Words to Know

A

Abscission:
The point at which a leaf meets a twig.
Acceleration:
The rate at which the velocity and/or direction of an object is changing with the respect to time.
Acid:
Substance that when dissolved in water is capable of reacting with a base to form salts and release hydrogen ions.
Acid rain:
A form of precipitation that is significantly more acidic than neutral water, often produced as the result of industrial processes.
Acoustics:
The science concerned with the production, properties, and propagation of sound waves.
Active solar energy system:
A solar energy system that uses pumps or fans to circulate heat captured from the Sun.
Adhesion:
Attraction between two different substances.
Aeration:
Shaking a liquid to allow trapped gases to escape and to add oxygen.
Aerobic:
Requiring oxygen.
Aerodynamics:
The study of the motion of gases (particularly air) and the motion and control of objects in the air.
Alga/Algae:
Single-celled or multicellular plants or plantlike organisms that contain chlorophyll, thus making their own food by photosynthesis. Algae grow mainly in water.
Alignment:
Adjustment to a certain direction or orientation.
Alkaline:
Having a pH of more than 7.
Alloy:
A mixture of two or more metals with properties different from those metals of which it is made.
Amine:
An organic compound derived from ammonia.
Amphibians:
Animals that live on land and breathe air but return to the water to reproduce.
Amplitude:
The maximum displacement (difference between an original position and a later position) of the material that is vibrating. Amplitude can be thought of visually as the highest and lowest points of a wave.
Anaerobic:
Functioning without oxygen.
Anemometer:
A device that measures wind speed.
Animalcules:
Life forms that Anton van Leeuwenhoek named when he first saw them under his microscope; they later became known as protozoa and bacteria.
Anthocyanin:
Red pigment found in leaves, petals, stems, and other parts of a plant.
Antibody:
A protein produced by certain cells of the body as an immune (disease-fighting) response to a specific foreign antigen.
Aquifer:
Underground layer of sand, gravel, or spongy rock that collects water.
Arch:
A curved structure spanning an opening that supports a wall or other weight above the opening.
Artesian well:
A well in which water is under pressure.
Asexual reproduction:
Any reproductive process that does not involve the union of two individuals in the exchange of genetic material.
Astronomers:
Scientists who study the positions, motions, and composition of stars and other objects in the sky.
Astronomy:
The study of the physical properties of objects and matter outside Earth's atmosphere.
Atmosphere:
Layers of air that surround Earth.
Atmospheric pressure:
The pressure exerted by the atmosphere at Earth's surface due to the weight of the air.
Atom:
The smallest unit of an element, made up of protons and neutrons in a central nucleus surrounded by moving electrons.
Autotroph:
An organism that can build all the food and produce all the energy it needs with its own resources.
Auxins:
A group of plant hormones responsible for patterns of plant growth.

B

Bacteria:
Single-celled microorganisms that live in soil, water, plants, and animals that play a key role in the decaying of organic matter and the cycling of nutrients. Some are agents of disease.
Bacteriology:
The scientific study of bacteria, their characteristics, and their activities as related to medicine, industry, and agriculture.
Base:
Substance that when dissolved in water is capable of reacting with an acid to form salts and release hydrogen ions; has a pH of more than 7.
Beriberi:
A disease caused by a deficiency of thiamine and characterized by nerve and gastrointestinal disorders.
Biochemical oxygen demand (BOD5):
The amount of oxygen that microorganisms use over a five-day period in 68° Fahrenheit (20° Celsius) water to decay organic matter.
Biological variables:
Living factors such as bacteria, fungi, and animals that can affect the processes that occur in nature and in an experiment.
Biomes:
Large geographical areas with specific climates and soils, as well as distinct plant and animal communities that are interdependent.
Bond:
The force that holds two atoms together.
Botany:
The branch of biology involving the study of plant life.
Braided rivers:
Wide, shallow rivers with pebbly islands in the middle.
Buoyancy:
The tendency of a fluid to exert a lifting effect on a body immersed in it.
By-products:
Something produced in the making of something else.

