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  • Alright, folks, welcome back. Okay. So, I will say a few words about science in general. Okay? So chemistry is one of the natural sciences. It has a long history, and I will spend about ten minutes

  • kind of motivating

  • why it is what we're doing and why we will be doing it in a particular way. This also holds for physics and for geoscience, whatever kind of natural science we're dealing with; same kind of methods,

  • same kind of history

  • so I like this picture- a lot of things are beautiful

  • and that you might have gotten those moments- I think we all have- where you look up at the sky or something else around you wonder, you know, how can all of this be? Why is it so beautiful and amazing?

  • One way to address that

  • and try to get a grip on all these

  • beauties around us is to kind of, you know, give a scientific description.

  • or put some elements in place.

  • and that is more easily said than done. And it's always

  • fun to realize

  • put yourself in this situation thousands of years ago. Say you're one of those

  • greek thinkers, and you lie on your back, and you look at the starry night, and you wonder, what is that light

  • in the far distance. What is that?

  • right now we know it's a star. Our sun

  • is a star, too. We know that.

  • back in the day, of course, they did not know. How do you actually come to such a conclusion? That is not trivial.

  • Right now in our lifetime we know so much

  • we take a lot of knowledge for granted. Knowledge has accumulated over the years.

  • But this is not trivial stuff. All of this knowledge took time to build up.

  • So it's fun to realize that sometimes it's actually

  • not trivial

  • to come to a certain conclusion.

  • So these people live in those

  • ages where not a lot of things were known.

  • And they were starting to think about

  • what is matter? I mean, how should we

  • think about matter? Can we give proper descriptions of matter?

  • So this person, Thales,

  • he says everything is made out of water

  • Okay? So he is trying to understand why do certain materials have certain properties, and his way of thinking about it had to do with, maybe it contains different amounts of water

  • Maybe water acts in a particular

  • way in different materials.

  • Democritus says, "well, I don't think that's the case." He says,

  • "I think materials are based on an accumulation of

  • many many many many tiny little particles

  • altogether

  • that are, in turn, indivisible ones.

  • stopped the most fundamental level there is a type of

  • pull that bring particles together

  • and they form the material on a larger scale

  • is the atomistic theory

  • that we know now is the best way to think about materials. But in those days, they did not know.

  • So Democritus is actually

  • a person that lived at a later time

  • than Thales,

  • however

  • Aristotle, you may know him,

  • was a person that lived at a later time than Democritus and

  • he did not believe this assertion

  • of Democritus

  • he said

  • he said something...about... (laughter)... the computer stopped.

  • he says speak the materials are not made out of indivisible particles, but they are composed of different amounts

  • of fire, earth, water,

  • air, and aether.

  • So it's a very different way of thinking about materials.

  • Now all these things, they can be true, I mean,

  • how do you know? I mean, at some

  • level you can say anything you want.

  • So how are you going to discriminate

  • between different

  • world views, if you want.

  • So how can we be sure that

  • if we know know Democritus is probably

  • the person that gave the best description but how do we know that?

  • Well, it turns out that if you actually start to do experiments and test things

  • over and over and over again, if you at some point can arrive at conclusions that make more

  • sense, more sense than other conclusions,

  • and then at some point you may be able to rule out

  • some hypotheses and confirm

  • others.

  • So basically, you take materials, you start to

  • look at them very carefully, manipulating them, do experiments on them,

  • a whole series, and

  • based on the logical kind of deduction, you can

  • arrive at certain conclusions.

  • So this is something people started to do

  • very early on, but really

  • they were ramping up

  • in medieval times

  • and accumulated

  • into kind of the method

  • that we now call the scientific method.

  • So this is kind of like the early days, and in terms of chemistry those were the alchemists

  • and they were really fascinated of course by

  • turning everything in to gold. Everybody wanted to be rich,

  • and if you're a poor person,

  • of course what you want is more money

  • and the way to get that is to have more gold.

  • If you can turn

  • dust to gold, then you are golden.

  • but you have to be able to do that, and so in order to

  • do that, motivated by this, say, pressure for economic

  • welfare, you start to experiment, sort of manipulate, and you actually

  • discover a whole bunch of properties of matter that you have not encountered before.

