The States of Matter
Matter can be found in only three states, or forms: solid, liquid, or
gas. Matter is defined by its properties—mass,
electricity, and magnetism. the mass of matter is the center of its
gravitational force. all matter can be broken down into elementary particles—neutrons,
protons, and electrons. these particles are the centers of electrical and
most things around you are solids. Rocks and buildings are solids. Though
solids may differ in many ways, they are alike in some ways. One way is that
solids have a shape of their own. The shape of a solid does not change by
itself. Solids are alike in another way. Suppose you change the shape of a
solid by bending or breaking it. The solid will still take up the same amount
of space. No matter how tightly you might wad up a piece of paper, it will
still take up the same amount of space.
is a reason why solids act the way they do. That reason has to do with their
molecules. All matter is made of molecules. These molecules are always moving.
The molecules that make up a solid are very close together. These molecules
are attracted to one another so strongly that they do not have much freedom to
move. This is why solids keep their own shape.
state of matter is the liquid state. Fruit juices and milk are some liquids.
Liquids are different from solids in many ways. Liquids do not have a shape of
their own. Liquids are like solids in one way. They cannot change in size by
themselves. One cupful of water may be poured in a pan or on
a table. But it is still one cupful of water. It still takes up the
same amount of space.
is a reason why liquids act as they do. Like solids, the reason has to do with
their molecules. The molecules that make up a liquid are not as strongly
attracted to one another as the molecules in a solid. Molecules in a liquid
have more freedom to move. They can move about to form many shapes. Although
liquids and solids are different in many ways, some things act like liquids
and solids. It may be hard to tell what these things are. In which state do
you think mayonnaise is? Egg yolks? Toothpaste?
are quite different from both solids and liquids. Gases have no color. They
cannot even be seen. Feeling gases is one way you may know that they are
around you. Gases, like liquids, have no shape of their own. Gases can be in
many shapes. Gases are different fro both solids and liquids in that gases can
move about by themselves. Therefore, a gas does not always take up the same
amount of space. The air which fills a small jar can spread out and take up
the space in a large room. The air you breathe can come together to fill the
space in a small balloon.
molecules which make up a gas make it act the way it does. These molecules are
much farther apart than the molecules of a liquid. This gives them much more
freedom to move. Because gases can move by themselves, they most often do not
stay in one place. When you open a window, some of the air in a room will move
outside. Some of the air outside will move inside. The air around you is made
up of many gases. One gas in the air is oxygen. Oxygen is the gas you need to
breathe in order to live. Another gas is called carbon dioxide. This gas is
needed by plants to grow.
Changing the States of Matter
The states of matter can be
changed. Some matter can change from a solid to a liquid. Some liquids can
change to a gas.
solid cannot change to a liquid by itself. Something has to cause this change.
The change from a solid to a liquid is called melting. Many solids can
malt when enough heat is added to them (examples: butter, ice, shortening,
wax). The molecules in a solid can hardly move because they are attracted to
one another very strongly. Heat can change the way the molecules are attracted
to one another. Heat can make them move faster and farther apart. So, the
molecules have more freedom to move. This is why a solid changes to a liquid
when heated. Some things need more heat than others in order to melt.
change of state is the change from a liquid to a gas. Since you cannot see a
gas, it is sometimes hard to see this kind of change. Just as heat can change
the way the molecules in a solid are attracted to one another, it can change
the way the molecules in a liquid are attracted to one another. When enough
heat is added to a liquid, the molecules move very fast and very far apart.
This is why a liquid changes to a gas when heated. When a liquid is changing
to a gas, it is sometimes called boiling. Most liquids can boil. Some
liquids need more heat than others to boil. People often think of steam as a
gas. But this steam is made up of droplets of water. Another way in which heat
changes liquids to gases is by evaporation. When a liquid evaporates,
its molecules become part of the air.
heated, a solid most often changes first to a liquid and then to a gas. A few
times, however, a solid can change straight to a gas. Sometimes a patch of ice
on a street or a sidewalk may “disappear.” This can happen even though the
air is not warm enough to melt the ice.
such as melting and boiling cannot take place without heat. But matter may
also change state when heat is taken away. Matter than can change from a solid
to a liquid can change back to a solid. This change of state is often called freezing.