C

Calibration:
Standardizing or adjusting a measuring instrument so its measurements are correct.
Capillary action:
The tendency of water to rise through a narrow tube by the force of adhesion between the water and the walls of the tube.
Carbohydrate:
A compound consisting of carbon, hydrogen, and oxygen found in plants and used as a food by humans and other animals.
Carnivore:
Meat-eating organism.
Carotene:
Yellowish-orange pigment present in most leaves.
Catalyst:
A compound that speeds up the rate of a chemical reaction without undergoing any change in its own composition.
Celestial:
Describing planets or other objects in space.
Cell:
The basic unit of a living organism; cells are structured to perform highly specialized functions.
Cell membrane:
The thin layer of tissue that surrounds a cell.
Cell theory:
The idea that all living things have one or more similar cells that carry out the same functions for the living process.
Centrifuge:
A device that rapidly spins a solution so that the heavier components will separate from the lighter ones.
Centripetal force:
Rotating force that moves towards the center or axis.
Channel:
A shallow trench carved into the ground by the pressure and movement of a river.
Chemical energy:
Energy stored in chemical bonds.
Chemical property:
A characteristic of a substance that allows it to undergo a chemical change. Chemical properties include flammability and sensitivity to light.
Chemical reaction:
Any chemical change in which at least one new substance is formed.
Chlorophyll:
A green pigment found in plants that absorbs sunlight, providing the energy used in photosynthesis, or the conversion of carbon dioxide and water to complex carbohydrates.
Chloroplasts:
Small structures in plant cells that contain chlorophyll and in which the process of photosynthesis takes place.
Chromatography:
A method for separating mixtures into their component parts (into their "ingredients," or into what makes them up).
Circuit:
The complete path of an electric current including the source of electric energy.
Cleavage:
The tendency of a mineral to split along certain planes.
Climate:
The average weather that a region experiences over a long period.
Coagulation:
The clumping together of particles in a liquid.
Cohesion:
Attraction between like substances.
Colloid:
A mixture containing particles suspended in, but not dissolved in, a dispersing medium.
Colony:
A mass of microorganisms that have been bred in a medium.
Combustion:
Any chemical reaction in which heat, and usually light, is produced. It is commonly the burning of organic substances during which oxygen from the air is used to form carbon dioxide and water vapor.
Complete metamorphosis:
Metamorphosis in which a larva becomes a pupa before changing into an adult form.
Composting:
The process in which organic compounds break down and become dark, fertile soil called humus.
Concave:
Hollowed or rounded upward, like the inside of a bowl; arched.
Concentration:
The amount of a substance present in a given volume, such as the number of molecules in a liter.
Condense/condensation:
The process by which a gas changes into a liquid.
Conduction:
The flow of heat through a solid.
Confined aquifer:
An aquifer with a layer of impermeable rock above it; the water is held under pressure.
Coniferous:
Refers to trees, such as pines and firs, that bear cones and have needle-like leaves that are not shed all at once.
Constellations:
Eighty-eight patterns of stars in the night sky.
Continental drift:
The theory that continents move apart slowly at a predictable rate.
Control experiment:
A set-up that is identical to the experiment but is not affected by the variable that will be changed during the experiment.
Convection:
The circulatory motion that occurs in a gas or liquid at a nonuniform temperature; the variation of the motion is caused by the substance's density and the action of gravity.
Convection current:
Circular movement of a fluid in response to alternating heating and cooling.
Convex:
Curved or rounded like the outside of a ball.
Corona:
The outermost atmospheric layer of the Sun.
Corrosion:
An oxidation-reduction reaction in which a metal is oxidized (reacted with oxygen) and oxygen is reduced, usually in the presence of moisture.
Cotyledon:
Seed leaves, which contain stored food for the embryo.
Crust:
The hard, outer shell of Earth that floats upon the softer, denser mantle.
Cultures:
Microorganisms growing in prepared nutrients.
Cyanobacteria:
Oxygen-producing, aquatic bacteria capable of manufacturing its own food; resembles algae.
Cycle:
Occurrence of events that take place the same time every year; a single complete vibration.
Cytology:
The branch of biology concerned with the study of cells.
Cytoplasm:
The semifluid substance inside a cell that surrounds the nucleus and the other membrane-enclosed organelles.

