The GED Science Exam - Physical Science

This chapter reviews the basic concepts of physical-science—the structure of atoms, the structure and properties of mat-ter, chemical reactions, motions and forces, conservation of energ

 T h e S t r u c t u r e o f A t o m s

You and everything around you are composed of tiny particles called atoms The book you are reading, the neu-rons in your brain, and the air you are breathing can all be described as a collection of various atoms

History of the Atom

The term atom, which means indivisible, was coined by Greek philosopher Democritus (460–370 B.C.) He dis-agreed with Plato and Aristotle—who believed that matter could infinitely be divided into smaller and smaller pieces—and postulated that matter is composed of tiny indivisible particles In spite of Democritus, the belief that matter could be infinitely divided lingered until the early 1800s, when John Dalton formulated a meaningful atomic theory It stated:

■ Matter is composed of atoms

■ All atoms of a given element are identical

■ Atoms of different elements are different and have different properties

■ Atoms are neither created nor destroyed in a chemical reaction

Physical Science

PHYSICAL SCIENCE includes the disciplines of chemistry (the

study of matter) and physics (the study of energy and how energy affects matter) The questions on the physical science section of the GED will cover topics taught in high school chemistry and physics courses This chapter reviews the basic concepts of physical-science—the structure of atoms, the structure and properties of mat-ter, chemical reactions, motions and forces, conservation of energy, increase in disorder, and interactions of energy and matter

23

■ Compounds are formed when atoms of more

than one element combine

■ A given compound always has the same relative

number and kind of atoms

These postulates remain at the core of physical science

today, and we will explore them in more detail in the

fol-lowing sections

Protons, Neutrons, and Electrons

An atom is the smallest unit of matter that has the

prop-erties of a chemical element It consists of a nucleus

sur-rounded by electrons The nucleus contains positively

charged particles called protons, and uncharged

neu-trons Each neutron and each proton has a mass of about

1 atomic mass unit, abbreviated amu An amu is

equiv-alent to about 1.66 × 10−24g The number of protons in

an element is called the atomic number Electrons are

negatively charged and orbit the nucleus in electron

shells

Electrons in the outermost shell are called valence

electrons Valence electrons are mostly responsible for

the properties and reaction patterns of an element The

mass of an electron is more than 1,800 times smaller

than the mass of a proton or a neutron When

calculat-ing atomic mass, the mass of electrons can safely be

neg-lected In a neutral atom, the number of protons and

electrons is equal The negatively charged electrons are

attracted to the positively charged nucleus This

attrac-tive force holds an atom together The nucleus is held

together by strong nuclear forces

A representation of a lithium atom (Li) It has 3 protons (p)

and 4 neutrons (n) in the nucleus, and 3 electrons (e) in the

two electron shells Its atomic number is 3 (p) Its atomic

mass is 7 amu (p + n) The atom has no net charge because

the number of positively charged protons equals the number

of negatively charged electrons.

Charges and Masses

of Atomic Particles

Charge +1 0 –1 Mass 1 amu 1 amu 18 1 00amu

Isotopes

The number of protons in an element is always the same

In fact, the number of protons is what defines an ele-ment However, the number of neutrons in the atomic nucleus, and thus the atomic weight, can vary Atoms that contain the same number of protons and electrons,

but a different number of neutrons, are called isotopes.

The atomic masses of elements in the periodic table are weighted averages for different isotopes This explains why the atomic mass (the number of protons plus the number of neutrons) is not a whole number For exam-ple, most carbon atoms have 6 protons and 6 neutrons, giving it a mass of 12 amu This isotope of carbon is called “carbon twelve” (carbon-12) But the atomic mass

of carbon in the periodic table is listed as 12.011 The mass is not simply 12, because other isotopes of carbon have 5, 7, or 8 neutrons, and all the isotopes and their abundance are considered when the average atomic mass

is reported

Ions

An atom can lose or gain electrons and become charged

An atom that has lost or gained one or more electrons is

called an ion If an atom loses an electron, it becomes a

positively charged ion If it gains an electron, it becomes

a negatively charged ion For example, calcium (Ca), a biologically important element, can lose two electrons to become an ion with a positive charge of +2 (Ca2+) Chlo-rine (Cl) can gain an electron to become an ion with a negative charge of−1 (Cl−)

The Periodic Table

The periodic table is an organized list of all known ele-ments, arranged in order of increasing atomic number, such that elements with the same number of valence electrons, and therefore similar chemical properties, are

found in the same column, or group For example, the

last column in the periodic table lists the inert (noble) gases, such as helium and neon—highly unreactive

ele-ments A row in the periodic table is called a period.

