The acceleration of an object is the rate of change of its velocity. An object is accelerated when its speed changes, when its direction of motion changes, or when both change.

An acid is a substance whose molecules contain hydrogen and whose water solution contains hydrogen ions.

The activation energy of a reaction is the energy that must be supplied initially for the reaction to start.

An alcohol is a hydrocarbon derivative in which one or more hydrogen atoms have been replaced by OH groups.

The alkali metals are a family of soft, light, extremely active metals with similar chemical properties. The alkali metals are lithium, sodium, potassium, rubidium, cesium, and francium in order of atomic number.

An alluvial fan is a deposit of sediments where a stream emerges from a steep mountain valley and flows onto a plain.

An alpha particle is the nucleus of a helium atom. It consists of two protons and two neutrons and is emitted in certain radioactive decays.

An amorphous solid is one whose constituent particles show no regularity of arrangement.

The amplitude of a wave is the maximum value of whatever quantity is periodically varying.

The angular momentum of a rotating object is a measure of its tendency to continue to spin. Angular momentum is a vector quantity. The angular momentum of an isolated object or system of objects is conserved (remains unchanged).

The antiparticle of a particle has the same mass and general behavior, but has a charge of opposite sign and differs in certain other respects. A particle and its antiparticle can annihilate each other with their masses turning entirely into energy. In the opposite process of pair production a particle-antiparticle pair materializes from energy.

Archimedes' principle states that the buoyant force on an object immersed in a fluid is equal to the weight of the fluid displaced by the object.

An asteroid is one of many relatively small bodies that revolve around the sun in orbits that lie between those of Mars and Jupiter.

The asthenosphere is a layer of rock capable of plastic deformation that is just below the lithosphere in the earth's mantle.

The earth's atmosphere is its gaseous envelope of air. The four regions of the atmosphere are, from the earth's surface upward, the troposphere, the stratosphere, the mesosphere, and the thermosphere.

Atmospheric pressure is the force with which the atmosphere presses down upon each unit area at the earth's surface. Its value is normally about 1.013 x 10⁵ Pa.

An atom is the ultimate particle of any element. It consists of a nucleus, composed of neutrons and protons, and a number of electrons that move about the nucleus relatively far away.

The atomic mass of an element is the average mass of its atoms expressed in atomic mass units (u).

The atomic number of an element is the number of electrons in each of its atoms or, equivalently, the number of protons in its nucleus. An element is a substance all of whose atoms have the same atomic number.

The Great Attractor is a point in space toward which our galaxy, along with several hundred other nearby galaxies, seems to be moving. Its nature is unknown.

An aurora is a display of changing colored patterns of light that appear in the sky, particularly at high latitudes. The aurora arises from the excitation of atmospheric gases at high altitudes by streams of protons and electrons from the sun.

A base is a substance whose molecules contain OH groups and whose water solutions contain OH^- ions.

A batholith is a very large body of intrusive rock, mainly granite, that extends downward as much as 10 km.

A benzene ring consists of six carbon atoms linked together in a flat hexagon.

The big bang theory holds that the universe originated about 15 billion years ago in the explosion of a hot, dense aggregate of matter. If the explosion was violent enough, the expansion of the universe will continue forever; if not, the universe will eventually begin to contract and will end up in a big crunch after which another cycle of expansion and contraction may occur.

A black hole is an old star that has contracted to so small a size that its gravitational field prevents the escape of anything, including light.

In the Bohr model of the atom, electrons are supposed to move around nuclei in circular orbits of definite size. When an electron jumps from one orbit to another, a photon of light is either emitted or absorbed whose energy corresponds to the difference in the electron's energy in the two orbits.

Boyle's law states that the volume of a gas is inversely proportional to its pressure when the temperature is held constant.

Brownian movement is the irregular motion of suspended particles due to molecular bombardment.

A carbohydrate is a compound of carbon, hydrogen, and oxygen whose molecules contain two atoms of hydrogen for each one of oxygen. Carbohydrates are manufactured in green plants from water and carbon dioxide in the process of photosynthesis. Sugars, starches, chitin, and cellulose are carbohydrates.

A catalyst is a substance that can alter the rate of a chemical reaction without itself being permanently changed by the reaction.

The Cenozoic Era refers to the past 65 million years of the earth's history; the term means "recent life."

The centripetal force on an object moving in a curved path is the inward force that must be exerted to produce this motion. It always acts toward the center of curvature of the object's path.

Charles's law states that the volume of a gas is proportional to its absolute temperature when the pressure is held constant.

Chlorophyll is the catalyst that makes possible the reaction of water and carbon dioxide in green plants to produce carbohydrates. The reaction is called photosynthesis.

Cleavage is the tendency of a substance to split along certain planes determined by the arrangement of particles in its crystal lattice.

Climate refers to average weather conditions in a region over a period of years.