Freezing takes place when a certain amount of heat is taken away from a
liquid. When heat is taken away from a liquid, its molecules slow down and
have less freedom to move.
enough heat is taken away from a liquid, the liquid changes to a solid. A gas
can also change when heat is taken away. It may change to a liquid. Maybe you
have seen tiny drops of water on grass or bushes in the morning. They may not
have been raindrops. They may have been drops of dew. Dew forms on
nights when there is hardly a cloud in the sky. Did you ever breathe on a
mirror and make it cloudy? If so, you were seeing water in the air you
breathed out. The air around you also has water in it. This water is in the
form of a gas. It is called water vapor. When water vapor is cooled, it
becomes water again. This is what makes a mirror cloudy and the grass wet with
dew. Sometimes, water vapor in the air helps to form clouds.
gases must be very cold before they can become liquids. Oxygen, the gas you
need to breathe, becomes a liquid at –297 degrees Fahrenheit. Liquid oxygen
is used to make part of the fuel for space rockets. Nitrogen, the gas
which makes up most of the air, becomes a liquid when it reaches –320
degrees Fahrenheit. Liquid nitrogen is used to make fertilizer to help grow
crops for your foot. Oxygen and nitrogen can also become solids.
on a window is a solid which has formed straight from a gas. Like dew, frost
is formed from water in the air. On cold nights, this water in the air most
often forms bits of ice on things near the ground.
Other Changes in Matter
Although you may see matter change around you each day, there are only
two kinds of changes. One kind of change is called a physical change.
When matter changes from one state to another, a physical change takes place.
In a physical change, matter may change in the way it looks. The matter,
however, does not change. That is, its molecules are not changed into
different kinds of molecules. When ice melts and becomes water, the molecules
do not change to other kinds of molecules. Even when water boils and becomes
water vapor, there is still no change in the kind of molecules. Water
molecules are still water molecules whether they are frozen or boiled. Another
kind of physical change takes place when matter changes in size. Heat can make
many things change in size. It can cause most of these things to become
bigger. Heat can make a sidewalk become a little bigger on a hot summer day.
The same sidewalk can become a little smaller on a cold winter night. Though
little, these changes in size may crack a sidewalk. (examples: popcorn and
cake batter) A change in the shape of matter is still another kind of physical
change. When you pour milk from a bottle into a glass, the molecules of milk
are not changed. The milk just changes from the shape of the bottle to the
shape of the glass. Another way in which you can change the shape of matter is
by breaking it apart. The glass in a window will change shape when a ball is
thrown through it. Many times, a change in the color of matter is also a kind
of physical change. Another kind of physical change is by mixing a powder to
milk to make a drink. Sometimes solids will dissolve or seem to melt
and disappear when they are mixed with a liquid. When you put salt in water,
the salt dissolves.
other kind of change in matter is called a chemical change. When a tree
trunk is made into paper, a chemical change takes place. In a chemical change,
matter is changed. That is, the molecules are changed into different kinds of
is a chemical change that is taking place in you all the time. It happens when
you breathe. When you breathe in, you breathe in oxygen. As your body uses
oxygen, some of the oxygen mixes with things your body no longer needs. One of
these things is carbon. The oxygen and carbon join to form a gas called carbon
dioxide. When you breathe out, you breathe out carbon dioxide. There is
another chemical change which puts oxygen into the air. This chemical change
takes place in green plants. Green plants take in the carbon dioxide you
breathe out. These plants use carbon dioxide. They need it to live and grow.
As green plants take in carbon dioxide, the carbon dioxide molecules become
broken down. The carbon is used to make food for the plant. The oxygen is
given off into the air.
in your mouth can change some of the food you eat. After you swallow your
food, it mixes with other liquids inside your stomach and other parts of your
body. These liquids also change the food you eat. They change the molecules of
food into different kinds of molecules. They food you eat must be changed so
that your body can use it.
oxygen in the air is near iron, the oxygen and the iron may join together.
When iron and oxygen join, their molecules change. The molecules change to
make a new kind of matter. This new matter is called rust.
changes molecules into different kinds of molecules. The molecules in the
match and the oxygen molecules in the air change into molecules of ashes,
carbon dioxide, and water. Oil and natural gas are often burned to heat homes
and other buildings. Gasoline is burned in cars and buses to make them run.
chemical changes take place when food is cooked.