  • and here is a person that was also in that tradition

  • one of the later ones, okay? So this is

  • almost leaving the medieval times here

  • and he says

  • 'why don't you just like, forget about

  • making gold but instead use the

  • methods that we've been

  • working with to do something good?"

  • For instance, finding medicines.

  • He is a person who was very

  • motivated by helping other

  • folks, other people, medically.

  • he said, 'the best way I can do that is by

  • actually using these methods like observing, experimenting, trying to understand

  • how the human body works, and then try to help this person.

  • This may seem very trivial, but this was not trivial because most

  • medical practices were based on mystic beliefs that did not necessarily have anything to do with a logical deduction of observations. Where you use the observations.

  • Okay, so he actually

  • used observation and

  • logical deduction

  • and did something good. For instance, he says

  • infection comes from outside the body, okay? Which is something that people did not know.

  • People thought that if you got an infection, it comes from you,

  • because you did something bad. Now we know it's a little microorganism that attacks you and sits there and causes problems.

  • He did through observing that, you take those states out of people in situations,

  • you actually have less infection.

  • He was still like a person of his time. For instance,

  • he still believed in bloodletting.

  • Cleansing through bloodletting. That was something disproven, but this

  • because the belief was so strong

  • we still thought it was a good practice.

  • Okay. So that leads us here

  • basically we have this method of trying to get closer to, um, I wouldn't say the

  • truth, necessarily, but to a better description of what matter is.

  • And so I want to make the distinction here that that method is

  • very different from

  • an opinion.

  • Scientific inquisition is not

  • coming up with better opinions.

  • So for instance, if this person says

  • "that's how it is"

  • he may say, "it's not."

  • he will say, "yes it is," he will say, "njet!" Okay? So these people have different opinions, and they will

  • never agree on anything.

  • But it's very hard to make sense of this.

  • Okay, so opinions are great

  • for a whole bunch of purposes, the driving force within our society, it does a lot of good, but then it comes to

  • finding the proper descriptions of nature, this is not a good

  • way to go about it.

  • Scientific inquisition is very different

  • and, you know, this is kind of like a very

  • short explanation of how the method works: you have a

  • clear observation of an event,

  • you give a very

  • proper description

  • of that event, which is frank.

  • Which is as frank as you can be.

  • Not biased

  • by what you want it to be

  • okay? This is really the difference between

  • opinion and scientific inquisition. You have to leave everything that you believe out of your system and just let the observation do its work. This is really hard for people, but that's really important. So,

  • a detailed description

  • of the phenomenon by frank observation

  • try not to be biased, or colored,

  • in your observation.

  • Then, you come to the interpretation. For instance,

  • This is Newton

  • you all know the example

  • apple falls, he gives a very detailed description

  • of what he saw,

  • the apple was sitting there, it was coming loose,

  • fell to the ground very close to him,

  • he gave a very detailed description, and then he says, well I can explain that if i come up

  • with this formula, of course the story is much longer than that,

  • but he has a formula and then -- very important,

  • you take that

  • interpretation

  • and you test it again

  • is this interpretation true under all circumstances?

  • it means that if an apple falls from this

  • tree,

  • is this also true for

  • another apple from a different tree?

  • Or from a different object? For instance, if I go and launch it from a building, Does it also obey this law?

  • If given,

  • It this law describes the falling of objects

  • throughout, then

  • you can give this a certain amount of credit, so to speak, in the sense that

  • this is a good description

  • of falling materials.

  • This is not an opinion. At all.

  • This is scientific inquisition.

  • Okay. So this of course is also the method that we

  • follow in chemistry

  • so let's say we have a beaker with chemicals, we do an experiment,

  • something changes.

  • you provide detailed descriptions of these

  • changes

  • of these phenomena

  • then we come up with an interpretation

  • For instance, this substance

  • must be a reducing agent. Okay? That is an interpretation, and with a hypothesis, you have to test it. Okay? We go back

  • change something,

  • for instance, say, if this were a reducing agent,

  • if I put a lot of, you know,

  • oxidizing agents in there,

  • the reaction must be different

  • and so you do that, and indeed if it is different,

  • then you actually strengthen your hypothesis.

  • okay, several times, until

  • you've exhausted all possibilities, and then you can

  • conclude that this must be an actual description of this

  • situation here.

  • again, this is not an opinion.

