shedding light on heat episode 2: changes of state
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Shedding Light on Heat Episode 2: Changes of State www.liacoseducationalmedia.com Page 1 of 8
Shedding Light on Heat
Episode 2:
Changes of State
The Heat is on! A huge amount of the technology and comforts that we have in our world just wouldn’t
exist if it wasn’t for what we’ve learned about the way heat behaves. So keep cool and use this excellent
series to teach your students everything that they need to know about heat, including its effect on things
and how it transfers from one thing to another.
In Episode 2, Changes of State, we look at the differences between solids, liquids, and gases, and at why
things change state when they absorb or lose heat energy. We also look at the strange example of carbon
dioxide, which doesn’t follow the same rules that that most other substances do!
Contents:
Part A: Introduction. A re-cap of the awesome Kinetic Theory.
Part B: Solids and Liquids: The atoms in a solid are held together in fixed positions. But how? Why
does a solid melt when it is heated? And do water molecules themselves actually change when ice melts
into water?
Part C: Liquids and Gases: You can compress air easily with a bicycle-tyre pump but you can’t do the
same thing if the pump was filled with water. Why not? And what exactly is happening to the water
molecules when liquid water is boiling?
Part D: Sublimation: Carbon dioxide is normally a gas, but if you cool it down to -78.5°C, it turns into a
solid. When it is re-heated, it doesn’t melt!
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Shedding Light on Heat Episode 2: Changes of State
Part A: Introduction
Solids, Liquids, and Gases. Pretty much everything on Earth
is either a solid, a liquid, or a gas. But what’s the difference,
at the atomic level, between these three states of matter?
Well let’s go back to the previous episode for a minute to see
what we already know.
So we’ve seen that in, for example, a solid piece of iron the
atoms are continuously vibrating.
The kinetic theory tells us that when the iron is cold, the iron atoms vibrate relatively slowly but when the
iron is hot (and you should therefore strike, no, that’s just a joke), the atoms are vibrating relatively
quickly. As evidence for this idea that atoms move at different speeds at different temperatures, we saw in
our last episode that food dye diffuses much more quickly in hot water than in cold water. The reason for
this is that the water molecules in hot water collide with the food dye molecules much more quickly and
with more force (and remember it’s all just random movement). This pushes the food dye molecules
around more quickly.
The kinetic theory also helps explain what solids, liquids, and gases are and why things change state. So
what happens exactly when something changes state from say a solid to a liquid, or from a liquid to a
gas? Well, that what we’re going to look at in this lesson. So let’s begin.
Part B: Solids and Liquids
By definition, a solid is composed of atoms, or groups of
atoms called molecules, that are held in fixed positions. The
atoms vibrate furiously in every direction, but they basically
stay where they are. So what holds them in place? To answer
that we need to look at atoms themselves.
Atoms are made up of even smaller particles called protons,
neutrons, and electrons. The protons and neutrons are each
about 2000 times more massive than electrons. The protons
and neutrons form the nucleus of the atom which is in the centre of the atom and the electrons kind of
move around the nucleus at really high speeds. Because of the way that electrons move, every atom is
basically spherical. The protons have what we call a positive charge and the electrons have a negative
charge.
Charge is kind of like magnetism. A
magnet has a North pole and a South pole
and the two poles attract each other
thanks to a magnetic force between them.
The positively charged protons and the
negatively charged electrons attract one
another with what’s called an electrostatic
force and so the atom maintains its form.
However, an electrostatic force of
attraction also exists between atoms,
because the protons of one atom can
attract the electrons of another atom. As a result, all atoms are a little bit sticky.
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Different atoms have a different amount of
stickiness depending on how many protons the
atom has and how the electrons move around
the atom. Molecules like water molecules and
sucrose molecules (sucrose is what we typically
call table sugar) are also a little bit sticky.
So in a solid, the atoms, or in this case the water
molecules, are vibrating but the electrostatic
forces between them are strong enough to keep
them in fixed positions. The molecules vibrate
but they don’t move around.
