>> host: Welcome back to
"Chemistry Matters."
We're on Unit 2:
"Introduction to Matter."
And by now you should have
had time to crush
all five chemicals
in the experiment.
Did you notice
any variables that were
difficult to control?
Were you able to identify
the unknown chemical?
Let's get back to our classroom
and see how our students'
results compare to yours.
>> professor: How did you all do
with this experiment?
Did anyone notice
any variables that were
difficult to control?
>> student: I had a hard time
getting by spoon to move
the same way each time.
>> student: It was hard to apply
the same amount of pressure
each time.
>> student: Yeah!
There's no easy way
to measure the pressure.
>> professor: Okay.
What potential sources
of error were there?
>> student: That's what
I mean.
It was hard to crush
the crystals the same way
every time,
so that's a potential
source of error.
>> professor: Good observation!
See, despite those possible
errors, who thinks
they may have figured out
what our unknown substance is?
>> student: We got it!
Our evidence shows that
the unknown is salt because
it broke into similar shaped
pieces, and they all had
a similar color.
>> professor: Okay.
Do you agree or disagree?
>> student: I disagree.
My evidence shows that
it could have been sugar,
but I'm not sure.
>> professor: Interesting.
Why do you think
it may be sugar?
>> student: The shape
of the crystals
looked just like sugar to me.
Are we allowed to taste it?
>> professor: Come on, guys.
You know it's never a good idea
to taste things in this lab
for safety reasons.
So-- but maybe we need
a little bit more investigation
before you can confirm
the unknown.
What else could you check?
>> student: Oh yeah,
boiling point,
melting point, density?
>> professor: Great.
You're on the right track.
In order to identify
this chemical, it will help
to look at some additional
physical properties.
>> host: Crushability
sounds like a funny word,
but it really is measured
by mining companies
and petroleum engineers
when they're drilling for oil.
Before they start mining
for metal ore
or drilling through stone
it's really important to know
the crushability of the rocks
in the area.
Crushability relates to
a chemical's ability to deform,
and that is a physical property.
[drilling]
But more specifically,
what we're talking about is
the chemical's brittleness.
Brittleness is a material's
ability to absorb energy
before fracturing.
And a great example of
a brittle material is glass.
[glass shattering]
By applying a lot of energy
with a swinging hammer,
we can see the glass fracture
into many different pieces.
This level of fracture--
which we can see from
the many shards
that are created
is related to its brittleness.
Other materials such as tin,
aluminum, and steel
are not considered brittle
due to their ability to absorb
the energy from the hammer.
[steel ringing]
However, when we look at metals,
we look at how malleable
these materials are.
The term malleability
relates to how readily
a material's shape
can be changed.
Take our fishing weight
for example.
When I hit it
with this hammer...
[hammering object]
...it doesn't shatter
like the glass.
Instead, I'm able
to change its shape,
at least a little bit.
In general, most metals
are malleable,
but non-metals are not.
Malleable materials
can be hammered, rolled
or pressed into thin sheets.
Steel is used
in the manufacturing
of car bodies
because it's strong,
but still malleable.
Now let's head back
to our classroom to learn more
about physical properties.
>> professor: Can you think of
another physical property
of matter?
>> student: Size!
>> professor: Sure.
What size-related measurement
could we make about these shoes?
>> student: Uh, length,
width, mass...
>> professor: Yes, and volume
is a physical property, too.
If I wanted to,
we could measure the volume
by filling these shoes
with water
and figure out
how much they'll hold.
But I don't think that would
be a very good idea.
[chuckling]
And let me ask you this,
what do you guys think about
magnetism?
Is that a physical property?
>> student: Well, sure it is,
because we can measure
magnetism without changing
the chemical composition
of the material.
>> professor: Good job.
And this is easily demonstrated
by holding this magnet
near this compass.
See how the compass needle
stays with the magnet?
Now that we know about
physical properties,
you should know that any time
a physical property changes,
we call that
a physical change.
A physical change is a change
that alters a substance
without altering
its composition.