  • You cannot just say, 'oh, I think this is a reducing agent. It must be so.' No,

  • you actually investigate that scientifically.

  • All right, here's a couple of people,

  • so after medieval times people started to use that method

  • consistently

  • Robert Boyle is one of these people,

  • and he, through that method, that scientific method,

  • actually uncovered a lot of different kinds of

  • laws that are enveloped into chemistry. For instance, the gas law

  • and the differences between

  • compounds and mixtures. That was one of Boyle's contributions.

  • also the preposterous theory of matter

  • was reaffirmed

  • he brought back this idea

  • from Democritus

  • long forgotten because Aristotle had dominated the way people thought about matter. He said, 'hold on a second

  • my experiments indicate that

  • if you assume that

  • materials are made out of indivisible particles, I can explain all my observations.' Therefore,

  • this must be

  • a better description

  • than the description based on, for instance,

  • earth, fire,

  • air,

  • and aether. Right? So,

  • based on that,

  • he could throw out Aristotle's description as

  • not being a good description

  • no hard feelings, just not a good description.

  • All right, another person

  • very important

  • to chemistry, Lavoisier.

  • he was the person

  • who discovered the conservation of mass during a chemical reaction

  • you'll hear about that in this class

  • he was also disproving

  • the phlogiston theory

  • what is that? That has to do with, for instance, if you burn something

  • where does the flame come from?

  • And so people thought that that was a material

  • that was actually baked in

  • in the matter that you burn, so the flames

  • were already in there. That material that

  • causes flames was in the

  • material itself. So people had different levels of wood and a lot of phlogiston. And water has none of this.

  • So that of course is not a good description

  • because now we know that

  • if something combusts, or is on fire,

  • it is a reaction with oxygen in the air.

  • It's nothing intrinsically tricky. Also, the

  • discovery of hydrogen, oxygen, and various other elements

  • can be

  • traced back to Antoine Lavoisier. Okay? So important contributions to the basics of chemistry. Thanks to this method of scientific inquisition.

  • People were starting to discover more and more materials and starting to organize these based on their properties.

  • This person, Dmitri Mendeleev,

  • was smart enough to put them in a kind of a table, organized in a table,

  • based on their properties. Their mass,

  • the way they reacted,

  • and so this is an early version

  • of the periodic table.

  • it is not the version that we have right now because this is, again, in the very early

  • stages. It's differently organized,

  • there's many more elements. But based on this

  • logical deduction,

  • taking the elements that you have

  • and then combining them logically

  • he could say that

  • scandium, which was unknown at the time, he said

  • based on my ordering there was a blank here. He says there must be a material

  • that has these properties and this mass. And, gallium

  • and germanium

  • were also not wrong at the time, and these guys, he predicted, because there were holes in this table. He said, based on the fact that

  • this table looks the way it is, and there are holes there,

  • there must be

  • elements

  • with these properties and he called them

  • well, after the fact he called them scandium, gallium, and germanium. So his is extremely powerful.

  • applying these,

  • this set of principles, organizing them,

  • trying to make sense of it, he could actually predict that

  • certain things existed

  • that had not been discovered yet. And he of course was right.

  • these are extremely encouraging

  • developments that basically told

  • people that they were on the right track.

  • Interpreting what

  • matter is

  • this is a quote from Mendeleev, he says:

  • We could live at the present day without a Plato,

  • but a double number of Newtons is required to

  • discover the secrets of nature, and to

  • bring life

  • into harmony with the laws of nature

  • What does that mean? He says look, Plato is a good guy.

  • He said beautiful things.

  • Nice philosopher. But what we really need

  • is something like Newton

  • who applies the scientific method

  • and actually investigates things and

  • comes to conclusions

  • that you cannot arrive at

  • by just sitting on a rock by a hill

  • thinking for yourself.

  • So this is the way the Greeks used to do it. They just would think. Just

  • by thinking you can create your own reality or your own interpretation of reality. He says, 'that's fine,

  • but that's not enough.'

  • We need scientific inquisition

  • hypothesis tested

  • in real life, and see if it is true.

  • and then go back

  • and strengthen your hypothesis. That is really, he says,

  • what will bring life into harmony with the laws of nature.