3 p

4 n e

e

e Nucleus

Electron

shells

Elements that share the same period have the same

num-ber of electron shells

Common Elements

Some elements are frequently encountered in

biologi-cally important molecules and everyday life Below you

will find a list of common elements, their symbols, and

common uses

H—Hydrogen: involved in the nuclear process that

produces energy in the sun

He—Helium: used to make balloons fly

C—Carbon: found in all living organisms; pure

car-bon exists as graphite and diamonds

N—Nitrogen: used as a coolant to rapidly freeze

food

O—Oxygen: essential for respiration (breathing)

and combustion (burning)

Si—Silicon: used in making transistors and solar

cells

Cl—Chlorine: used as a disinfectant in pools and as

a cleaning agent in bleach

Ca—Calcium: necessary for bone formation

Fe—Iron: used as a building material; carries oxygen

in the blood

Cu—Copper: a U.S penny is made of copper; good

conductor of electricity

I—Iodine: lack in the diet results in an enlarged

thy-roid gland, or goiter

Hg—Mercury: used in thermometers; ingestion can

cause brain damage and poisoning

Pb—Lead: used for X-ray shielding in a dentist

office

Some elements exist in diatomic form (two atoms of

such an element are bonded), and are technically

mole-cules These elements include hydrogen (H2), nitrogen

(N2), oxygen (O2), fluorine (F2), chlorine (Cl2), bromine

(Br2), and iodine (I2)

 S t r u c t u r e a n d P r o p e r t i e s

o f M a t t e r

Matter has weight and takes up space The building

blocks of matter are atoms and molecules Matter can

interact with other matter and with energy These

inter-actions form the basis of chemical and physical reactions

Molecules

Molecules are composed of two or more atoms Atoms are held together in molecules by chemical bonds Chemical bonds can be ionic or covalent Ionic bonds form when one atom donates one or more electrons to another Covalent bonds form when electrons are shared between atoms The mass of a molecule can be calculated

by adding the masses of the atoms of which it is com-posed The number of atoms of a given element in a molecule is designated in a chemical formula by a sub-script after the symbol for that element For example, the glucose (blood sugar) molecule is represented as

C6H12O6.This formula tells you that the glucose mole-cule is contains six carbon atoms (C), twelve hydrogen atoms (H), and six oxygen atoms (O)

Organic and Inorganic Molecules

Molecules are often classified as organic or inorganic Organic molecules are those that contain both carbon and hydrogen Examples of organic compounds are methane (natural gas, CH4), glycine (an amino acid,

NH2CH2COOH), and ethanol (an alcohol, C2H5OH) Inorganic compounds include sodium chloride (table salt, NaCl), carbon dioxide (CO2), and water (H2O)

States of Matter

Matter is held together by intermolecular forces—forces between different molecules Three common states of matter are solid, liquid, and gas Matter is an atom, a molecule (compound), or a mixture Examples of mat-ter in solid form are diamonds (carbon atoms), ice (water molecules), and metal alloys (mixtures of differ-ent metals) A solid has a fixed shape and a fixed volume The molecules in a solid have a regular, ordered arrange-ment and vibrate in place, but are unable to move far Examples of matter in liquid form are mercury (mer-cury atoms), vinegar (molecules of acetic acid), and per-fume (a mixture of liquids made of different molecules) Liquids have a fixed volume, but take the shape of the container they are in Liquids flow, and their density (mass per unit volume) is usually lower than the density

of solids The molecules in a liquid are not ordered and can move by sliding past one another through a process

called diffusion.