For example, breaking glass,
crushing salt,
flattening out metal,
those are what we consider
physical changes.
And of course, a physical change
you guys are very familiar with,
cutting paper.
[scissors cutting paper]
Does that make sense?
>> student: Yeah.
>> student: Mmm-hmm.
>> professor: Okay. What about
balling up aluminum foil?
[aluminum foil crinkling]
>> student: Um, physical change.
Because aluminum foil
is malleable
and can change shape easily.
>> professor: Sure.
So let's think about
the clean water we saw
at the beginning of this unit
at the water treatment plant.
Well, that clean water
they are producing
is known as effluent.
Can you think of
some examples of
a physical property of water?
>> student: Well, I don't think
water is malleable,
or has crushability.
But I know that water boils
at 212 degrees Fahrenheit.
>> professor: That's right.
And what temperature is that
in Celsius?
Our metric measurement
for temperature?
>> student: 100 degrees Celsius.
Water also freezes
at 0 degrees Celsius.
>> professor: That's correct!
Water has a melting
and boiling point of 0
and 100 degrees Celsius,
respectively.
These two characteristics
are physical properties
of matter.
But do you think that water
at 0 degrees Celsius
has any crushability?
>> student: Sure! That's ice!
Lots of people like
crushed ice.
>> professor: You're right!
Water has crushability
when it's frozen.
Now here's a puzzle:
would you say that
freezing water
and turning into ice
is a chemical change?
>> student: No, because
the ice is still made
out of water.
It didn't change
into another chemical.
>> professor: Correct!
Nice job explaining
your reasoning.
>> host: By now, I hope
you've got a pretty good handle
on the physical properties
and physical changes of matter.
The importance of
physical properties
and physical changes
can't be overstated,
because they make life
on our planet possible.
Everything from
chopping broccoli,
to freezing our ice cream,
to the formation
of rain clouds--
those are all physical changes.
Think about
how important this is.
Life is possible on this planet
because of water.
And water undergoes
physical changes
in a continuous cycle.
It can change from
its liquid state into ice,
which can melt
back into water,
which can then be evaporated
into water vapor.
What I'm describing here
is phase changes.
A phase change
is a special type of change
in which a substance transitions
among the states
of solid, liquid and gas,
but the chemical properties
of the substance
remain the same.
Now we'll take a closer look
at solids, liquids, and gases,
which are all phases of matter.
In many ways,
the easiest cycles to observe
are cycles of matter,
especially water.
Cycling of matter
always involves energy,
which might be heat
as we're about to learn
in our classroom.
>> professor: Look at this
boiling water.
You see that the water
is evaporating--
that's a phase change.
So would you classify
the phase change of water
as a physical
or a chemical change?
>> student: It's got to be
a physical change,
because the chemical
properties stay the same.
It's still water.
>> professor: Good job.
Think of it this way--
solid water, or ice,
is chemically still water,
so the phase change of matter
would be a physical property.
Even when water evaporates
into a gas,
the gas is still water vapor,
and therefore is still water.
>> host: Remember,
a physical property is one
that a sample of matter
displays without changing
its composition.
And physical properties
can be observed without
chemically changing matter.
Therefore, phase changes
are physical changes.
Phase changes often occur
in response
to temperature changes
or pressure.
As temperature increases
in a system of molecules,
the enthalpy increases.
Enthalpy is a fancy word
that simply means
the amount of heat in a system
at constant pressure.
As the enthalpy or the heat
in the system increases,
the molecules or atoms
begin to vibrate faster.
>> professor: If we add heat
to a system that contains
a solid, the atoms
or molecules within the solid
will begin to vibrate faster
and become a liquid.
So what do you call
this kind of phase change?
>> student: Melting?
>> professor: Correct.
Melting is the phase change
that we're describing.
But now, if you keep adding heat
to the system,
what would happen
to the molecules?
>> student: The molecules
in the liquid move even faster.
>> professor: That's right,
the molecules will begin
to vibrate more
and kinda slide past each other.
So what do we call
this process?
>> student: Is it boiling?