A cloud is composed of water droplets or ice crystals small enough to remain suspended aloft.

Coal is a fuel, largely carbon, that was formed chiefly from plant material that accumulated under conditions preventing complete decay.

Combustion is the rapid combination of oxygen with another substance during which heat and light are given off.

Comets are objects, largely ice, dust, and frozen gases, that follow regular orbits in the solar system and appear as small, hazy patches of light accompanied by long, filmy tails when near the sun.

A compound is a homogeneous combination of elements in definite proportions. The properties of a compound are different from those of its constituent elements.

A conductor is a substance through which electric current can flow readily.

A constellation is a group of stars whose arrangement in the sky suggests a particular pattern.

According to the theory of continental drift, today's continents were once part of two primeval supercontinents called Laurasia (North America, Greenland, Europe, and most of Asia) and Gondwanaland (South America, Africa, Antarctica, India, and Australia), which were separated by the Tethys Sea. The continents are still in motion.

Convection currents result from the uneven heating of a fluid. The warmer parts of the fluid expand and rise because of their buoyancy, while the cooler parts sink.

In the copernican system the earth and the other planets revolve around the sun, the moon revolves around the earth, and the earth rotates on an axis passing through Polaris.

The earth's core is a spherical region around the center of the earth whose radius is about 3500 km. It probably consists mainly of molten iron and has a small solid inner core.

The coriolis effect is the deflection of winds to the right in the northern hemisphere, to the left in the southern as a consequence of the earth's rotation.

The sun's corona is a vast cloud of extremely hot, rarefied gas that surrounds the sun. It is visible during solar eclipses.

Cosmic rays are atomic nuclei, mostly protons, that travel through the galaxy at speeds close to that of light. They probably were ejected during supernova explosions and are trapped in the galaxy by magnetic fields.

Coulomb's law states that the force between two electric charges is directly proportional to both charges and inversely proportional to the distance between them. The force is repulsive when the charges have the same sign, attractive when they have different signs.

In a covalent bond between atoms, the atoms share one or more electron pairs. The atoms in a molecule are held together by covalent bonds. A covalent crystal consists of atoms that share electron pairs with their neighbors.

The crust of the earth is its outer shell of rock. The crust averages about 35 km in thickness under the continents and about 5 km under the oceans.

A crystalline solid is one whose atoms or molecules are arranged in a definite pattern.

An electric current is a flow of charge from one place to another. A direct current is one that always flows in one direction; an alternating current periodically reverses its direction of flow. The unit of current is the ampere (A), which is equal to the flow of 1 coulomb per second (C/s).

Cyclones are weather systems centered about regions of low pressure. In the northern hemisphere cyclones are characterized by couterclockwise winds, in the southern hemisphere by clockwise winds. Anticyclones are weather systems centered about regions of high pressure; the characteristic winds of anticyclones are opposite in direction to those of cyclones.

The matter waves that correspond to a moving object have a de Broglie wavelength inversely proportional to its momentum.

The law of definite proportions states that the elements that make up a chemical compound are always combined in the same definite proportions by mass.

A delta is a deposit of sediments where a stream flows into a lake or the sea.

The density of a substance is its mass per unit volume.

Diffraction refers to the ability of waves to bend around the edge of an obstacle in their path.

A dike is a wall-like mass of igneous rock that cuts across existing rock layers.

An ionic compound dissociates into free ions when dissolved in water.

The doppler effect is the change in perceived frequency of a wave due to relative motion between its source and an observer. Light and sound waves exhibit the doppler effect.

An earthquake consists of rapid vibratory motions of rock near the earth's surface usually caused by the sudden movement of rock along a fault. Earthquake P (for primary) waves are longitudinal oscillations in the solid earth like those in sound waves; earthquakes S (for secondary) waves are transverse oscillations in the earth like those in a stretched string; earthquake surface waves are oscillations of the earth's surface.

A lunar eclipse occurs when the earth's shadow obscures the moon. A solar eclipse occurs when the moon obscures the sun.

Electric charge is a fundamental property of certain of the elementary particles of which all matter is composed. There are two kinds of charge, positive and negative; charges of like sign repel, unlike charges attract. The unit of charge is the coulomb (C). The principle of conservation of charge states that the net electric charge in an isolated system remains constant.

In an electrochemical cell such as a battery, electric current is produced by an oxidation-reduction reaction whose two half-reactions take place at different locations.

Electrolysis refers to the liberation of free elements from a liquid when an electric current passes through it.

An electrolyte is a substance that separates into free ions when dissolved in water.

An electromagnet is a current-carrying coil of wire with an iron core to increase its magnetic field.

Electromagnetic induction refers to the production of a current in a wire when there is relative motion between the wire and a magnetic field.