Examples of matter in gaseous form include helium

gas used in balloons (helium atoms), water vapor

(mol-ecules of water), and air (mixture of different mol(mol-ecules

including nitrogen, oxygen, carbon dioxide, and water)

Gases take the shape and volume of their container They

can be compressed when pressure is applied The

mole-cules in gases are completely disordered and move very

quickly Gas density is much lower than the density of a

liquid

Phase Changes

Change of phase involves the transition from one state of

matter into another Freezing water to make ice for

cool-ing your drink, condensation of water vapor as morncool-ing

dew, and sublimation of dry ice (CO2) are examples of

phase change A phase change is a physical process No

chemical bonds are formed or broken Only the

inter-molecular (physical) forces are affected

Freezing is the process of changing a liquid into a solid

by removing heat The opposite process whereby heat

energy is added to the solid until it changes into a liquid

is called melting Boiling is the change of phase from a

liquid to a gas and also requires the input of energy

Con-densation is the change from gas to liquid Some

sub-stances sublime—change directly from the solid phase to

the gas phase without forming the liquid state first

Car-bon dioxide is such a substance Solid carCar-bon dioxide,

called dry ice, evaporates into the gas phase when heated

When gas changes directly into a solid, the process is

called deposition.

Phase changes between the three states of matter

The stronger the intermolecular forces are, the easier

it is for the molecule to exist in one of the condensed

states (liquid or gas) Molecules in which intermolecular

forces are strong tend to have high boiling points, since

more energy is needed to turn the molecules into the gaseous state where molecular interactions are low

Compounds and Mixtures

A compound is a homogeneous substance composed of

two or more elements, united chemically Examples of compounds include carbon dioxide (a product of respi-ration), sucrose (table sugar), seratonin (a human brain chemical), and acetic acid (a component of vinegar) In each of these compounds, there is more than one type of atom, chemically bonded to other atoms in definite pro-portion Compounds are made of molecules

A mixture is a physical combination of its

compo-nents In a homogeneous mixture, the components can’t

be visually separated Homogeneous mixtures also have the same composition (ratio of components) through-out their volume An example is a mixture of a small amount of salt in water A uniform mixture is often

called a solution In a solution, one substance (solute) is

dissolved in another (solvent) In the salt and water mix-ture, the salt is the solute, and the water is the solvent In

a heterogeneous mixture, the components can often be visually identified, and the composition may vary from one point of the mixture to another A collection of dimes and pennies is a heterogeneous mixture A mix-ture of sugar and flour is also heterogeneous While both components (sugar and flour) are white, the sugar crys-tals are larger and can be identified

Miscibility is the term used to describe the ability of two substances to form a homogeneous mixture Water and alcohol are miscible They can be mixed in such a way that the mixture will be uniform throughout the sample At each point, it will look, smell, and taste the same Oil and water are not miscible A mixture of these two substances is not homogeneous, since the oil floats

on water In a mixture of oil and water, two layers con-taining the two components are clearly visible Each layer looks, smells, tastes, and behaves differently

Gas

Liquid Vaporization

Solid

Condensation

Melting Freezing

 C h e m i c a l R e a c t i o n s

Removing stains from clothes, digesting food, and

burning wood in a fireplace are all examples of chemical

reactions Chemical reactions involve changes in the

chemical arrangement of atoms In a chemical reaction,

the atoms of reactants combine, recombine, or dissociate

to form products The number of atoms of a particular

element remains the same before and after a chemical

reaction The total mass is also preserved Similarly,

energy is never created or destroyed by a chemical

reaction If chemical bonds are broken, energy from

those bonds can be liberated into the surroundings as

heat However, this liberation of energy does not

consti-tute creation, since the energy only changes form—from

chemical to heat

Writing Chemical Reactions

A chemical reaction can be represented by a chemical

equation; the reactants are written on the left side and

the products on the right side of an arrow, indicating the

direction in which the reaction proceeds The chemical

equation below represents the reaction of glucose

(C6H12O6) with oxygen (O2) to form carbon dioxide

(CO2) and water (H2O) Your body runs this reaction all

the time to obtain energy

(C6H12O6) + 6 (O2) → 6 (CO2) + 6 (H2O)

The numbers in front of the molecular formulas

indi-cate the proportion in which the molecules react No

number in front of the molecule means that one

mole-cule of that substance is reacting In the reaction above,

one molecule of glucose is reacting with six molecules of

oxygen to form six molecules of carbon dioxide and six

molecules of water In reality, there are many molecules

of each of the substances and the reaction tells you in

what proportion the molecules react So if you had ten

molecules of glucose react with 60 molecules of oxygen,

you would obtain 60 molecules of carbon dioxide and 60

molecules of water In many ways, chemical equations

are like food recipes

2 Bread + 1 Cheese + 2 Tomato → Sandwich

With two slices of bread, one slice of cheese, and two slices of tomato, you can make one sandwich If you had six slices of bread, three slices of cheese, and six slices of tomato, you could make three sandwiches The same principles of proportion apply in chemical reactions