Facts and Theories of Matter
Matter and energy always go
together. In order to understand more about energy, we must first examine some
facts and theories about matter.
is the name for all the substances of the world. Water, air, rock, iron,
elephants, soap bubbles, trees, bugs—everything that has any mass
at all and takes up any space is matter.
matter is made of tiny particles called molecules. A molecule is
the smallest single whole particle of any pure substance. Iron, coal,
grass, gold, air, water—all the liquids, solids, and gases in the
world—everything is made of molecules.
are different substances because there are different kinds of molecules.
are made of one or more atoms. If you break apart a water molecule into
its atoms you have 2 atoms of hydrogen and 1 atom of oxygen. But then you
do not have water. You have different atoms which can be used to make
different substances, or which can join again to make a water molecule.
are 103 different kinds of atoms in the world. Each different kind is
called an element. So there are 103 known elements in the world.
(Carbon, oxygen, nitrogen, hydrogen, gold, silver, copper, and led are
some of the elements.)
atoms of different kinds are combined in a substance, the substance is a
compound. Water, nylon, iron rust, salt are a few compounds.
you could possibly look further into the atom, scientists believe you
would find these particles: a nucleus made of still smaller parts
called neutrons and protons; one or more electrons
whirling around the nucleus. Atoms are like the letters of the alphabet.
Molecules are like words. Molecules are different because they are made up
of different combinations of atoms, just as words are made up of different
combinations of letters.
The Motion of Molecules
Another key fact about matter is this: all molecules are in motion.
all three states of matter—solid, liquid, and gas—the molecules are moving
solids, the motion is vibration. The molecules vibrate rapidly back and forth.
But even though they are vibrating, the molecules of a solid hold together and
keep their positions.
liquids, the molecules vibrate, too, but they are not locked together. They
are free to roll and slide around each other.
gas, the molecules move rapidly in all directions. They zip around and bump
and bounce at enormous speeds. Because of their rapid motion, the molecules in
a gas are much further apart than in a solid or liquid.
are so small that a tiny grain of sand is made of trillions of them.
you smell different odors (such as cologne, food cooking, or cleaning
products), you are smelling invisible molecules. Perhaps you have never
thought of an odor as being a real thing, but it is.
the pleasant and unpleasant odors of the world are whirling, jiggling, bouncing
You can see a bit of iron attracted toward a magnet. Scientists have
found that molecules attract each other, too. You cannot see it happen with
separate molecules because they are too small.
walls, the floor, ropes and wires, you yourself—all are held together by the
attraction of molecules.
molecule has attraction for other molecules. In some materials the attraction
is stronger than in other materials.
hang a picture with steel wire rather than cotton thread. The molecules in
steel wire attract each other more strongly than the molecules in cotton
molecules of the thin thread of spider webs attract each other so strongly
that even a heavy insect cannot easily tear it.
Using Molecular Attraction
We can use materials whose molecules have a strong attraction for other
kinds of molecules as well as for each other.
and ink have strong attraction for one another. Otherwise your writing would
fall off the paper if the paper were tipped.
rubber cement, paste, and mucilage are useful to us because their molecules
have strong attraction.
roads and buildings are made mostly of sand and gravel. These loose materials
are held together by the strong attraction of the molecules in cement.
strong material is one in which the molecules have a strong attraction for
all substances—solids, liquids, and gases—molecules attract each other.
Heat energy comes from the separate motion of molecules. In solids and
liquids the motion is vibration. The molecules vibrate rapidly back and forth.
In gases the molecules fly in all directions, bumping and bouncing as they go.
In any substance, the more heat energy it gets, the more rapidly its molecules
Measuring Heat Energy
Most people use the words “temperature” and “heat” as though
they had the same meaning. Scientists have to use the words more carefully,
because the words have different meanings.
tells us something about the speed of the moving molecules. Water molecules
that come out of the hot-water faucet in your home are a certain temperature.