  • Okay. Last example just to kind of, like, close the story on this,

  • This is Sherwood Rowland,

  • who recently

  • passed away, sadly

  • He was here in the chemistry department,

  • he won the Nobel prize,

  • with the discovery of, basically,

  • the depletion of ozone in the ozone layer. He said certain

  • materials, CFK's, will deplete

  • the ozone

  • in our atmosphere

  • and so he set out, actually, on a campaign

  • to prevent

  • the release of these CFK's in our atmosphere. Successfully,

  • and thanks to him, the ozone layer is

  • no longer

  • I would say,

  • the hole in the ozone layer is no longer expanding, but actually shrinking. So this is a very important development.

  • He was a very

  • good atmospheric chemist.

  • done many, many experiments, very meticulous, very

  • scientifically accurate, so to speak, okay, took the scientific method

  • very serious

  • so this is a statement that says

  • chemicals expelled into the atmosphere can have pronounced climate effects

  • which is something that he observed through his experiments.

  • and just to contrast that with something else, this person says

  • the greenhouse effect is utter nonsense.

  • now, whether he is right

  • on the level of- on the metaphorical level, you can say

  • who is right? I don't know,

  • but I can tell you that this is

  • abiding by the scientific method. And it is trying to do the best you can

  • based on scientific evidence

  • and this is not necessarily so.

  • This is much more colored and based on

  • opinion, like his.

  • Things are good in our society.

  • but those appeals do not have

  • necessarily anything to do with scientific inquisition.

  • It's a very different way of truth seeking. Okay?

  • so distinguish these two things.

  • The scientific inquisition method is one of the best methods, in my opinion,

  • we have

  • to get to a better description of the world around us. We cannot just simply base it on our own opinion, you know.

  • only thinking.

  • okay. So,

  • it's all about matter.

  • so let's look at this glass of...I don't know what it is. Pineapple juice. Orange juice.

  • It looks particularly delicious today because it's kind of hot

  • I would classify this as a

  • homogeneous mixture

  • a homogenous mixture is a

  • mixture because I know there are multiple things

  • in this glass. Okay? For instance, there is water in there

  • H2O.

  • there's also sugar; sucrose, in there. Another compound

  • water is a compound

  • sucrose is a

  • compound they are all in this glass. Or, citric acid.

  • citric acid is a

  • component that fits in oranges and it gives it

  • a slightly acidic flavor.

  • or vitamin c; another

  • molecule, a compound

  • that sits in the glass.

  • together

  • they form a mixture

  • which is homogeneously

  • mixed in the sense that they will not

  • segregate out.

  • so on the molecular level they form a homogenous mixture.

  • Nowhere in this glass do I have one chunk here that is sugar, one chunk here

  • that is citric acid, and one chunk here that is water, They all come together and mix on a molecular level and thus it is a homogenous mixture of

  • compounds

  • these are compounds

  • okay, so these compounds, they turn out to be pure substances

  • in and of themselves. If I just have the water--

  • in and of themselves. If I just have the water--

  • water, I have a pure substance. So a compound can be a pure substance when it is just by itself.

  • pure water, or if I just have sugar, a sugar cube, I have

  • pure sugar. It's a pure substance. There is nothing else in it but sugar.

  • So these compounds

  • once isolated, are pure substances.

  • but, as you can see,

  • a water molecule,

  • this compound, has

  • still multiple things in it. It has hydrogen atoms and oxygen atoms

  • So, I can separate water out and also sucrose, which is in it,

  • into individual elements

  • in this case, carbon

  • all these

  • compounds here are made out of these atoms, either carbon, oxygen, or hydrogen.

  • okay? So these are elements

  • and then the elements themselves

  • are composed of individual atoms

  • and the atoms are composed of electrons, protons, and neutrons.

  • so we have a flow here from mixtures

  • to pure substances

  • which can be compounds

  • the compounds can be separated out into elements

  • elements are composed of atoms and atoms are composed of electrons, protons and neutrons.

  • So let me classify this or put this into

  • a different scheme, which is a little bit more

  • easy to understand.

  • So I have matter here,

  • I can separate matter roughly into mixtures

  • things that are mixed, and things that are pure substances.

  • So let me first look at the mixtures

  • I can have heterogeneous mixtures. What is that?

  • Well let's say if I have in one hand some salt and in the other hand some sand, and I put it together,

  • into a small pile,

  • then I have a mixture. But I know this is not a homogeneous mixture.