Heat of Reaction (Enthalpy)

Breaking molecular bonds releases energy stored in those bonds The energy is released in the form of heat Simi-larly, forming new bonds requires an input of energy Therefore, a chemical reaction will either absorb or give off heat, depending on how many and what kind of bonds are broken and made as a result of that reaction A

reaction that absorbs energy is called endothermic A

container in which an endothermic reaction takes place gets cold, because the heat of the container is absorbed by the reaction A reaction that gives off energy is called

exothermic Burning gasoline is an exothermic

reaction—it gives off energy

Increase in Disorder (Entropy)

Disorder, or entropy, is the lack of regularity in a system.

The more disordered a system, the larger its entropy Dis-order is much easier to come by than Dis-order Imagine that you have 100 blue beads in one hand and 100 red beads

in the other Now place all of them in a cup and shake What are the chances that you can pick out 100 beads in each hand so that they are separated by color, without looking? Not very likely! Entropy and chaos win There

is only one arrangement that leads to the ordered sepa-ration of beads (100 blue in one hand, 100 red in the other), and many arrangements that lead to mixed-up beads (33 blue, 67 red in one hand, 33 red and 67 blue in the other; 40 blue, 60 red in one hand, 60 blue, 40 red in the other ) The same is true of atoms Sometimes, arrangement and order can be achieved Atoms and mol-ecules in solids, such as snowflakes, have very regular, ordered arrangements But given enough time (and tem-perature), the snow melts, forming less ordered liquid water So, although reactions that lead to a more ordered state are possible, the reactions that lead to disorder are more likely The overall effect is that the disorder in the universe keeps increasing

Often, a reaction needs help getting started Such help

can come from a catalyst A catalyst is a substance or

form of energy that gets a reaction going, without being

changed or used up in the reaction A catalyst acts by

lowering what is called the activation energy of a reaction.

The activation energy is often illustrated as a hill

sepa-rating two valleys that needs to be crossed in order to get

from one valley to the other (one valley representing the

reactants, and the other the products) The catalyst acts

by making the hill lower

A catalyst acts by lowering the activation energy barrier (Ea)

to product formation In the diagram, the top hill represents

a high activation energy The catalyst acts to make the hill

smaller, so that the bottom hill represents the activation

energy in the presence of a catalyst.

Light is a catalyst for the photosynthesis reaction In

living systems, reactions are catalyzed by special protein

molecules called enzymes.

Reversible and Irreversible

Reactions

Some reactions can proceed in both

directions—reac-tants can form products, which can turn back into

reactants These reactions are called reversible Other

reactions are irreversible, meaning that reactants can

form products, but once the products form, they can not

be turned back into reactants While wood can burn

(react with oxygen) to produce heat, water, and carbon

dioxide, these products are unable to react to form wood

You can better understand reversibility if you look at the

activation energy diagram in the previous section The

hill that needs to be crossed by reactants to form

prod-ucts is much lower than the hill that needs to be crossed

by products to form reactants Most likely, such a

reac-tion will be irreversible Now look at the diagram below

The hill that needs to be crossed is almost the same for

reactants and for products, so the crossing could take

place from both sides—the reaction would be reversible

The activation energies (Ea) for the forward reaction

(reac-tants forming products) and for the reverse reaction (prod-ucts forming reactants) are about the same Such a reaction

is reversible.

 M o t i o n s , F o r c e s , a n d

C o n s e r v a t i o n o f E n e r g y

A force is a push or a pull Objects move in response to

forces acting on them When you kick a ball, it rolls A force is also required to stop motion The ball stops rolling because of the frictional force What happens here? First, your body breaks the chemical bonds in the food you have eaten This supplies your body with energy You use up some of that energy to kick the ball You apply a force, and as a result, the ball moves, carry-ing the energy your foot supplied it with But some of that energy is transferred from the ball to the ground it rolls on in the form of heat, through frictional force As energy is lost this way, the ball slows down When all the energy is used up through friction, the ball stops moving This example illustrates the concept of conservation of energy, as well as Newton’s first law—the Law of Inertia