They are vibrating at a certain speed. The water that comes out of the
cold-water faucet has a lower temperature. The molecules are moving more
can’t easily measure the motion of molecules. They are too tiny and they are
moving too fast. So we do the next best thing—we measure the effect of their
motion on something we can see. We use a thermometer to measure the
and slower, still colder and slower—is there a temperature at which
molecules stop vibrating altogether? There is, and it is called absolute
zero. It is about 459 degrees below the zero on the Fahrenheit scale. It
is about 373 degrees below the zero of the centigrade scale.
has to do with the total amount of energy of all the molecules of a
substance. Two quarts of boiling water have twice as many molecules as one
quart of boiling water. So there is twice as much heat energy in two quarts as
in one quart.
the U.S. and in England the usual unit of heat energy is the B.T.U. this
stands for British thermal unit. It is the amount of heat that will raise the
temperature of one pound of water by one degree Fahrenheit.
are other ways of measuring heat energy, such as calories in food. This is a
measure of the heat energy in a food. To measure calories in food, the food is
burned as fuel to heat a certain quantity of water. The temperature rise of
the water tells us the amount of heat energy in the food burned.
we say that a slice of bread has 63 calories, we mean that burning the bread
as fuel will make the temperature of a liter of water rise 63 degrees
Transferring Heat Energy
Most heating systems transfer energy from a fire in the furnace to the
radiators in the rooms. An air conditioner transfers heat energy from the
rooms of your house to the outside air. An automobile radiator and fan cool
the engine by transferring heat energy from the engine to the outside air.
engineers who designed these machines had to solve problems in heat transfer.
For example, an automobile engine must not get too hot or the metal will
crack. But it must not get too cool or the engine won’t work well. So the
fan and radiator must transfer the right amount of heat. They must be the
right size—not too big and not too small. To figure the right size, the
engineer must calculate how many B.T.U.’s must be transferred from the
engine every minute.
Heat to Mechanical Energy
The most useful way of getting work from heat energy is to transfer it
to mechanical energy. Mechanical energy is energy in large numbers of
molecules moving together. This is done by gasoline engines, diesel engines,
and steam engines.
Chemical and Nuclear Energy
Most molecules are made of two or more atoms combined with each other.
They are held together by a very powerful force. This force is called a chemical
energy, we can break the bonds between the atoms. We can separate the
molecules of water into its atoms. We can combine the separate atoms again
into a molecule of water. Or we can combine the atoms with other atoms to make
different substances. For example, oxygen can combine with iron to form a new
substance, rust (iron oxide).
of these is a chemical change. When we separate, or combine, or shift
atoms from one molecule to another, we make a chemical change.
changes don’t just happen. Breaking chemical bonds and shifting atoms around
is work. It takes energy. The energy in every chemical change is chemical
takes energy to change a solid into a liquid. It takes even more energy to
change a liquid to a gas.
we use heat energy to make this change of state. Heat energy can change ice to
water. Heat energy can change water to steam. But chemical energy by itself
can also produce changes of state.
have the energy to combine and change. This energy is chemical energy.
most important example of chemical energy is you. Your body works by chemical
energy. Breathing, moving, growing, eating, digesting—all these require
molecules of substances to be taken apart, shifted, and combined into new
substances. All these chemical changes involve chemical energy.
kinds of atoms combine with other atoms very easily. Others do not. Rusting
takes place when molecules of oxygen combine with molecules of iron. There are
oxygen molecules in the air, and there are iron molecules in iron or steel.
When they combine, they form a crumbly, reddish material, which we call rush
or iron oxide.
and oxygen do not combine easily. Iron and oxygen combine very readily,
because they have a strong attraction for each other. Because of this strong
attraction, they hold on to each other very firmly. We say there are strong
chemical bonds between them. When they have combined, it is very difficult to
separate them. Lots of energy must be put in to break the chemical bonds.
Energy for Chemical Change
In iron mines we dig out iron
ore, which is mainly iron oxide. But iron oxide itself is useless. It is a
reddish, crumbly powder. To change iron oxide into useful iron, we have to
separate the oxygen from the iron. Lots of input energy is needed to break the
chemical bonds. The input energy is heat. If we heat the iron oxide molecules
to a very high temperature, we cause them to vibrate very rapidly. This rapid
vibration breaks the chemical bonds. Other substances are added to help in the
separation. The main point is that heat energy is put in to break a chemical
Energy from Chemical Change
To produce iron from iron ore, the bonds must be broken between the
atoms of iron and oxygen. To break the bond, energy must be taken in.