Electromagnetic waves are coupled periodic electric and magnetic disturbances that spread out from accelerated electric charges. Gamma rays, e-rays, ultraviolet radiation, visible light, infrared radiation, millimeter waves, microwaves, and radio waves are all electromagnetic waves that differ only in their frequencies. Electromagnetic waves all travel in a vacuum with the speed of light.

An electron is a tiny, negatively charged particle found in matter.

The electronvolt (eV) is a unit of energy equal to 1.6 x 10^-19 J, which is the amount of energy acquired by an electron accelerated by a potential difference of 1 volt. A MeVis 10⁶ eV and a GeV is 10⁹ eV.

An element is a substance all of whose atoms have the same number of protons in their nuclei. Elements can neither be decomposed nor transformed into one another by ordinary chemical or physical means. There is a limited number of elements, and all other substances are combinations of them in various proportions.

An endothermic reaction is one that must be supplied with energy in order to occur.

Energy is the property something has that enables it to do work. The unit of energy is the joule (J). The basic forms of energy are kinetic energy, potential energy, and rest energy. The law of conservation of energy states that energy can be neither created nor destroyed, although it may change from one form to another (including mass).

Entropy is a measure of the disorder of the particles that make up a body of matter. In a system of any kind isolated from the rest of the universe, entropy cannot decrease.

A chemical equation expresses the result of a chemical change. When the equation is balanced, the number of each kind of atom is the same on both sides of the equation.

A chemical equilibrium occurs when a chemical reaction and its reverse reaction both take place at the same rate.

At the vernal and autumnal equinoxes the sun is directly overhead at noon at the equator. The periods of daylight and darkness on these days are the same everywhere on the earth.

Erosion refers to all processes by which rock is disintegrated and worn away and its debris removed.

The escape speed is the minimum speed an object needs to permanently escape from the gravitational pull of a particular astronomical body.

An ester is an organic compound formed when an alcohol reacts with an acid.

The Pauli exclusion principle states that no more than one electron in an atom can have a particular set of quantum numbers.

An exothermic reaction is one that liberates energy.

The expanding universe refers to the fact that all of the galaxies visible to us seem to be rapidly moving apart from one another. The evidence for the recession is the observed red shift in galactic spectra, which is a shift of their spectral lines toward the red end of the spectrum that is interpreted as a doppler effect.

A fault is a rock fracture along which movement has occurred.

The ferromagnesian minerals are a group of minerals that are silicates of iron and magnesium and green to black in color.

A field of force is a region of altered space (for example, around a mass, an electric charge, a magnet) that exerts a force on appropriate objects in that region.

A flood plain is the wide, flat floor of a river valley produced by the sidewise cutting of the stream when its slope has become too gradual for further downcutting.

Foliation refers to the alignment of flat or elongated mineral grains characteristic of many metamorphic rocks.

A force is any influence that can cause an object to be accelerated. The unit of force is the newton (N).

Fossils are the remains or traces of organisms preserved in rocks.

The frequency of a wave is the number of waves that pass a given point per second. The unit of frequency is the hertz (Hz). The frequency of a sound wave is its pitch.

A frontal surface is the surface separating a warm and a cold air mass; a front is where this surface touches the ground. A cold front generally involves a cold air mass moving approximately eastward. A warm front generally involves a warm air mass moving approximately eastward.

A functional group is a group of atoms in an organic molecule whose presence determines its chemical behavior to a large extent. An example is the OH group that characterizes the alcohols.

The four fundamental interactions, in order of decreasing strength, are the strong, electromagnetic, weak, and gravitational. They give rise to all the physical processes in the universe. The electromagnetic and weak interactions, and probably the strong as well, are closely related.

Our galaxy is a huge, rotating, disk-shaped group of stars that we see as the Milky Way from our location about two thirds of the way out from the center. Spiral galaxies are other collections of stars that resemble our galaxy. The universe apparently consists of widely separated galaxies of stars.

Gamma rays are very high frequency electromagnetic waves.

A glacier is a large mass of ice formed from compacted snow that gradually moves downhill.

Globular clusters are roughly spherical assemblies of very old stars outside the plane of our galaxy but associated with it.

Gondwanaland was the ancient supercontinent which later split up to form what are today South America, Africa, Antarctica, India, and Australia.

Newton's law of gravity states that every particle in the universe attracts every other particle with a force that is directly proportional to both of their and inversely proportional to the square of the distance between them.

The greenhouse effect refers to the process by which a greenhouse is heated: sunlight can enter through its windows, but the infrared radiation the warm interior gives off cannot penetrate glass, so the incoming energy is trapped. The earth's atmosphere is heated in a similar way, with its carbon dioxide and water vapor content absorbing infrared radiation from the warm earth.

Groundwater is rainwater that has soaked into the ground. The water table is the upper surface of the part of the ground whose pore spaces are saturated with water. A spring is a channel through which groundwater comes to the surface.