>> professor: Well, boiling
is a physical change.
What's the phase change
that we're describing?
>> student: Evaporation?
>> professor: Great word.
And you're right.
But a more general word
for liquids changing into gases
is vaporization.
When liquid transitions
to the gas phase,
we say it has vaporized.
The liquid will begin
to change into a gas.
If vaporization happens
on a surface,
like the surface of a lake
or a towel,
we call that evaporation.
And, evaporation can take place
at any temperature.
So back to your answer,
does water have to boil
before it evaporates?
>> student: No.
Water on a driveway
evaporates all summer long,
and it doesn't boil.
>> professor: Right.
Evaporation can happen
at any time, while boiling
is a bulk transformation
that only happens
when a liquid turns to gas
and bubbles are formed.
Boiling will only happen
when the temperature
is at or above
the boiling point.
Now, if we remove the heat
from a system
and cooled down the matter,
you would see the actual gas
turn into a liquid.
We call this phase change
condensation.
Condensation
is the phase change
that occurs when water vapor
cools down to form liquid water.
If we keep lowering
the temperature to get
the liquid into a solid form,
we would call this phase change,
what?
>> student: Freezing!
>> professor: Great job!
>> host: Now, let's examine
some phase changes
with a demonstration activity.
Much of mathematics and science
has to do with how change
can be created and controlled.
In this activity,
we'll examine phase changes
in water as they're caused
by changing temperatures.
Freezing point
is the temperature at which
a liquid turns into a solid.
You can think of putting
trays full of liquid water
into the freezer
and taking out trays
of solid water or ice cubes
like we have in this beaker.
Right now,
the temperature is right at
water's freezing point
of 0 degrees Celsius.
Let's heat things up
and start to melt the ice.
As you see, it's going to take
some time to melt the ice
completely and the temperature
continues to stay
at 0 degrees Celsius
as the majority of ice
hasn't melted.
It's sometimes confusing
to consider
that the freezing point
and the melting point
for a given substance
are exactly the same.
Think of it this way.
Zero degrees Celsius
is the only temperature
between solid water
and liquid water.
It's as if you have a door
that represents
zero degrees Celsius.
Everything on one side
of the door is solid water
and everything
on the other side of the door
is liquid water.
If you heat up
the solid water,
when it crosses through
the doorway
of zero degrees Celsius,
it turns into a liquid.
In the same way,
if you cool down liquid water,
when it crosses through
the doorway
of zero degrees Celsius,
it turns into solid water.
Changing temperature
from 1 degree Celsius
to zero degrees turns a liquid
to a solid.
Ice can close schools,
burst pipes,
and cause accidents,
and that's pretty powerful.
When school districts
make decisions
about closing schools
due to bad weather,
they're often concerned
with decreasing temperatures.
They want to know
whether the liquid
on the roads
will turn into solid ice.
>> professor: What are we
measuring when we're talking
about temperature?
>> student: Temperature
means heat, which makes
molecules move more,
and they don't hold on
to each other as much.
>> student: Temperature
can also mean cold.
Does that mean the molecules
move around less?
>> professor: Yes, it does.
Temperature is a measurement
of the random kinetic energy
in a sample of matter.
Now let's do a little
demonstration
where you all will actually
act like molecules.
Can you guys step to the front
of the lab bench really quickly
for me? It's okay.
Now, everyone, link arms.
Right now, you're connected
to each other like a solid,
you can move around some,
but you're not moving much.
Okay, if I raise the temperature
in the room,
show me what happens.
>> student: This is just like
what happens when ice melts
and turns into a liquid.
>> professor: Okay,
now let's raise
the temperature even more.
Show me vaporization.
[laughing]
>> student: This is where
all the molecules turn into gas
and fly away.
>> professor: Right.
That's vaporization.
How did you just demonstrate
increasing temperature?
>> student: Well,
when we moved around more,
that was because we had
more energy, which means
we had a higher temperature.
>> student: And each of us
eventually broke apart
from each other
because of the movement.
>> professor: Good!
The reverse of vaporization
will be condensation.