  • because on the molecular level,

  • these two substances are not mixed. Okay? it's a heterogeneous mixture.

  • On the other hand, the glass of lemonade

  • is a homogenous mixture.

  • there's no locations in this

  • glass where somebody could have

  • only water or only sugar. It is mixed on the molecular level.

  • all these mixtures are made out of pure substances

  • mixed together

  • so, the pure substance then

  • can be separated into

  • elements. For instance,

  • sugar contains the elements carbon, oxygen, and hydrogen

  • so the compound sugar is

  • containing elements.

  • and the elements themselves can also be a material. For instance,

  • carbon, diamond

  • is made of only elements. Only carbon elements. It's an

  • elemental material

  • so, diamond

  • is a material, a pure substance,

  • which only contains the element carbon.

  • sugar

  • is a pure substance, but in a

  • compound because it contains

  • multiple elements.

  • it contains carbon, hydrogen, and oxygen.

  • both are called pure substances.

  • Okay. So therefore, pure substances can be distinguished into two categories. One is the

  • compounds

  • which means a material composed of multiple elements,

  • or elemental materials, a material composed of only one type of element. Think of diamonds. They only contain carbon.

  • So both of them of course are composed of atoms.

  • elemental materials only contain one type of atom.

  • and compounds contain typically different types of atoms.

  • now atoms

  • are

  • composed of

  • a nucleus

  • in the middle of the atom

  • and then electrons around it

  • and then finally the nucleus is composed

  • of a neutron and a proton.

  • We'll look at these particles individually

  • a little bit better

  • okay? So these are the fundamental particles, the elemental particles with the neutron, the proton,

  • and the electron that

  • constitute mass.

  • you can go even deeper and talk about quarks, but we won't do that in this class.

  • we don't talk about quarks here.

  • here in this chemistry class we stop at this particular level: neutrons, protons, and electrons,

  • provides enough information to explain the different properties of materials. Okay. Now we have the classification of materials, these materials can exist in different states

  • we all know that. Think of

  • water. Water can exist

  • as ice, which is a solid

  • as a liquid

  • liquid water, and as a gas. Water steam or water vapor.

  • in all cases

  • the water molecule is the same. It's the same water molecule. It is just organized differently.

  • Here, they are all very close together, organized into a lattice

  • here they are still close,

  • but not organized

  • as well

  • they look more chaotically

  • and here they are not close at all.

  • but realize that the molecules

  • themselves are just water.

  • no change; no chemical change

  • between a solid, a liquid, and a gas. Same material, it's just organized differently.

  • it's in a different phase.

  • here's another rendering of that

  • you see in the gas; you see little

  • ping-pong balls floating around, those are molecules

  • they are moving around

  • in the liquid, they do too, but much more closely

  • and then finally in a solid, they are very closely packed

  • they still move around a little bit

  • they're not fully tight; they're still shaking, and they bounce a little bit, but definitely not as much as they could in liquid; nowhere close to gas.

  • but the properties of the individual spheres here

  • don't change. It's just a packing

  • in the closeness relative to the other particles.

  • That sets the difference between phases.

  • Okay. Now let's look at a couple of fundamental properties of materials. And I'm sure you're completely familiar with this.

  • but let's just do this anyway.

  • Mass is an important property of matter.

  • and mass relates to the quantity

  • of the material.

  • The more you have,

  • the more it's mass.

  • usually we indicate mass by this letter 'm'.

  • Volume is another property of materials,

  • volume of course relates to the amount of space occupied by matter.

  • Now, sometimes that also relates to how much you have.

  • Because if I have more

  • of this

  • particular metal, then it's

  • volume will be larger.

  • but

  • not always so. For instance,

  • if I take a balloon,

  • which is

  • containing gas inside,

  • and if I put the balloon into

  • a fridge, the temperature will get colder, lower, and then the gas will shrink, and hence the volume of the balloon will shrink too.

  • but, the amount of gas, the amount of molecules inside, hasn't changed.

  • So the volume is not necessarily

  • dependent only on the quantity of the material.

  • the mass is.

  • this balloon and that balloon have

  • the same amount of gas molecules inside, and yet their volumes are different. Just because the temperature is different.

  • Okay. Now density is another very important property. Density is the ratio

  • between these last two properties

  • the mass and the volume. Mass over volume. That is the definition of density.