What happens though if we heat a solid? The
molecules (a) vibrate faster and faster as the
temperature increases and (b) as a result of the
extra vibration, they move a little further apart
which weakens the size of the electrostatic force
between them, just like the force between two
magnets weakens as they get further and further
apart. Eventually the water molecules are
vibrating so fast that the forces holding them in
place are no longer strong enough to hold them in
place and they break free of their fixed positions. The solid becomes a liquid, which is free to slosh and
splash around. This process of a solid turning into a liquid (I’m sure you already know) is called melting
and the temperature at which something melts is called its melting point.
The same thing happens if you heat up solid tin,
which is made up entirely of tin atoms. The
atoms vibrate faster and faster until the
electrostatic forces can’t hold them in place
anymore and the tin melts, turning into a liquid.
A liquid is a substance where the atoms or
molecules are free to slide around each other
although they’re still in contact with each other.
Liquids have no fixed shape but take the shape
of whatever container they’re in.
Now melting doesn’t change what the
substance is. The water molecules are still
the same water molecules whether they are
in the form of solid ice or of liquid water.
They’re all still H2O molecules, but in the
liquid state, the molecules have enough
energy to break away from their fixed
positions.
Likewise the tin atoms are still tin atoms
when the tin melts. No new substance
forms when a solid melts into a liquid.
Now what happens if we cool a liquid down?
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If a liquid is cooled down, the atoms or molecules
vibrate and move around more and more slowly and at
a certain point they’re no longer vibrating fast enough
to overcome the electrostatic forces between them and
they stick together again in fixed positions. The liquid
becomes a solid. We call this process freezing,
although if the substance is usually a solid, like rock,
then we also call it solidifying. It would seem weird to
say that when lava cools down it freezes and that these
volcanic rocks are frozen lava, but in a sense they are! The temperature at which a solid melts (or which a
liquid freezes) depends on how strong the electrostatic forces between the atoms or molecules are. It’s a
bit like magnets. Some magnets can be separated easily, while other magnets need much more force.
Water needs to be cooled down to 0°C before it starts freezing and, of course, ice that is below 0°C has to
warm up to 0°C before it starts melting. Generally, the melting point and the freezing point of a pure
substance are the same.
Table sugar (which is technically called sucrose) melts at 186°C.
The electrostatic forces between the sucrose molecules are
obviously much stronger than they are between water molecules.
Sulfur melts at 115°C so the electrostatic forces between the sulfur
atoms are stronger than they are between water molecules but not
as strong as the forces between sucrose molecules. Tin melts at
232°C. This is one of the lowest melting points of any metal. Most
metals melt at a much higher temperature because the electrostatic forces between the atoms are much
stronger.
Iron melts at 1538°C. This is liquid iron that is above this
temperature. When it cools to below its melting point it
solidifies.
Now some things, like certain types of plastic, don’t actually
have a definite melting point, they just get softer and softer as
they’re heated until they become liquids. Here you can see what
hot gooey plastic looks like. Plastics can easily be moulded into
whatever shape you want if they’re heated up to the right
temperature.
In a process called injection moulding, small plastic pellets
are heated until they’re soft enough to be injected into a steel
mould. The plastic then cools down, hardens, and the part is
ejected. The process can be repeated again and again.
Injection moulding is just one of many processes used to create a huge number of plastic parts. The
reason plastics don’t have a definite melting point has to do with the size of the molecules that make them
up.
Polyethylene for example, the most commonly used plastic in the world (it’s used to make milk bottles
and cling wrap), is made of long tangled chains of carbon atoms with two hydrogen atoms attached to
each carbon atom. Here I’ve colour-coded the chains. Each chain might be thousands of carbon atoms
long. The atoms are attracted to each other, which ordinarily keeps the plastic solid, but when the plastic
Melting Points of Some Common Plastics
Type of Plastic Melting Point
Polyethylene 115 - 135°C
Polypropylene 130 - 171°C
PVC 100 - 260°C
nylon 190 - 350°C
polystyrene about 240°C
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is heated and the atoms vibrate faster and faster,
this section might break free, but this section
might stay solid. As more and more parts of the
long molecules break free the plastic gets softer
and softer. So instead of having a definite melting
point, polyethylene (and many plastics) melt over
a range of temperatures.
Glass is similar. It too gets runnier and runnier as
it heats up. Above temperatures of about 1500°C,
depending on the type of glass, it’s so runny that
it’s considered a liquid. However, it doesn’t have a definite, clear cut temperature where it melts.
