a good thing because that's how the brakes on your car work a

05:22

squirrel with a death wish runs out into the street you mash the brake pedal and

05:26

the pressure from your foot is delivered via break fluid to the wheels [Car brakes rapidly]

05:31

and sure that's simplifying things a bit but I'm a magician not a car explainer

05:36

guy all right well where were we oh yeah doing my magic aquatic disappearing

05:40

trick by which I mean boiling water and the molecules were starting to go

05:45

bonkers the water gets hotter and hotter and finally reaches its boiling point

05:49

when boiling starts the molecules have enough energy to break the bonds holding

05:53

them together and they're finally free able to move around completely

05:56

independent of each other because they've transition to become a

05:59

gas just like when a solid starts to melt when a liquid starts to turn into [Magician holding pan of water]

06:04

gas the liquid stays at the same temperature even though it continues to

06:07

take on heat well the extra energy allows the molecules to break free

06:10

by definition water above 100 degrees Celsius becomes vapor so if it's liquid

06:16

it can't be hotter than that right only when all liquid has been converted to

06:20

gas can the thermometers start going up again and this amount of heat required

06:25

to turn a liquid into gas is called the heat of vaporization now for the second [Magician holding act 2 sign]

06:31

part of my act I'm gonna reverse all of this magic and turn this vapor back into

06:35

a solid quick question though how good is the air conditioning here hmm see

06:41

we're gonna need to get things cold enough that all the water we heated up

06:45

now has to give up heat to the system around it the temperature where a gas

06:50

turns into a liquid is called the condensation point and that temperature

06:54

is exactly the same as the boiling point for whatever substance we're talking

06:58

about well the process just works the other way around instead of heat going [Molecules appear]

07:02

into this substance getting these molecules all jazzed up heat comes out

07:07

of the substance think about a pot of boiling water the lid of the pot will

07:11

have a lot of water condensed on it and that's because the lid is cooler than

07:14

the water so when the water vapor comes into contact it cools down suddenly and

07:18

cools down enough to turn back into a liquid and you might have a couple of [Magician holding an ice cube tray]

07:23

trays like these in your freezer right when you've used the last ice cube and

07:27

you don't want to get yelled after leaving an empty ice tray in the freezer

07:30

well you fill it up with liquid water and pop it back in the water has a

07:35

higher temperature than the air surrounding it so it starts giving off

07:38

heat but once the waters temperature reaches the freezing point which fyi

07:42

is the same as the melting point well those molecules start to get back in

07:47

line and just like before the water can't get [Ice cube tray in a freezer]

07:50

colder than the freezing point until all of it has been converted to a solid

07:55

abracadabra what once was wet now is dry all thanks to my magic and physics but

08:02

you know mostly my magic well there's one other state of matter which we

08:06

haven't talked about yet and we know if we give a solid enough heat it'll

08:10

eventually change into a gas but what happens if we keep heating that gas up

08:14

does it just stay gassy no as things get hotter and hotter the molecules get more

08:21

and more worked up eventually there's so much energy that electrons start popping

08:26

off their atoms creating electrically charged molecules called ions this [ions appear beside water molecules]

08:32

ionized gas becomes the fourth state of matter called plasma plasma is what

08:38

stars are mostly made up of and we use it on earth for things like welding

08:42

making computer chips and most importantly yes TVs yep this odd state

08:48

of matter is responsible for delivering sweet mind-numbing reality entertainment [Woman flicking through TV channels]

08:53

right into our brains and although we don't run into it very often here on

08:57

earth almost all of the matter in the universe is actually plasma but we can't

09:02

really make plasma on our kitchen stove so we're not gonna get too deep into

09:06

that right now well one thing that might seem like

09:09

magic is how all these different states of matter can exist at the same

09:14

temperature when it's 72 degrees outside water is liquid oxygen is gas and pizza

09:19

is solid how is that possible people everything is made up of the same

09:23

building blocks right? well yes and no different substances have different

09:28

chemical properties some molecules have electrons that aren't too attached to [Electron spinning round orbital of molecule]