The half-life of a radioactive nuclide is the period of time required for one-half of an original sample to decay.

The halogens are a family of highly active nonmetals with similar chemical properties. The halogens are fluorine, chlorine, bromine, iodine, and astatine in order of atomic number.

Heat is energy of random molecular motion. The heat that a body possesses depends upon its temperature, its mass, and the kind of material of which it is composed. The unit of heat is the joule (J).

The heat of fusion of a substance is the amount of heat needed to change 1 kg of the substance from solid to liquid at its freezing point. It is also the heat that 1 kg of the substances give up when it changes from vapor to liquid at its boiling point.

The heat of vaporization of a substance is the amount of heat needed to change 1 kg of the substance from liquid to vapor at its boiling point. It is also the heat that 1 kg of the substance gives up when it changes from vapor to liquid at its boiling point.

The Hertzsprung-Russell diagram is a graph on which are plotted the intrinsic brightness and spectral type of individual stars. Most stars belong to the main sequence, which follows a diagonal line on the diagram, but there are also small, hot white dwarf stars and large cool red giant stars occupying other positions not on the main sequence.

Humidity refers to the moisture content of air. Relative humidity is the ratio between the amount of moisture in a volume of air and the maximum amount of moisture that volume of air can hold when completely saturated. It is usually expressed as a percentage.

A hydrocarbon is an organic compound containing only carbon and hydrogen. An unsaturated hydrocarbon is one whose molecules contain more than one bond (that is, shared electron pair) between adjacent carbon atoms. In a saturated hydrocarbon there is only one bond between adjacent carbon atoms.

The hydronium ion consists of a water molecule with a hydrogen ion attached. Its symbol is H₃O⁺. (Sometimes more than one water molecule may be attached to a single hydrogen ion; the notion of the hydronium ion is for convenience only.)

A hypothesis is an interpretation of scientific results as first proposed. After thorough checking, it becomes a law if it states a regularity or relationship, or a theory if it uses general considerations to account for specific phenomena. (These terms are often used in senses slightly different from the ones indicated, but the definitions stated here correspond to their usual meanings.)

The ice ages were time of severe cold in which ice sheets covered much of Europe and North America.

An ideal gas is one that obeys the formula pV/T = constant, which is a combination of Boyle's and Charles's laws. The behavior of actual gases corresponds approximately to that of an ideal gas.

Igneous rocks are formed from a molten state by cooling.

The inert gases are a family of almost totally inactive elements consisting of helium, neon, argon, krypton, xenon, and radon in order of atomic number.

Inertia is the apparent resistance an object offers to any change in its state of rest or of motion.

Insolation is an acronym for incoming solar radiation. Insolation is the energy that powers weather phenomena.

An insulator is a substance through which electric charges can move only with difficulty.

Interference occurs when two or more waves of the same kind pass the same point in space at the same time. If the waves are "in step" with each other, their amplitudes add together to produce a strong wave; this situation is called constructive interference. If the waves are "out of step" with each other, their amplitudes tend to cancel out and the resulting wave is weaker; this situation is called destructive interference.

Intrusive rocks are igneous rocks that flowed into regions below the surface already occupied by other rocks and gradually hardened there.

An ion is an atom or molecule that has an electric charge because it has gained or lost one or more electrons. The process of forming ions called ionization.

In an ionic bond, electrons are transferred from one atom to another and the resulting ions then attract each other. An ionic crystal consists of individual ions in an array in which attractive and repulsive forces balance out.

The ionosphere is a region in the upper atmosphere that contains layers of ions. The ionosphere reflects radio waves and so makes possible long-range radio communication.

Isomers are compounds whose molecules contain the same atoms but in different arrangements.

The isotopes of an element have atoms with the same atomic number but different atomic masses; their nuclei contain the same number of protons but different numbers of neutrons.

Kepler's laws of planetary motion state that (1) the paths of the planets around the sun are ellipses, (2) the planets move so that their radius vectors sweep out equal areas in equal times, and (3) the ratio between the square of the time required by a planet to make a complete revolution around the sun and the cube of its average distance from the sun is a constant for all the planets.

Kinetic energy is the energy a body has by virtue of its motion. The kinetic energy of a moving body is equal to _ mv², one-half the product of its mass and the square of its speed.

According to the kinetic theory of matter, all matter consists of tiny individual molecules that are in constant random motion. Heat is the kinetic energy of these random molecular motions.

A laser is a device that produces an intense beam of monochromatic, coherent light from the cooperative radiation of excited atoms. (The waves in a coherent beam are all in step with one another)

The latitude of a place on the earth's surface is the angular distance of the place north or south of the equator. The equator is a circle around the earth midway between its poles.

Laurasia was the ancient supercontinent which later split up into what are today the continents North America, Greenland, Europe, and most of Asia.

Lava is molten rock on the earth's surface.

