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The States of Matter

            Matter can be found in only three states, or forms: solid, liquid, or gas. Matter is defined by its propertiesmass, electricity, and magnetism. the mass of matter is the center of its gravitational force. all matter can be broken down into elementary particlesneutrons, protons, and electrons. these particles are the centers of electrical and nuclear forces.
           
Probably 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.
           
There 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.
           
Another 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.
           
There 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?
           
Gases 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.
           
The 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.
           
A 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.
           
Another 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.
           
When 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.
           
Changes 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.
           
When 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.
           
Most 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.
           
Frost 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.
           
The 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 molecules.
           
There 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.
            The liquid 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.
           
When 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.
            Fire 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.
            Other 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.

  1. 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.
  2. All 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.
  3. There are different substances because there are different kinds of molecules.
  4. 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.
  5. There 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.)
  6. If atoms of different kinds are combined in a substance, the substance is a compound. Water, nylon, iron rust, salt are a few compounds.
  7. If 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.
           
In all three states of matter—solid, liquid, and gas—the molecules are moving rapidly.
           
In 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.
           
In liquids, the molecules vibrate, too, but they are not locked together. They are free to roll and slide around each other.
           
In a 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.
           
Molecules are so small that a tiny grain of sand is made of trillions of them.
           
When 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.
           
All the pleasant and unpleasant odors of the world are whirling, jiggling, bouncing molecules.

 

Molecular Attraction

            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.
           
The walls, the floor, ropes and wires, you yourself—all are held together by the attraction of molecules.
           
Every molecule has attraction for other molecules. In some materials the attraction is stronger than in other materials.
           
We 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 thread.
           
The 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.
           
Paper and ink have strong attraction for one another. Otherwise your writing would fall off the paper if the paper were tipped.
           
Glue, rubber cement, paste, and mucilage are useful to us because their molecules have strong attraction.
           
Concrete 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.
           
A strong material is one in which the molecules have a strong attraction for each other.
           
In all substances—solids, liquids, and gases—molecules attract each other.

 

Heat Energy

            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 move.

 

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.
           
Temperature 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 slowly.
           
We 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 temperature.
           
Colder 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.
           
Heat 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.
           
In 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.
           
There 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.
           
When 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 centigrade.

 

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.
           
The 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 bond.
           
With 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).
           
Each of these is a chemical change. When we separate, or combine, or shift atoms from one molecule to another, we make a chemical change.
           
Chemical 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 energy.
           
It takes energy to change a solid into a liquid. It takes even more energy to change a liquid to a gas.
           
Usually 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.
           
Substances have the energy to combine and change. This energy is chemical energy.
           
The 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.

Combining Atoms
           
Some 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.
           
Tin 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 bond.

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.

Oxidation
           
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 change.
           
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 muscles move.

Photosynthesis
           
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 oxygen.
           
This process is called photosynthesis. Photo means “light”; synthesis means “put together.” Photosynthesis is the most important energy transfer in the world.

Nuclear Energy
           
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 balloons.

Elements
           
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:

  1. All atoms have one or more electrons whirling around the nucleus.
  2. Atoms have one or more tiny particles called protons in the nucleus.
  3. Every atom has exactly as many electrons as protons.
  4. Every kind of atom, with the exception of ordinary hydrogen, has neutrons in its nucleus.
  5. The 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 of carbon.

            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 orbit.
           
Scientists have wondered what keeps the parts together. They have discovered some facts that seem to give part of the answer:

  1. Protons attract electrons. The protons in the nucleus attract the whirling electrons with just enough force to keep them in their orbits.
  2. Electrons repel other electrons. Therefore the whirling electrons do not bump into their neighboring electrons when their orbits cross.
  3. Neutrons 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.
  4. Protons repel other protons. Now there’s a puzzle! The nucleus contains a cluster of protons. The protons repel each other, yet the nucleus stays together.

            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
   
         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 nucleus together!
           
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.

Mass-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.

Fusion
           
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 powerhouses.

Mass-energy Transfers
           
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 another.”
           
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 the other.
           
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.”