Law of Inertia

The velocity of an object does not change unless a force is applied

For velocity of motion to change, either the speed and/or the direction must change and a net or unbal-anced force must be applied To summarize, an object at rest (whose speed is zero) remains at rest, unless some force acts on it—a person pushes it, the wind blows it away, gravity pulls it down A moving object contin-ues to move at the same speed in the same direction, unless some force is applied to it to slow it down, speed

it up, or change its direction The amount of speed an object gains (acceleration) or loses (deceleration) is directly proportional to the force applied The harder

reactants

products

Ea for

forward reaction

Ea for

reverse reaction

reactants

products

Ea with a

catalyst

Ea without

a catalyst

you kick the ball, the faster it will move The mass of the

ball will also determine how much it will accelerate Kick

a soccer ball Now kick a giant ball made of lead with the

same force (watch your foot!) Which ball moves faster as

a result of an equal kick? These observations constitute

Newton’s second law—the Law of Acceleration

Law of Acceleration

The acceleration of an object depends on its

mass and on the force applied to it The greater

the force, the greater the acceleration The

greater the mass, the lower the acceleration

Or, mathematically, force = mass × acceleration.

A good way to learn about the laws of motion is to

shoot pool What happens to billiard balls if you miss

and fail to hit any of them? Nothing They stay at rest

What happens when you hit the cue ball with the cue? It

moves in the direction you hit it in The harder you hit

it, the faster it moves Now, what happens when the cue

ball collides with another ball? The other ball starts

moving The cue ball slows down The energy is

trans-ferred from the cue ball to the ball it collided with When

an object exerts a force on a second object, the second

object exerts an equal force in the opposite direction on

the first object This is Newton’s third law—the Law of

Interaction

Law of Interaction

For every action, there is an equal and opposite

reaction

Types of Forces

Newton’s laws do not depend on the type of force

applied Some types of forces include gravitational,

elec-tromagnetic, contact, and nuclear

G RAVITATIONAL FORCE

Gravitation is an attractive force that each object with mass exerts on any other object with mass The strength

of the gravitational force depends on the masses of the objects and on the distance between them When we think of gravity, we usually think of Earth’s gravity, which prevents us from jumping infinitely high, keeps our homes stuck to the ground, and makes things thrown upward fall down We, too, exert a gravitational force on the Earth, and we exert forces on one another, but this is not very noticeable because our masses are very small in comparison with the mass of our planet The greater the masses involved, the greater the gravita-tional force between them The sun exerts a force on the Earth and the Earth exerts a force on the sun The moon exerts a force on the Earth, and the Earth on the moon The gravitational force of the moon is the reason there are tides The moon’s gravity pulls the water on Earth The sun also exerts a force on our water, but this is not

as apparent because the sun, although more massive than the moon, is very far away As the distance between two objects doubles, the gravitational force between them decreases four times

Gravitation

Gravitation is an attractive force that exists between all objects It is proportional to the masses of the objects and inversely propor-tional to the square of the distance between them

E LECTROMAGNETIC FORCE

Electricity and magnetism are two aspects of a single electromagnetic force Moving electric charges produce magnetic forces, and moving magnets produce electric forces The electromagnetic force exists between any two charged or magnetic objects, such as a proton and an electron or two electrons Opposite charges attract (an electron and a proton), while like charges repel (two pro-tons or two electrons) The strength of the force depends

on the charges and on the distance between them The greater the charges, the greater the force The closer the charges are to each other, the greater the force between them

C ONTACT FORCE

Contact forces are forces that exist as a result of an

inter-action between objects that are physically in contact with

one another They include frictional forces, tensional

forces, and normal forces

The friction force opposes the motion of an object

across a surface For example, if a glass moves across the

surface of the dinner table, there exists a friction force in

the direction opposite to the motion of the glass Friction

is the result of attractive intermolecular forces between

the molecules of the surface of the glass and the surface

of the table Friction depends on the nature of the two

surfaces For example, there would be less friction

between the table and the glass if the table was moistened

or lubricated with water The glass would glide across the

table more easily Friction also depends on the degree to

which the glass and the table are pressed together Air

resistance is a type of frictional force

Tension is the force that is transmitted through a rope

or wire when it is pulled tight by forces acting at each

end The tensional force is directed along the rope or

wire and pulls on the objects on either end of the wire

The normal force is exerted on an object in contact

with another stable object For example, the dinner table

exerts an upward force on a glass at rest on the surface of

the table

Nuclear forces are very strong forces that hold the

nucleus of an atom together If nuclei of different atoms

come close enough together, they can interact with one

another and reactions between the nuclei can occur

Forms of Energy

Energy is defined as the ability to do work We have

already stated that energy can’t be created or destroyed;