The left-hand rule states that when a current-carrying wire is grasped so that the thumb of the left hand points in the direction in which the electrons move, the fingers of that hand point in the direction of the magnetic field around the wire.

A lens is a piece of glass or other transparent material shaped to produce an image by refracting light that comes from an object. A converging lens brings parallel light to a single point at a distance called the focal length of the lens. A diverging lens spreads out parallel light so that it seems to come from a point behind the lens.

A light-year is the distance light travels in 1 year; it is about 9.46 x 10¹² km.

Lines of force are imaginary lines used to describe a field of force. They are closest together where the field is strongest, farthest apart where the field is weakest.

Fats, oils, waxes, and sterols are lipids, which are synthesized in plants and animals from carbohydrates.

The lithosphere is the earth's outer shell of rigid rock. It consists of the crust and the outermost part of the mantle.

The longitude of a place on the earth's surface is the angular distance of the place east or west of the prime meridian. The prime meridian is a semicircle on the earth's surface from the north to the south pole that passes through Greenwich, England.

Magma is molten rock below the earth's surface.

A magnet is a body that can attract iron objects. When freely suspended, the north pole of a magnet points north while its south pole points south. Like poles repel each other; unlike poles attract.

A magnetic field is present around every moving charge and electric current. A magnetic field exerts a sideways force on any other moving charge or electric current in its presence.

Magnetic forces are exerted by moving charges (such as electric currents) on one another. The magnetic force between two moving charges is a modification of the electric force between them due to their motion; it is a relativistic effect.

The earth's mantle is the solid part of the earth between the core and the crust; it is about 2900 km thick.

The mass of a body is the property of matter that manifests itself as inertia. It may be thought of as the quantity of matter in the body. The unit of mass is the kilogram (kg).

A matter wave is associated with rapidly moving objects, whose behavior in certain respects resembles wave behavior. The matter waves associated with a moving object are in the form of a group, or packet, of waves which travel with the same speed as the object. The quantity that varies in a mater wave is called the object's wave function, and its square is the object's probability density. The greater the probability density at a certain place and time, the greater the likelihood of finding the object there at that time.

The Mesozoic Era is the period in the earth's history from 225 million years ago to 65 million years ago; the term means "intermediate life."

The metallic bond that holds metal atoms together arises from a "gas" of electrons that can move freely through a solid metal. These electrons are also repsonsible for the ability of metals to conduct heat and electricity well.

Metals possess a characteristic sheen (metallic luster) and are good conductors of heat and electricity. They combine with nonmetals more readily than with one another. A metal atom has one or several electrons outside closed shells or subshells and combines chemically by losing these electrons to nonmetal atoms.

Metamorphic rocks are rocks that have been altered by heat and pressure deep under the earth's surface.

Meteoroids are pieces of matter moving through the solar system; meteors are the flashes of light they produce when entering the earth's atmosphere; and meteorites are the remains of meteoroids that reach the ground.

Minerals are the separate homogeneous substances of which rocks are composed. The most abundant minerals are silicates; also common are carbonates and oxides.

A molecule is an electrically neutral combination of two or more atoms held together strongly enough to be experimentally observable as a particle.

The linear momentum of a body is the product of its mass and its velocity. The law of conservation of momentum states that when several objects interact with one another (for instance, in an explosion or a collision), if outside forces do not act upon them, the total momentum of all the objects before they interact is exactly the same as their total momentum afterward. (See also angular momentum.)

A moraine is the pile of debris around the end of a glacier left as a low, hummocky ridge when the glacier melts back. The deposited material is called till.

Newton's first law of motion states that every object continues in its state of rest or of uniform motion in a straight line if no force acts upon it. The second law of motion states that when a net force F acts on an object of mass m, the object is given an acceleration of F/m in the same direction as that of the force. The third law of motion states that when an object exerts a force on another object, the second object exerts an equal but opposite force on the first. Thus for every action force there is an equal but opposite reaction force; the two forces act on different objects.

The neutralization of an acid by a base in water solution is a reaction between hydrogen and hydroxide ions to form water.

The neutron is an electrically neutral elementary particle whose mass is slightly more than that of the proton. Neutrons are present in all atomic nuclei except that of ordinary hydrogen.

A neutron star is an extremely small star, smaller than a white dwarf, which is composed almost entirely of neutrons.

Nonmetals have an extreme range of physical properties; in the solid state they are usually lusterless and brittle and are poor conductors of heat and electricity. Some nonmetals form no compounds whatever; the others combine more readily with active metals than with one another. A nonmetal atom lacks one or several electrons of having closed shells or subshells and combines chemically by picking up electrons from metal atoms or by sharing electrons with other nonmetal atoms.

Nuclear fission occurs when a large nucleus splits into two or more smaller nuclei Considerable energy is given off each time fission occurs.

Nuclear fusion occurs when two small nuclei unite to form a large one. Considerable energy is given off each time fusion occurs.