  • so here's an example

  • this is an example of a high density material. Lead.

  • This is a chunk of lead. It's quite heavy.

  • it's quite heavy for it's volume.

  • So the mass is high for it's volume, that means that the density is a large number.

  • So here is -- bless you --

  • here are a couple of bullets; these are lead bullets, okay? The mass of these bullets is much less than this big chunk of lead. Is it's density

  • also less?

  • No. The density is exactly the same. The density is independent- completely independent- from the quantity.

  • okay? So the quantity doesn't matter.

  • any amount of lead has the

  • same density.

  • it is mass over volume.

  • the mass per volume unit is the same

  • no matter how much of the lead you have

  • here is an example of a material that has a low density

  • this is bromine gas in a flask.

  • the molecules are very far apart

  • which means you don't have a lot of material per volume unit

  • so per volume you don't have a lot

  • you don't have a lot of mass, that means the density is a low number.

  • this is a low density material.

  • so we talked about the different phases in which materials can be,

  • we also know that materials can

  • change

  • their phase

  • so, water can melt, meaning it can go from a solid state to a liquid state.

  • So: if you go from the solid to the liquid phase, we call that melting

  • you know that; this guy is

  • experiencing that right now

  • liquid to the gas phase is called vaporization

  • so liquid to the gas phase is called vaporization, and vaporization is a

  • typical word we use for this process.

  • gas to the liquid phase

  • is called condensation.

  • you've probably heard of that

  • liquid to the solid phase is called freezing, same when it happens with water

  • when it freezes

  • it goes from the liquid to the solid phase

  • and from the solid to the gas phase directly, so, let's say a chunk of ice

  • is directly changing into water vapor

  • that is called sublimation.

  • you see a chunk of frozen CO2

  • okay, so this is a big block of CO2

  • you can keep that at low temperatures and it will stay solid. But if you take that out, to room temperature,

  • suddenly the CO2 molecules will quickly

  • go into the gas phase without going to the liquid phase first. Okay? So that is an example of

  • sublimation.

  • this little..kind of looks like

  • vapor you see on top actually is not CO2. That actually is water that condenses on the surface. That gives you little water vapor droplets. That's what you can see.

  • the CO2 itself you can't see in the gas phase.

  • Okay. So here's a diagram you'll see in the book, as well,

  • the solid changes into liquid, that is of course called melting,

  • the opposite is called freezing. You are very familiar with those words.

  • liquid to gas

  • is called vaporization; you typically do not call it boiling.

  • You call it vaporization. The opposite is called condensation. And then,

  • going directly from the solid to the gas

  • sublimation and vice versa

  • from the gas to a solid: deposition. That's the last word. Deposition.

  • So recognize these words and know what they mean. This is a picture from the book so you can look it up.

  • let's look at some

  • changes that take place when

  • one of these phase changes takes place. That's a...very ugly sentence.

  • Here's an ice cube

  • when it melts, it turns into water.

  • and then when it vaporizes

  • it turns into steam.

  • but during these processes, the density of the material will change.

  • the separation

  • of heated water molecules

  • will change if they go from the

  • solid phase to the liquid phase.

  • and from the liquid phase to the gas phase.

  • heated, they'll be extremely far apart, meaning the water now has a very low density.

  • here

  • it is a pretty high density.

  • and here it is also a fairly high density.

  • Water is an interesting material because it

  • turns out that the

  • solid form of water actually has a lower density

  • than the liquid form of water which is

  • quite unique because most materials have their highest densities

  • in the solid form

  • that's where they are the closest.

  • And a slightly lower density in the liquid form, and by

  • far the lowest density in the vaporous form; the gas phase.

  • Okay? So that makes a lot of sense.

  • If you think about how far these things are apart,

  • that means the density goes down.

  • You can also have phase changes between

  • two forms of solid.

  • Let's look at graphite, for instance. Graphite is composed solely of carbon.

  • and if you compress this with very high pressures,

  • you can turn it into a diamond. It's a very hard process; extremely high pressures are required. You can't do it.

  • in both cases, the element

  • that makes up this material has not changed, okay?

  • this is pure carbon, this is pure carbon. The way in which they're

  • organized is slightly different.

