20、 Obama
B1 Intermediate US 192 Folder Collection
After playing the video, you can click or select the word to look it up in the dictionary.
Loading...
Report Subtitle Errors
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.
    You must  Log in  to get the function.
Tip: Click on the article or the word in the subtitle to get translation quickly!

Loading…

B1 Intermediate US

Preparation for General Chemistry 1P. Lecture 02. Classification of Matter.

192 Folder Collection
Amy.Lin published on August 12, 2019

Learning Words

loading

Comments

Loading…
More Recommended Videos

  1. 1. Search word

    Select word on the caption to look it up in the dictionary!

  2. 2. Repeat single sentence

    Repeat the same sentence to enhance listening ability

  3. 3. Shortcut

    Shortcut!

  4. 4. Close caption

    Close the English caption

  5. 5. Embed

    Embed the video to your blog

  6. 6. Unfold

    Hide right panel

  1. Listening Quiz

    Listening Quiz!

  1. Click to open your notebook

  1. UrbanDictionary 俚語字典整合查詢。一般字典查詢不到你滿意的解譯，不妨使用「俚語字典」，或許會讓你有滿意的答案喔