A nuclear reactor is a device in which fissions occur at a controlled rate.

Nucleic acid molecules consist of long chains of nucleotides whose precise sequence governs the structure and functioning of cells and organisms. DNA has the form of a double helix and carries the genetic code. One form of the simpler RNA acts as a messenger in protein synthesis.

The nucleus of an atom is its small, heavy core, containing all the atom's positive charge and most of its mass. The nucleus of ordinary hydrogen consists of a single proton; nuclei of other atoms consist of protons and neutrons.

All the atoms of a nuclide have the same atomic and mass numbers.

Ohm's law states that the current in a metallic conductor is equal to the potential difference between the ends of the conductor divided by its resistance; symbolically, I = V/R.

Organic chemistry refers to the chemistry of carbon compounds.

Oxidation is the chemical combination of a substance with oxygen. More generally, an element is oxidized when its atoms lose electrons and reduced when its atoms gain electrons.

Ozone is a form of oxygen whose molecules consist of three oxygen atoms each. Ozone is present in the upper atmosphere where it absorbs solar ultraviolet radiation.

Pair production is the materialization of a particle-antiparticle pair (for instance, an electron and a positron) when a sufficiently energetic gamma ray passes near an atomic nucleus.

The Paleozoic Era is the period in the earth's history from 570 million years ago to 225 million years ago; the term means "ancient life."

The parallax of a star is the apparent shift in its position relative to more distant stars as the earth revolves in its orbit.

Mendeleev's periodic law in modern form states that if the elements are listed in the order of their atomic numbers, elements with similar properties recur at regular intervals. Such similar elements make up groups; the halogens are an example. A tabular arrangement of the elements showing this recurrence of properties is called a periodic table.

Petroleum is a naturally occurring liquid mixture of hydrocarbons believed to have originated from plant and animal matter buried long ago.

The pH of a solution expresses its degree of acidity or basicity in terms of its hydrogen-ion concentration. A pH of 7 signifies a neutral solution, a smaller pH than 7 signifies an acidic solution, and a higher pH than 7 signifies a basic solution.

The phases of the moon occur because the area of the illuminated side of the moon visible to us varies with the position of the moon in its orbit.

The photoelectric effect is the emission of electrons from a metal surface when light shines on it.

Electromagnetic waves transport energy in tiny bursts called photons that resemble particles in many respects. The energy of a photon is related to the frequency f of the corresponding wave by E = hf, where h is Planck's constant.

Photosynthesis is the reaction between water and carbon dioxide to produce carbohydrates that occurs in green plants with the help of sunlight and the catalyst chlorophyll.

A planet is a satellite of the sun that appears in the sky as a bright object whose position changes relative to the stars.

According to the theory of plate tectonics, the lithosphere of the earth is divided into seven huge plates and a number of smaller ones. Continental drift occurs because the plates can move relative to one another.

A pluton is a body of intrusive rock formed by the solidification of magma under the surface.

In a polar covalent compound one part of each molecule is relatively negative and another part relatively positive.

A polar molecule is one that behaves as if it were negatively charged at one end and positively charged at the other. A polar liquid is a liquid whose molecules are polar whereas a nonpolar liquid has molecules whose charge is symmetrically arranged.

A polymer is a long chain of simple molecules, called monomers, that have joined together, usually under the influence of heat and catalysts.

A positron is a positively charged electron.

The potential difference between two points is the work needed to take a positive electric charge of 1 C from one of the points to the other. The unit of potential difference is the volt (V), equal to 1 J/C.

Potential energy is the energy an object has by virtue of its position. The gravitational potential energy of an object of mass m that is a height h above some reference level is PE = mgh, where g is the acceleration of gravity. Since mg is the object's weight w, an alternative formula is PE = wh.

Power is the rate at which work is being done. The unit of power is the watt (W), equal to 1 J/s.

Precambrian time is the name given to the period in the earth's history preceding the Paleozoic Era, which began 570 million years ago.

A precipitate is an insoluble solid that forms as a result of a chemical reaction in solution.

The pressure on a surface is the perpendicular force per unit area that acts on it. The unit of pressure is the pascal (Pa), equal to 1 N/m². Another common pressure unit is the bar, equal to 10⁵ Pa, which is very close to atmospheric pressure.

Proteins are the chief constituents of living matter and consist of long chains of amino acid molecules. The sequence of amino acids in a protein molecule together with the form of the molecule determines its biological role.

The proton is a positively charged elementary particle found in all atomic nuclei.

The ptolemaic system is an incorrect hypothesis of the astronomical universe in which the earth is at the center with all of the other celestial bodies revolving around it in more or less complex orbits.

A pulsar is a star that emits extremely regular flashes of light and radio waves. Pulsars are believed to be neutron stars that are spinning rapidly.