To make bonds between iron and oxygen, heat energy is given off.
When oxygen combines with iron, chemical bonds are made and energy is
given off. In this chemical change, heat energy is given of.
You use this kind of chemical change all the time. Everything that
burns—slowly or quickly—is combining with oxygen. In such chemical changes
heat energy is given off.
The combining of oxygen with any substance is called oxidation.
We say the substance is being oxidized. During oxidation, heat energy is
usually given off.
Oxidation can take place slowly or rapidly. The rusting of iron is an
example of slow oxidation. The burning of wood, coal, and other fuels is rapid
oxidation. In some kinds of oxidation the fuel burns up all at once. This
extremely rapid oxidation is called an explosion.
Energy in Substances
Gasoline is a good fuel for running a car because it is a high-energy
substance. Gasoline is made of hydrogen and carbon atoms, loosely bonded to
each other. These bonds can be broken very easily by the heat of an electric
spark or match. Then the hydrogen and carbon atoms are ready to form strong
bonds with oxygen. As they combine with oxygen they give off lots of heat
energy. In the engine, the heat energy is transferred to mechanical energy.
Water is not a good fuel because it is a low-energy substance. That is,
water molecules cannot readily combine with oxygen and give off energy. This
is because they already contain oxygen atoms. These oxygen atoms are strongly
bonded to hydrogen atoms. These bonds cannot easily be broken.
There are millions of different kinds of substances in the world. Some
are high-energy. Some are low-energy. Some are in-between. High-energy
substances make chemical changes readily. Low-energy substances are hard to
Of course burning is only one of the many chemical changes that take
place. In every chemical change there is an energy transfer. High-energy
substances can give off lots of energy. Low-energy substances can give off
little or none.
Chemical Energy in Living Things
A fuel is a high-energy substance. You take in a fuel called food. In
your body the fuel combines with oxygen. The fuel is oxidized. Burning the
oxidation there is an output of heat energy. This is what keeps your body
warm. There is also an output of mechanical energy. This is what makes your
Living things get their energy by feeding on high-energy substances. No
matter what food chain you follow, you come to green plants. Green plants make
food for all other living things. They make it out of substances that animals
cannot use as food.
Plants take molecules of water from the soil and molecules of carbon
dioxide from the air. These are low-energy substances. With the energy of
sunlight, the plants separate these molecules into their atoms and then
combine them into new molecules with high energy—molecules of sugar and
This process is called photosynthesis. Photo means
“light”; synthesis means “put together.” Photosynthesis is the
most important energy transfer in the world.
Every atom has two main groups of parts. There is an outer group of
parts called electrons. There is an inner group of parts called the nucleus.
The energy we get from the nucleus is called nuclear energy. It is also
called atomic energy, but nuclear is a more exact word. The energy does not
come from the entire atom, but only from the nucleus.
Nuclear energy is caused by changes in the nuclei of atoms. The nucleus
is at the center of the atom. It consists of one or more tiny particles called
protons. Almost all nuclei also contain other tiny particles called neutrons.
The only exception is hydrogen. Hydrogen is the simplest kind of atom. Its
nucleus is a single proton. A single electron whirls around the nucleus. With
only two tiny particles, a hydrogen atom is the lightest of the atoms.
Helium is the next heavier atom. There are two electrons whirling
around the nucleus. There are two protons and two neutrons in the nucleus. So
helium atoms are heavier than hydrogen atoms, because they are made of more
particles. But helium is still a very light substance. It is a gas used in
Altogether, there are 103 different kinds of atoms that we know of.
These 103 different kinds of atoms are called elements.
How the atoms of all these elements are alike:
atoms have one or more electrons whirling around the nucleus.
have one or more tiny particles called protons in the nucleus.
atom has exactly as many electrons as protons.
kind of atom, with the exception of ordinary hydrogen, has neutrons in its
number of neutrons in the atoms of an element is not always the same. For
example, most carbon atoms have six neutrons, but some kinds have seven or
eight. The different kinds of carbon atoms are called the isotopes
Isotope means “same place.” The three isotopes of carbon
belong in the same place on a list of elements because all three are forms of
the same element, carbon. Most of the elements have several isotopes.