However, all elements (that is, substances made up of only one type of atom) or substances which are
made of small groups of atoms (like water and glucose, a type of sugar) have definite melting points.
Part C: Liquids and Gases
So, the atoms and molecules in a solid are held together by electrostatic forces but if the solid is heated
the atoms or molecules break free of their fixed positions. The solid melts into a liquid. However, the
electrostatic forces are still strong enough to keep the atoms or molecules in contact.
When you continue to heat up a liquid, the atoms or
molecules that make it up continue to vibrate faster and faster.
Eventually the vibration is so fast that the electrostatic forces
are no longer strong enough to keep the atoms or molecules
together and they shoot off into the air. The liquid turns into a
gas. A gas is made up of extremely fast moving atoms or
molecules travelling at literally 1000s of kilometres per hour,
that bounce around all over the place, crashing into each other
and into things that are around them. Of course they don’t move very far between collisions, we’re talking
billionths of a metre.
The fact that we can
easily compress a gas
is strong evidence
that the atoms or
molecules in the gas
have completely
separated from each
other. A simple
bicycle-tyre pump can push the atoms and molecules that make up the air closer together. Even with a
small force, I can easily compress the amount of air in the pump by more than half of its original volume.
The simulation, which I’m controlling with my mouse shows
what’s happening.
Solids and liquids, however, can’t be compressed easily, since
the atoms that make them up are already really close together.
The stones at the bottom of a stone wall, for example, are
weighed down by tonnes of stones above them, but they only
compress by something like millionths of a millimetre.
Shedding Light on Heat Episode 2: Changes of State www.liacoseducationalmedia.com Page 6 of 8
The process of a liquid changing to a gas is called “boiling” and the temperature at which a liquid
becomes a gas is called the liquid’s boiling point. When water turns into a gas, the gas is given a special
name: steam. The bubbles in boiling liquid water are steam: little by little the liquid water turns into
gaseous water.
The boiling point of a liquid again depends on the
electrostatic forces between the atoms or molecules
that make it up. When the electrostatic forces are
weaker, it doesn’t take as much energy to separate
the atoms or molecules apart, and so the boiling
point is lower.
When the electrostatic forces are stronger, the
boiling point is higher because it takes more energy
to separate the atoms or molecules apart.
The boiling point of water is 100°C, and of methylated spirits is 78°C. Sulfur boils at about 444°C.
Evaporation is similar to boiling but can occur even at temperatures below boiling point. The water I
poured on the paper towel on the left had completely evaporated after about 1 hour on this warm spring
day. So why does water evaporate?
I said earlier that water molecules in cold water move and vibrate on average more slowly than water
molecules in hot water. They don’t move very far of course because they’re surrounded by other water
molecules.
However, regardless of the temperature, a water molecule will occasionally just randomly be bumped
with enough force to send it flying off into the air (there goes one now) even if the temperature is less
than 100°C. That’s evaporation. (There goes another one.)
Evaporation occurs because water molecules are all moving at different speeds, and the faster ones quite
often gain enough energy (just through random collisions) to break free. The hotter the water, the faster
the evaporation, because, on average, the water molecules are moving faster.
About 1% of the atmosphere around us is actually
water that has evaporated. This water is called water
vapour. Vapour (spelled vapor in US English) and
evaporation (pronounced evaporation of course) are
obviously related words.
If a gas is cooled, the atoms or molecules slow down to
the point where they are aren’t moving fast enough
anymore to overcome the electrostatic force of
attraction between them, and the gas turns back into a
liquid. This process is called condensation and the temperature at which this happens is called the
condensation point. The condensation point is the same as the boiling point.
Quite often, water itself that has condensed is called
condensation. This ice-cream tub cools the water molecules that
were in the air to the point where the molecules stick together
and condense. People will often say something like
“condensation has formed on the plastic”. So the word
condensation can mean both the process and the stuff that has
formed as a result of the process.
Shedding Light on Heat Episode 2: Changes of State www.liacoseducationalmedia.com Page 7 of 8
The breath that we exhale contains quite a lot of water molecules that have evaporated from our lungs and
from our mouths. When it’s cold, it’s only about 1°C at the moment, the water condenses as soon as it
leaves our bodies and forms tiny tiny water droplets which we see as a kind of cloud.