Quantum mechanics is a theory of atomic phenomena based on the wave nature of moving things. Newtonian mechanics is an approximate version of quantum mechanics valid only on a relatively large scale of size.

Four quantum numbers are needed to specify completely the physical state of an atomic electron. These are the total quantum number, n, which governs the energy of the electron; the orbital quantum number, l, which determines the magnitude of the electron's angular momentum; the magnetic quantum number, m_l, which determines the direction of the electron's angular momentum; and the spin magnetic quantum number, m_s, which determines the orientation of the electron's spin.

According to the quantum theory of light, light travels in tiny bursts (or quanta) of energy called photons. The higher the frequency of the light, the more energy each of its photons has.

A quasar is a distant celestial object that emits both light and radio waves strongly and that may be powered by a super-massive black hole at its center.

A radio telescope is a directional antenna connected to a sensitive radio receiver. Radio waves from space are produced by extremely hot gases, by fast electrons that move in magnetic fields, and by atoms and molecules excited to radiate.

In radioactive decay, certain atomic nuclei spontaneously emit alpha particles (helium nuclei), beta particles (electrons or positrons), or gamma rays (high-frequency electromagnetic waves).

Radiocarbon dating is a procedure for establishing the approximate age of once-living matter on the basis of the relative amount of radioactive carbon it contains.

Reflection occurs when a wave bounces off a surface. Light is reflected by a mirror, sound by a wall.

Refraction occurs when a wave changes direction on passing from one medium to another in which its speed is different.

Einstein's general theory of relativity interprets gravity as a warping of space and time; this theory correctly predicted that the path of a light ray is bent when it passes through a gravitational field. The special theory of relativity deals with situations in which there is relative motion between an observer and what is being observed. Compared with their measurements when there is no relative motion, lengths are shorter, time intervals are longer, and masses are greater. Special relativity accounts for the relationships between electricity and magnetism. Another conclusion of special relativity is that nothing can travel faster than the speed of light.

The rest energy of an object is the energy it possesses by virtue of its mass alone. An object's rest energy is given by the product of its mass measured when it is at rest and the square of the speed of light, namely m₀c².

A salt is one of a class of ionic compounds most of which are crystalline solids at ordinary temperatures and most of which consist of a metal combined with one or more nonmetals. Any salt can be formed by mixing the appropriate acid and base and evaporating the solution to dryness.

A satellite is, in general, an astronomical body revolving about some other body. Usually the term refers to the satellites of the planets; thus the moon is a satellite of the earth.

A scalar quantity has magnitude only. Mass and speed are examples.

The scientific method of studying nature can be thought of as consisting of four steps; (1) formulating a problem; (2) observation and experiment; (3) interpretation of the results; (4) testing the interpretation by further observation and experiment.

Sedimentary rocks consist of materials derived from other rocks that have been decomposed by water, wind, or glacial ice. They may be fragments cemented together or material precipitated from water solution.

A seismograph is a sensitive instrument designed to detect earthquake waves.

An electron shell in an atom consists of all the electrons having the same principal quantum number n. When a particular shell contains all the electrons possible it is called a closed shell. An electron subshell in an atom consists of all the electrons having both the same principal quantum number n and the same orbital quantum number l. When a particular subshell contains all the electrons possible it is called a closed subshell. Closed shells and subshells are exceptionally stable.

A sill is a sheetlike mass of igneous rock lying parallel to preexisting rock layers.

Soil is a mixture of rock fragments, clay minerals, and organic matter. The organic matter consists largely of partially decomposed plant debris called humus.

A solar prominence is a large sheet of luminous gas projecting from the solar surface.

The summer solstice occurs when the north pole is tilted toward the sun; the period of daylight in the northern hemisphere is longest on this day. The winter solstice occurs when the north pole is tilted away from the sun; the period of daylight in the northern hemisphere is shortest on this day.

A solution is a homogenous mixture of elements or compounds without any fixed proportions. The substance present in larger amount is the solvent, the other is the solute. When a solid or gas is dissolved in a liquid, the liquid is always considered the solvent. The solubility of a substance is the maximum amount that can be dissolved in a given quantity of solvent at a given temperature.

A saturated solution is one that contains the maximum amount of solute that can be absorbed at a given temperature.

A spectrum is the band of different colors produced when a light beam passes through a glass prism or is diffracted by a device called a grating. An emission spectrum is one produced by a light source alone. It may be a continuous spectrum with all colors present, or a bright-line spectrum, in which only a few specific frequencies characteristic of the source appear. An absorption spectrum is one produced when light from a glowing source passes through a cool gas. It is also called a dark-line spectrum because it appears as a continuous band of colors crossed by dark lines corresponding to characteristic frequencies absorbed by the gas.

The speed of a moving object is the rate at which it covers distance. The unit of speed is the m/s. The terminal speed of a falling body is the speed at which its weight is balanced by the force of air resistance, so it cannot fall any faster. This limit arises because air resistance increases with speed.