  • Okay? So in graphite, the carbon atoms are organized in sheets that are stacked,

  • and in diamonds they are organized in this very nice

  • lattice. The diamond structure is a lattice.

  • In all these cases, all these little

  • balls there, the spheres, are carbon atoms.

  • So this is a solid-to-solid phase transition.

  • Same ingredients,

  • different properties based on

  • a different packing of the atoms.

  • another very important phase change is the change from

  • something that is a solid

  • to something that dissolves in water

  • okay? So here's using an example

  • look at this example several times. This is an

  • ionic lattice composed of ions

  • negative ions, positive ions.

  • This is a piece of salt. Let's say rock salt.

  • or sodium chloride. For instance,

  • if you put it in water it will dissolve

  • that means that the

  • individual

  • ions

  • this is a chlorine ion

  • this is a sodium ion

  • will be encapsulated by water molecules.

  • and they take it out of the lattice and thus floating away from the salt lattice.

  • the process continues until there's

  • no ions left

  • in the lattice

  • so that means that the lattice is gone

  • that means you have no salt left.

  • this material is dissolved

  • in water.

  • and the way that we write that chemically is the following:

  • Sodium chloride

  • it has an 's,' that means it is in the solid phase

  • I put a grain of the stuff in

  • a cup of water and then,

  • after a little while, the lattice is gone

  • all of the individual ions are now

  • floating about

  • in the aqueous phase. In the water phase.

  • and therefore, the individual ions now are written down separately with this

  • as a specifier.

  • Okay? This is

  • aqueous, it means they are dissolved.

  • They are encapsulated by water molecules. Individually.

  • Okay. So these are

  • phase changes

  • and they should be discriminated from chemical changes.

  • in a chemical change, it's not just

  • a difference in arrangement. Okay? So a physical change, a phase change, means that I have particles that are the same, I'm just going to reorganize them. Put them in different places. Stack them differently. That is a phase, or physical, change. A chemical change is different.

  • In a chemical change,

  • you are actually forming

  • new materials

  • here's an example

  • a pretty brutal one

  • of a chemical reaction. This is the combustion

  • of organic materials

  • the tree is on fire

  • and that is basically organic material-wood turning in to CO2 and H2O

  • under the release of heat.

  • another example

  • this is

  • basically the deterioration of metal compounds. Corrosion.

  • and here is a lovely lady doing

  • experiments in the lab

  • something you will be doing, as well.

  • In all these examples

  • the materials you are working with

  • are not just to be organized in space, they are actually making new chemical bonds.

  • Okay? So different atoms now

  • are making new bonds, forming

  • new materials.

  • So, we will talk about it very extensively

  • but just to give you an idea, some very simple examples will help to

  • clarify the difference.

  • what happens if a silver spoon tarnishes? Is that a physical change or a chemical change? Who says physical?

  • Who says chemical? Chemical.

  • It is a chemical change because basically it is the reaction of silver,

  • which is the spoon,

  • with sulfur to form a new compound, a new chemical, on the surface.

  • which is this silver sulfide. Water

  • freezes to become ice. What is that? That is physical.

  • The water molecule itself doesn't change. It stays water. It just organizes itself differently.

  • and goes from a liquid phase to a solid phase. How about this?

  • sugar cube result in your coffee.

  • chemical or physical? Physical. Yes. Because

  • the sugars stay sugar.

  • It doesn't change.

  • The molecular structure of sugar stays the same, it is just

  • surrounded by water molecules. It doesn't form a lattice, it's

  • rearranged

  • into the solution

  • and it requires this 'aq' sign.

  • and this-

  • tums

  • it neutralizes your stomach acid. Is that a chemical reaction of physical? That's a chemical reaction. That's right.

  • That's actually an acid-based reaction

  • this OH compound then is reacting with the acidic component in your stomach

  • creating new materials.

  • All right! Well I think that's

  • probably enough for today.

  • I'll see you again on Wednesday.

Alright, folks, welcome back. Okay. So, I will say a few words about science in general. Okay? So chemistry is one of the natural sciences. It has a long history, and I will spend about ten minutes

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B1 中級 美國腔

普通化學1P的準備。第02講。物質的分類。 (Preparation for General Chemistry 1P. Lecture 02. Classification of Matter.)

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    Amy.Lin 發佈於 2021 年 01 月 14 日
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