So, link arms one more time.
You're representing
the condensation of a gas
back into liquid.
Now hold very still...
hold very still...
you're now illustrating
the freezing of a liquid
into a solid.
Your arms are sort of
holding you close together.
You're still breathing,
so you're moving a little--
like the molecules of water.
But you're not moving much.
We can easily separate you,
but your arms
do hold you together.
In molecules,
we call the forces
that may hold one molecule
next to another
intermolecular forces.
If a chemical has stronger
intermolecular forces,
it will be really hard to melt.
>> host: One additional
experiment we suggest you try
is to measure temperature
during phase changes
in a compound like water.
Start with ice
at zero degrees Celsius
and add heat to melt the ice
into liquid water.
Then record the temperature
as the melting and evaporation
processes occur.
The result will be a graph
known as a heating curve,
as shown here.
This graph shows
the uptake of heat energy
by 1 kilogram of water
as it passes from
the ice phase
at minus 50 degrees Celsius,
to steam at a temperature of
more than 100 degrees Celsius.
Looking at the area labeled A
on this heating curve,
notice that the temperature
rises as the ice absorbs heat.
Notice where B starts
once the ice has reached
its melting point
of zero degrees Celsius.
That the graph flattens out,
indicating that none of
the heat being added
is actually changing
the temperature of the water.
All of the heat being added
is causing the phase change
from solid ice to liquid.
In the section labeled C,
the ice has been completely
converted to liquid water
and it absorbs heat
and gets warmer and warmer
along C until the start
of the section labeled D,
where liquid water
starts undergoing
a phase change and is
absorbing heat energy
without changing temperature.
At this point
in the heating curve,
on average,
the water molecules
have absorbed enough heat
to overcome
the intermolecular forces
between them.
As well as overcome
atmospheric pressure
and water begins to vaporize
into a gas.
At the start of the section
labeled E,
all of the liquid water
has been converted to vapor.
The reason it takes more energy
to convert the same amount
of liquid water to vapor
compared to the ice melting
to a liquid, is that
when a substance vaporizes
it also has to overcome
atmospheric pressure.
The temperature of this gas
can then reach temperatures
higher than 100 degrees
Celsius-- water's boiling point.
You should try this experiment
yourself to confirm
how temperature changes occur
during phase changes.
With some substances,
like metals, you'd have to add
a lot of energy to make
the molecules unstick
and move away from each other.
But if the substance
has really weak
intermolecular forces,
then it's very easy
to make it melt.
Our students were demonstrating
a chemical that melts
at lower temperatures,
like chocolate.
The crosscutting concept
we've been utilizing
is known as Scale, Proportion
and Quantity.
We're studying an easily
observable process,
melting and freezing,
by using a system model.
In this case the model
is representing
intermolecular forces
between atoms.
Now let's return
to our classroom
to get more information
on condensation.
>> professor: If I take this
glass and hold it over
the beaker
where the gas is rising,
it will actually start
to collect water.
See, the water
in the gaseous phase
is actually condensing
into liquid water droplets
on the outside of the glass
because it's cooling down.
So remember this,
condensation point
is the temperature at which
a gas turns into a liquid
at standard pressure.
Now you've also seen
gaseous water from the air
turn into liquid water
when a glass you're drinking
from starts to sweat.
How do you think water
got on the outside
of these glasses?
>> student: Did the water
leak through tiny holes
in the glass?
>> student: Isn't there water
in the air?
>> professor: Correct.
The condensation is from
water vapor in the air.
Water vapor in the room
is actually
sticking to the glass
because that water vapor
is cooling down on contact
with the glass.
>> host: With a good
understanding
of physical properties,
phase change
and intermolecular forces,
you're now ready to explore
why certain chemicals
act the way they do.
And one of the elements
used to demonstrate
these concepts
is liquid nitrogen.
So join me at the top
of the next video,
where I'll have a special guest
from Atlanta's
Fernbank Science Center,
Adrian Elliot,
to help execute a very cool
demonstration of what I mean.
We'll see you there.