The spin of an elementary particle such as an electron refers to its intrinsic angular momentum, which is the same for all particles of a given kind. Electron spin plays an important part in atomic and molecule structures.

A star is a large, self-luminous body of gas held together gravitationally and obtaining its energy from nuclear fusion reactions in its interior.

Stellar brightness. The apparent brightness of a star is its brightness as seen from the earth; the intrinsic brightness of a star is a measure of the total amount of light it radiates into space.

The structural formula of a compound is a diagram that show the bonds between the atoms in its molecules.

A sunspot is a dark marking on the solar surface. Sunspots range up to some thousands of km across, last from several days to over a month, and have temperatures as much as 1000°C cooler than the rest of the solar surface. The sunspot cycle is a regular variation in the number and size of sunspots whose period is about 11 years.

Superconductivity refers to the loss of all electrical resistance by certain materials at very low temperatures.

A supernova is a heavy star that explodes with spectacular brightness and emits vast amounts of material into space.

Temperature is that property of a body of matter that gives rise to sensations of hot and cold; it is a measure of average molecular kinetic energy

The first law of thermodynamics is the law of conservation of energy. The second law of thermodynamics states that some of the heat input to a heat engine must be wasted in order for the engine to operate.

A thermometer is a device for measuring temperature. In the celsius scale, the freezing point of water is given the value 0 °C and the boiling point of water the value of 100 °C. In the fahrenheit scale these temperatures are given the values 32 °F and 212 °F.

The tides are twice-daily rises and falls of the ocean surface. They are due to the different attractive forces exerted by the moon and, to a smaller extent, the sun on different parts of the earth. Spring tides have a large range between high and low water. They occur when the sun and moon are in line with the earth and thus add together their tide-producing actions. Neap tides have small ranges and occur when the sun and moon are 90° apart relative to the earth and thus their tide-producing actions tend to cancel each other out.

Till is the unsorted material deposited by a glacier and left behind when the glacier recedes.

A transformer is a device that transfers electric energy in the form of alternating current from one coil to another by means of electromagnetic induction.

The uncertainty principle states that it is impossible at the same time to determine accurate values for the position and momentum of a particle. Hence, in dealing with electrons within atoms, all we can consider are probabilities rather than specific positions and states of motion.

An unconformity is an uneven surface separating two series of rock beds. It is a buried surface of erosion that involves at least four geologic events: formation of the older rocks; diastrophic uplift of these rocks; erosion; and the deposit of sediments on the eroded landscape.

According to the principle of uniform change, the geologic processes that shaped the earth's surface in the past are the same as those active today.

Van der Waals forces arise from the electrical attraction between nonuniform charge distributions in atoms and molecules. They enable atoms and molecules to form solids without sharing or transferring electrons.

A variable star is one whose brightness changes regularly.

A vector is an arrowed line whose length is proportional to the magnitude of some quantity and whose direction is that of the quantity. A vector quantity is a quantity that has both magnitude and direction.

Veins consist of minerals deposited from solutions along cracks in rocks.

The velocity of an object specifies both its speed and the direction of its motion. A car has a speed of 30 m/s; its velocity is 30 m/s to the northwest.

Viscosity is the resistance of fluids to flowing motion; liquids are more viscous than gases.

A volcano is an opening in the earth's crust through which molten rock (called magma while underground, lava above ground) comes out, usually at intervals rather than continuously.

The wavelength of a wave is the distance between adjacent crests, in the case of transverse waves, or between adjacent compressions, in the case of longitudinal waves.

Waves carry energy from one place to another by a series of periodic motions of the individual particles of the medium in which the waves occur. (Electromagnetic waves are an exception.) There is no net transfer of matter in wave motion. In a longitudinal wave the particles of the medium vibrate back and forth in the direction in which the waves travel. In a transverse wave the particles vibrate from side to side perpendicular to the wave direction. Sound waves are longitudinal; waves in a stretched string are transverse; water waves are a combination of both since water molecules move in circular orbits when a wave passes.

Weather refers to the state of the atmosphere in a particular place at a particular time, whereas climate refers to the weather trends in a region through the years.

Weathering is the surface disintegration of rock by chemical decay and mechanical processes such as the freezing of water in crevices.

The weight of an object is the force with which gravity pulls it toward the earth. The weight of an object of mass m is mg, where g is the acceleration of gravity.

Work is a measure of the change, in a general sense, a force causes when it acts upon something. The work done by a force acting on an object is the product of the magnitude of the force and the distance through which the object moves while the force acts on it. If the direction of the force is not the same as the direction of motion, the projection of the force in the direction of motion must be used. The unit of work is the joule (J).

X-rays are high-frequency electromagnetic waves given off when matter is struck by fast electrons.