it can only change form Forms of energy include

poten-tial energy and kinetic energy

Potential energy is stored energy Kinetic energy is the

energy associated with motion Look at the following

illustration As the pendulum swings, the energy is

con-verted from potential to kinetic, and back to potential

When the hanging weight is at one of the high points, the

gravitational potential energy is at the maximum, and

kinetic energy is at the minimum At the low point, the

kinetic energy is maximized, and gravitational potential

energy is minimized

The change of potential energy into kinetic energy, and kinetic energy into potential energy, in a pendulum

Examples of potential energy include nuclear energy and chemical energy—energy is stored in the bonds that hold atoms and molecules together Heat, hydrodynamic energy, and electromagnetic waves are examples of kinetic energy—energy associated with the movement of molecules, water, and electrons or photons (increments

of light)

 I n t e r a c t i o n s o f E n e r g y

a n d M a t t e r

Energy in all its forms can interact with matter For example, when heat energy interacts with molecules of water, it makes them move faster and boil Waves— including sound and seismic waves, waves on water, and light waves—have energy and can transfer that energy when they interact with matter Consider what happens

if you are standing by the ocean and a big wave rolls in Sometimes, the energy carried by the wave is large enough to knock you down

Waves

Energy is also carried by electromagnetic waves or light waves The energy of electromagnetic waves is related to their wavelengths Electromagnetic waves include radio waves (the longest wavelength), microwaves, infrared radiation (radiant heat), visible light, ultraviolet radia-tion, X-rays, and gamma rays The wavelength depends

on the amount of energy the wave is carrying Shorter wavelengths carry more energy

When a wave hits a smooth surface, such as a mirror,

it is reflected Sound waves are reflected as echoes Mat-ter can also refract or bend waves This is what happens when a ray of light traveling through air hits a water sur-face A part of the wave is reflected, and a part is refracted

Maximum Potential Energy

Maximum Potential Energy

Maximum Kinetic Energy

Each kind of atom or molecule can gain or lose energy

only in particular discrete amounts When an atom gains

energy, light at the wavelength associated with that

energy is absorbed When an atom loses energy, light at

the wavelength associated with that energy is emitted

These wavelengths can be used to identify elements

Nuclear Reactions

In a nuclear reaction, energy can be converted to matter

and matter can be converted to energy In such processes,

energy and matter are conserved, according to Einstein’s

formula E = mc 2 , where E is the energy, m is the mass

(matter), and c is the speed of light A nuclear reaction is

different from a chemical reaction because in a nuclear

reaction, the particles in nuclei (protons and neutrons)

interact, whereas in a chemical reaction, electrons are lost

or gained by an atom Two types of nuclear reactions are

fusion and fission

Fusion is a nuclear process in which two light nuclei

combine to form one heavier nucleus A fusion reaction

releases an amount of energy more than a million times

greater than the energy released in a typical chemical

reaction This gain in energy is accompanied by a loss of

mass The sum of the masses of the two light nuclei is

lower than the mass of the heavier nucleus produced

Nuclear fusion reactions are responsible for the energy

output of the sun

Fission is a nuclear process in which a heavy nucleus

splits into two lighter nuclei Fission reaction was used in

the first atomic bomb and is still used in nuclear power

plants Fission, like fusion, liberates a great amount of

energy The price for this energy is a loss in mass A heavy

nucleus that splits is heavier than the sum of the masses

of the lighter nuclei that result

Key Concepts

This chapter gave you a crash course in the basics of physical science Here are the most important concepts to remember:

➧All matter is composed of tiny particles called atoms

➧Atoms combine with other atoms to form molecules

➧In a chemical reaction, atoms in molecules rearrange to form other molecules

➧The three common states of matter are solid, liquid, and gas

➧The disorder in the universe is always increasing

➧Mass and energy can’t be created or destroyed

➧Energy can change form and can be trans-ferred in interactions with matter