We will begin this Topic by examining the different phases of matter; solid, liquid and
gas. Next, we will see how heat is exchanged between these phases and the resulting effects on heating and cooling curves.
We will also explore the Kinetic Molecular Theory and discover how it predicts the behavior of gases.
Solids can be made up of many things. They can have
or compounds inside. They can also be made up of mixtures, or combinations of different elements and compounds.
Most of the solids you see are mixtures. Most rocks are mixtures of many elements and compounds. Concrete is a good example
of a man-made solid mixture. CHARACTERISTICS OF SOLIDS
First let's explain that characteristics are the traits or features that something might have. One characteristic of a
solid is that it might be hard. That idea is pretty straight forward.
One of the main characteristics of solids is
that they hold their own shape. So if you put a solid in a container it won't change its shape... No matter how much you move
or slide it around.
You can even grind a solid up so that it fills up a container. If you look at the powder under a microscope you will still
see little tiny solids that you couldn't change. You know that liquids are different because if you put a liquid into a container
it will fill it up as much of the container as it can.
In the same way that a solid holds its shape the atoms inside of a solid are not allowed to move around much. This is a physical
characteristic of all solids. It happens no matter how small the pieces are. The atoms in liquids and gases move around in
all directions. The solid atoms and molecules are trapped in their places. The atoms still spin and the electrons still move
but the entire atom doesn't go anywhere. They just kind of jiggle in place. SO YOU WANT TO BE A SOLID
Obviously not everything is a solid. If you look around you'll see solids,
liquids, and especially gases (remember the air around you). Sometimes liquids feel a physical need to become a solid. Then
look out! Phase changes are about to happen.
Scientists use something called a FREEZING POINT
to measure when
a liquid turns into a solid. There are physical effects that can change the freezing point. Pressure is one of those effects.
When the pressure surrounding a substance goes up the freezing point also goes up. That means it's easier to freeze the substance
at higher pressures. When it gets colder, most solids shrink in size. There are a few which expand but most shrink. CRYSTALS
If a solid is made up of pure elements or compounds something special happens. It can freeze into a very specific structure.
This structure is called a CRYSTAL LATTICE
. A crystal lattice is a very exact organization of atoms which creates a
specific place for every molecule or atom in the solid. It is very neat and very compact. A great example of a crystal lattice
is a diamond.
By now you know what a solid is. If you wave your
arms around you can find a gas. But what about liquids? Not that we suggest it but you know you've got some spit. That's a
liquid. What about your blood? That's a liquid too. The main thing is to figure out what makes those things liquids.
are an in-between phase of matter. They are right between solids and gases. One characteristic of a liquid is that it fills
the shape of any container. So you pour some water in a cup. It fills up the bottom of the cup first and then fills the rest.
It also takes the shape of the inside of the cup. It starts filling at the bottom because of GRAVITY
. When it is in
that cup it also has a flat surface. That's because of gravity too.
One other characteristic of liquids is that they
are very hard to COMPRESS
. When you compress something you take a certain amount and force it in a smaller space. Solids
are tough to compress too but gases are easy. When you compress something you squeeze it so the atoms in the substance are
closer together. When pressure goes up... Substances are compressed. Liquids already have their atoms close together so it's
hard to push them even closer. SO YOU WANT TO BE A LIQUID
If you want to be a liquid you could start out as two different things. You could be a solid or you could be a gas. Each
of them has a different way of becoming a liquid.
Let's say you're a solid. That's you. A handsome cube of ice sitting
on a counter. All you do is dream of becoming liquid water. What you need is some ENERGY
. Atoms in a liquid have more
energy than the atoms in a solid. The easiest energy around is probably heat. There is a magic temperature for every substance
called the MELTING POINT
. When a solid reaches the temperature of its melting point... It can become a liquid. For
water the temperature has to be a little over zero degrees Celsius. If you were salt, sugar or wood your melting point would
be higher than water.
So solids need more energy. The reverse is true if you are a gas. You need to lose some energy from your very excited gas
atoms. The easy answer is to lower the surrounding temperature. When the temperature drops, energy will be sucked out of your
gas atoms. When you get to the CONDENSATION POINT
, that's the temperature when you become a liquid. If you were the
steam of a boiling pot of water and you hit the wall, the wall would be so cool that you would quickly become a liquid. EVAPORATION
Sometimes a liquid can be sitting
there and its molecules will become a gas. That's called EVAPORATION
. You might be wondering how that can happen when
the temperature is low. It turns out that all liquids can evaporate at room temperature and pressure. Evaporation is when
there are atoms or molecules escaping from the liquid and turning into a vapor. You see... Not all of the molecules in a liquid
actually have the same energy. The energy you can measure is really an AVERAGE
of all the molecules. There are always
a few molecules with a lot of energy and some with barely any energy at all. It is those with a lot of energy that build up
enough power to become a gas and leave the liquid. When it leaves it has evaporated.