Energy in the Nucleus
In the center of the atom is a cluster of neutrons and protons called
the nucleus. Around the nucleus, the electrons are whizzing, each in its own
Scientists have wondered what keeps the parts together. They have
discovered some facts that seem to give part of the answer:
attract electrons. The protons in the nucleus attract the whirling
electrons with just enough force to keep them in their orbits.
repel other electrons. Therefore the whirling electrons do not bump
into their neighboring electrons when their orbits cross.
do not attract or repel protons, electrons, or other neutrons. So even
though there are neutrons in the nucleus, they seem to make no difference
in keeping the parts of the atom together.
repel other protons. Now there’s a puzzle! The nucleus contains a
cluster of protons. The protons repel each other, yet the nucleus stays
What force binds the protons together? Scientists don’t know, but
they do know that the binding force is enormously powerful. It is the most
powerful force in the world. It is trillions of times as powerful as gravity
or magnetism. Even though scientists don’t know what stored-up energy keeps
the nucleus together, they have learned how to release it and use it. Nuclear
energy can be released in two ways—by fission and by fusion.
Fission means “breaking
apart.” Some elements have nuclei with a great many particles. For instance,
uranium nuclei are densely packed with particles. The most common
isotope of uranium has 238 particles in each nucleus. This isotope is called
U-238. there is another isotope of uranium called U-235, with 235 particles in
each nucleus. This isotope has a strange behavior.
It can easily break into two smaller nuclei. This breaking apart is
nuclear fission. Here is the important thing about nuclear fission: a certain
amount of binding energy holds a U-235 nucleus together. When the nucleus
breaks into two parts, each of the parts is also held together by binding
energy. But when we add up the binding energy of the two parts, we find that
it comes to less than the binding energy that held the whole big
What happened to the rest of the binding energy? In the law of
conservation of energy, energy cannot be destroyed, but only transferred. The
missing binding energy was transferred. It was sent out as heat, light, and
other forms of energy. An input of nuclear energy (the binding energy of the
U-235 nucleus) was transferred to an output of these other forms of energy.
Scientists weighed the U-235 before fission. Then after fission they
weighed all the broken parts. The broken parts together weighed less than the
U-235. a scientist would say they had less mass. The missing mass had been
converted into energy.
There seemed to be some relationship between mass and energy. This
relationship between mass and binding energy was first predicted by Albert
Einstein in 1905. since then scientists have done many experiments in
converting mass to energy by nuclear fission.
Scientists have also found another way of releasing energy from the
nucleus—by putting together the nuclei of atoms. Putting together is call
fusion. They found that certain kids of hydrogen nuclei can be forced to
combine. When this happens, they form larger nuclei with more particles. These
larger nuclei have less binding energy than the hydrogen nuclei added
together. So there is some missing energy to be accounted form these larger
nuclei also have less mass than the hydrogen nuclei added together. So there
is some missing mass to be accounted for.
What happened to the rest of the energy? What happened to the rest of
the mass? Just as in the fission of U-235, mass was transferred to energy.
Both in fission and in fusion, mass is transferred to heat, light, and other
forms of energy. This explanation of transfer of mass to energy is only a
beginning explanation of a very complicated idea.
Scientists are finding ways to use nuclear energy. The most successful
way so far has been to transfer it to heat energy. The heat energy is used to
drive engines in ships and submarines and to drive electric generators in
Scientists continue to make discoveries about the world of matter and
energy. Often, a new discovery means that they must give up a former belief.
Years ago, the law of conservation of energy seemed to be complete: “Energy
cannot be created or destroyed; it can only be transferred from one form to
Years ago, there was a similar belief about matter, called the law of
conservation of matter (or mass). It stated that “Mass can neither be
created nor destroyed; it can only be transferred from one form to another.”
Were these two laws true, or false, or incomplete? Now we know that
separately the two laws are incomplete, and together they are true. Mass and
energy may seem to disappear, but they have only been transferred from on to
Scientists no longer speak about mass and energy as entirely different.
They speak about mass-energy. The law of conservation of mass-energy:
“Mass-energy can neither be created nor destroyed; it can only be
transferred from one form to another.”