Actual clouds are made up of tiny water
droplets that have condensed from all the water
that has evaporated mostly from the oceans.
The air is made of about 80% nitrogen gas and
about 20% oxygen gas. If you cool down air to
minus 196°C the nitrogen gas will condense and
you’ll end up with liquid nitrogen which is what
you’re seeing here. As it heats back up, liquid
nitrogen boils very quickly and turns back into
a gas.
Part D: Sublimation
This is dry ice. It’s solid carbon dioxide (CO2) made by
cooling carbon dioxide gas down to minus 78.5°C. Carbon
dioxide gas is unusual in that it doesn’t condense into a liquid
when it’s cooled down, it turns directly from a gas into a
solid.
When the dry ice warms up again, it doesn’t melt like water
ice does, it turns directly from a solid to a gas. It’s just one of
those things in nature. Here I’ve placed some dry ice and
some water ice into two beakers and I’ve placed the two beakers onto a hot plate. The water ice is
melting, but the dry ice is said to be “subliming”, not melting.
Sublimation is the name given to the process of
a substance changing state directly from a solid
to a gas without first turning into a liquid. Solid
water (that is, water ice) melts at 0°C while
solid carbon dioxide sublimes at -78.5°C. The
opposite of sublimation is deposition. Solid
carbon dioxide is called dry ice because it looks
like water ice, but it doesn’t from a liquid. The
solid carbon dioxide turns directly into gaseous
carbon dioxide.
About 15 minutes later, the water ice had completely melted into liquid water and the dry ice had
completely sublimed into carbon dioxide gas which had then spread out into the air of the room that we
were filming in.
If I place two small pellets
of dry ice into a conical
flask and place a balloon
over the opening, I can
trap the carbon dioxide
gas that forms as the dry
ice sublimes. The balloon
keeps getting bigger and bigger as more and more carbon dioxide is produced. Whenever a solid or a
Shedding Light on Heat Episode 2: Changes of State www.liacoseducationalmedia.com Page 8 of 8
liquid changes state into a gas, the gas takes up much much more space than the original solid or liquid
(when the gas is at normal atmospheric pressure of course).
This because in a gas,
the atoms or
molecules spread out
and they’re not in
contact with each
other except when
they collide. In fact,
just 1 litre of dry ice
expands out to over 800 litres of carbon dioxide gas when it fully sublimes. About 13 minutes later, the
balloon had ballooned right out and couldn’t take any more.
Here you can see what’s left of the two dry ice pellets that I started with.
Dry ice is so cold that it cools the air around it to the point where all the water vapour in the air first
condenses and then freezes on the beaker.
Dry ice is used for many industrial processes and it’s commonly used to
create mist in theatres by dropping it into water. The clouds or mist that
you can see are actually made of water droplets. The dry ice cools the
air around it, so the water in the air condenses and forms, basically,
clouds of water droplets. The actual carbon dioxide gas being produced
isn’t visible. Yep, dry ice is pretty cool stuff. Literally.
So the Kinetic Theory of Matter combined with the fact that atoms are sticky thanks to being made up of
positively charged protons and negatively charged electrons, neatly explains what solids, liquids and
gases are and how and why substances can change state.
But things don’t just change temperature and state when they’re heated up or cooled down, they can also
change size. They can literally expand and therefore take up more space, and they can contract. This
affects everything from the natural world of oceans and rivers, to the modern industrial world of bridges
and buildings. And so, it’s the expansion and contraction caused by heating and cooling that we’ll be
looking at in our next episode. See you then.
CREDITS:
Written, directed, and presented by Spiro Liacos
The simulations of the solids, liquids, and gases were created by
PhET Interactive Simulations
University of Colorado Boulder
https://phet.colorado.edu
https://phet.colorado.edu/sims/html/states-of-matter-basics/latest/states-of-matter-basics_en.html
cooling metal by liquid nitrogen by Konstantin Soin. Licensed under a Creative Commons License.
Precious Plastic - Building machines by davehakkens. Licensed under a Creative Commons License.
Precious Plastic - Build the injection by davehakkens. Licensed under a Creative Commons License.
Extrude beams from plastic waste by davehakkens. Licensed under a Creative Commons License.
Plastic Injection Molding by engineerguy. Licensed under a Creative Commons License.
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