LOOKING FOR A GAS
You want to identify a gas? Maybe you're standing
next to your friend and you hear him make a little farting noise. Not here buddy. We're not talking about that kind of gas.
Flatus is not funny. Maybe it is but it has no place here. Not yet anyway.
Gas is everywhere. There is something called
the ATMOSPHERE. That's a big layer of gas that surrounds the Earth. Gases are random groups of atoms. There are solids
where atoms and molecules are really compact. Liquids have them a little more spread out. But gases are really really spread
out and the atoms and molecules are full of ENERGY, bouncing around constantly.
One of the physical characteristics
is that a gas can fill a container of any size or shape. Think about a balloon for a minute. No matter what shape you make
the balloon it will be completely filled with the gas. The atoms and molecules are spread equally throughout the entire balloon.
Liquids can only fill the bottom of the container while gases can fill it entirely.
SO YOU WANT TO BE A GAS
We're betting that you want to be a gas right now. Trust us... It's easier
to make gas than to become one. First you need some beans. We digress. Actually you need energy to become a gas. The atoms
in a gas have more energy than the atoms in solids and liquids. The easiest way to think of energy is to think of temperature.
When you increase the temperature of a system you are pouring energy into that system.
When you want to be a gas you
usually start out as a liquid. If you add energy to a liquid the atoms get all excited. When you boil water the steam you
see is small water droplets being carried by the rising water gas/vapor. You can also find water vapor in fog and clouds.
The special temperature when a liquid becomes a gas is called the BOILING POINT. When you cool a gas it liquefies.
When a gas becomes a liquid the speed and energy in the molecules drop and ATTRACTION forces allow the molecules to
You might hear the term VAPOR. Vapor and gas mean the same thing. The word vapor is used to
describe gases that are usually found as liquids (like water). A compound like CO2 is usually a gas, so it described
that way. But water (H20) is usually found as a liquid at room temperature. So when it becomes a gas scientists
use the term vapor.
Sometimes a liquid
can be sitting there and its molecules will become a gas. That's called EVAPORATION. You might be wondering how that
can happen when the temperature is low. It turns out that all liquids can evaporate at room temperature and pressure. Evaporation
is when there are atoms or molecules escaping from the liquid and turning into a vapor. You see... Not all of the molecules
in a liquid actually have the same energy. The energy you can measure is really an AVERAGE of all the molecules. There
are always a few molecules with a lot of energy and some with barely any energy at all. It is those with a lot of energy that
build up enough power to become a gas and leave the liquid. When it leaves it has evaporated.
Plasmas are a lot like gases but the atoms are different because they are made up of free ELECTRONS
of the element. You don't really find plasmas too often when you walk around. They aren't something that happen regularly
on Earth. If you have ever heard of the Northern Lights or Ball Lightning you might know that those are types of plasmas.
It takes a very special ENVIRONMENT
to keep plasmas going. Scientists say that they are the fourth state of matter.
They are different and unique from the other states of matter. SO YOU WANT TO BE A PLASMA
So imagine you're a gas. Floating around and you say "Hmmmm, I'd like
to become a plasma. They are too cool!" You're already half-way there being a gas. But you need more. You need to tear off
a bunch of electrons from your atoms. Eventually you'll have bunches of positively and negatively charged particles in almost
. When the ions are in equal amounts the charge of the entire plasma is close to NEUTRAL
(A whole bunch of positive particles will cancel out the charge of an equal bunch of negatively charged particles.)
Now you know what you need to have. The question is how do you make it happen? The answer is ENERGY
! A plasma can be
made from a gas if a lot of energy is pushed inside. All of this extra energy makes the neutral atoms break apart into positively
and negatively charged ions and free electrons. The electrons are pulled off of neutral atoms. They leave ions and electrons
in a big gaseous ball. And there you are. You are a big glowing ball of plasma running around and impressing your friends.
FINDING A PLASMA
We said you wouldn't find plasmas anywhere. But maybe there are some right in front of you. Think about a fluorescent light
bulb. They are not like regular light bulbs. Inside the long tube is a gas. When the light is turned on ELECTRICITY
flows through the tube. This electricity acts as that special energy and charges up the gas. This charging and exciting of
the atoms creates a glowing plasma inside the bulb.
Another example of plasma that you may have seen is a neon sign.
Just like a fluorescent light, neon signs are glass tubes filled with gas. When turned on the electricity flows through the
tube. The electricity charges the gas, possibly neon, and creates a plasma inside of the tube. The plasma glows a special
color depending on what kind of gas is inside.
You also see plasma when you look at stars. They aren't easy to find
if you live in a big smoggy city. Look hard. Stars are big balls of gases at really high temperatures. The high temperatures
charge up the atoms and create plasma. Stars are another good example of how the temperature of plasmas can be very different.
Fluorescent lights are cold compared to really hot stars. But still... They are both forms of plasma.
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