09:32

them so energy is able to flow more easily between those molecules and the

09:35

shape size and mass of the molecules all affect how it reacts to heat and how it

09:41

changes phases well polarity also matters polarity is

09:46

when one side of a molecule has more electrons creating a partial electrical

09:50

charge and pressure can change boiling and melting points too.. generally speaking

09:55

if pressure increases so does the melting and boiling point and if we

09:59

lower pressure and a substance will melt and boil at a lower temperature too.. All of

10:05

these factors combine to make the wonderful diverse universe we live in

10:08

everything looks a little more magical now doesn't it we can graph all these [A field and pool appear]

10:13

phase changes too - take a look at this graph this is a heat curve for water

10:18

okay it's not the sexiest of curves heat curve is a more of a name rather than a [Magician discussing heat curve graphs]

10:23

description there all right well we've got temperature on the y-axis and heat

10:28

on the x-axis yeah not time which is different than a lot of graphs that

10:31

we've worked on before usually the x-axis is time right but we don't care

10:35

about time in this case, this is all about how much heat it takes to

10:39

change the state of a substance and heat is measured in joules or kilo joules as

10:45

we have on our graph here right there now let's actually look at the line on [Line on water heat curve graph shown]

10:49

this thing we have periods where the temperature increases and times when it

10:53

plateaus well what's going on in these different segments? well like we said

10:58

when something melts its temperature doesn't increase until it's all the way

11:01

melted so these flat sections are where the transitions are happening and in

11:06

these parts where the temp is going up well the state of the matter is staying

11:10

the same we'll start at the left when things are pretty chill we know that

11:14

we've got a solid chunk of ice when things are this cold and it can take a

11:17

bunch of heat before anything interesting happens I'm guessing [People stood by an ice cube block]

11:21

watching ice get warmer is right up there with watching paint dry in the

11:24

pantheon of boring activities but as we get it up to zero we hit plateau number

11:29

one at the end of that section the thermometer starts going up again so we

11:33

know the phase transition is over we're all wet now then we have another flat

11:38

spot which you guessed it means another transition after a whole bunch more heat [Graph pointer moves along heat curve line]

11:42

or into gas land and this graph doesn't go all the way to plasma so we're not

11:47

gonna sweat that transition alright so we have

11:50

our melting point right here at zero degrees but what if we're reading this

11:53

graph from right to left like it's Hebrew or something well then we've got

11:57

freezing going on instead of melting it just depends on which direction we're

12:02

going and it's the same thing with boiling and condensing it's a matter of

12:06

which way we're you know walking the line all right

12:10

now what is our heat of fusion remember that's how much energy it takes to get [Heat of fusion graph appears]

12:14

from solid to liquid all the heat we pump in for this first flat section is

12:19

the heat of fusion if we had actual numbers on this graph we could just

12:23

subtract the lower heat amount here on the right from the higher heat amount

12:27

here on the left but we don't have numbers so you know just remember that

12:31

process and it's the same for the heat of vaporization we could calculate the

12:36

exact amount of heat by subtracting the heat at the start of the transition from

12:39

the heat at the end and we can do this kind of thing for any substance at all

12:43

this is a graph or some unknown substance yeah all right we can see that [heat of fusion graph for unknown substance]

12:47

it doesn't take a whole lot of heat to make its temperature increase when it's

12:50

a solid then it hits a phase transition at 55 degrees and it takes about 15

12:56

kilojoules of heat to completely turn into a liquid when it reaches 65 degrees

13:02

we get to phase transition number two and it requires 20 kilojoules to turn

13:07

our mystery heat into gas then the temp increases more gradually as we pump in

13:13

more thermal energy give us a thermometer a blowtorch and a bunch of [Thermometer and blowtorch appear]

13:17

different materials and we could make graphs like this all day which actually

13:21

sounds like a fun afternoon.... all right well things these days are

13:26

kind of slow turns out that when you advertise yourself as the magic [Melto magician advert appears]

13:30

physicist well people expect you to do a little

13:32

more than make ice melt and you don't get a lot of those harde gigs

13:36

with your amazing boiling water act... sorry maybe I should consider a phase [Kids throw tomatoes at magician]

13:41

transition of my own well now for my next trick I'm gonna turn potatoes into

13:46

French fries yeah I should have stayed in college