D

Decanting:
The process of separating a suspension by waiting for its heavier components to settle out and then pouring off the lighter ones.
Decibel (dB):
A unit of measurement for sound.
Deciduous:
Plants that lose their leaves at some season of the year, and then grow them back at another season.
Decomposition:
The breakdown of complex molecules—molecules of which dead organisms are composed—into simple nutrients that can be reutilized by living organisms.
Decomposition reaction:
A chemical reaction in which one substance is broken down into two or more substances.
Denaturization:
Altering of an enzyme so it no longer works.
Density:
The mass of a substance compared to its volume.
Density ball:
A ball with the fixed standard of 1.0 g/l, which is the exact density of pure water.
Dependent variable:
The variable in a function whose value depends on the value of another variable in the function.
Deposition:
Dropping of sediments that occurs when a river loses its energy of motion.
Desert:
A biome with a hot-to-cool climate and dry weather.
Desertification:
Transformation of arid or semiarid productive land into desert.
Dewpoint:
The point at which water vapor begins to condense.
Dicot:
Plants with a pair of embryonic seeds that appear at germination.
Diffraction:
The bending of light or another form of electromagnetic radiation as it passes through a tiny hole or around a sharp edge.
Diffraction grating:
A device consisting of a surface into which are etched very fine, closely spaced grooves that cause different wavelengths of light to reflect or refract (bend) by different amounts.
Diffusion:
Random movement of molecules that leads to a net movement of molecules from a region of high concentration to a region of low concentration.
Disinfection:
Using chemicals to kill harmful organisms.
Dissolved oxygen (DO):
Oxygen molecules that have dissolved in water.
Distillation:
The process of separating liquids from solids or from other liquids with different boiling points by a method of evaporation and condensation, so that each component in a mixture can be collected separately in its pure form.
DNA:
Abbreviation for deoxyribonucleic acid. Large, complex molecules found in nuclei of cells that carry genetic information for an organism's development.
Domain:
Small regions in an iron object that possess their own magnetic charges.
Dormancy:
A state of inactivity in an organism.
Dormant:
Describing an inactive organism.
Drought:
A prolonged period of dry weather that damages crops or prevents their growth.
Dry cell:
An electrolytic cell or battery using a non-liquid electrolyte.
Dynamic equilibrium:
A situation in which substances are moving into and out of cell walls at an equal rate.

E

Earthquake:
An unpredictable event in which masses of rock shift below Earth's surface, releasing enormous amounts of energy and sending out shock waves that sometimes cause the ground to shake dramatically.
Eclipse:
A phenomenon in which the light from a celestial body is temporarily cut off by the presence of another body.
Ecologists:
Scientists who study the interrelationship of organisms and their environments.
Ecosystem:
An ecological community, including plants, animals and microorganisms considered together with their environment.
Electric charge repulsion:
Repulsion of particles caused by a layer of negative ions surrounding each particle. The repulsion prevents coagulation and promotes the even dispersion of such particles through a mixture.
Electrical energy:
The motion of electrons within any object that conducts electricity.
Electricity:
A form of energy caused by the presence of electrical charges in matter.
Electrode:
A material that will conduct an electrical current, usually a metal; used to carry electrons into or out of an electrochemical cell.
Electrolyte:
Any substance that, when dissolved in water, conducts an electric current.
Electromagnetic spectrum:
The complete array of electromagnetic radiation, including radio waves (at the longest-wavelength end), microwaves, infrared radiation, visible light, ultraviolet radiation, X rays, and gamma rays (at the shortest-wavelength end).
Electromagnetic waves:
Radiation that has properties of both an electric and a magnetic wave and that travels through a vacuum at the speed of light.
Electromagnetism:
A form of magnetic energy produced by the flow of an electric current through a metal core. Also, the study of electric and magnetic fields and their interaction with charges and currents.
Electron:
A subatomic particle with a mass of about one atomic mass unit and a single electrical charge that orbits the nucleus of an atom.
Electroscope:
A device that determines whether an object is electrically charged.
Elevation:
Height above sea level.
Elliptical:
An orbital path that is egg-shaped or resembles an elongated circle.
Embryo:
The seed of a plant, which through germination can develop into a new plant; also, the earliest stage of animal development.
Embryonic:
The earliest stages of development.
Endothermic reaction:
A chemical reaction that absorbs energy, such as photosynthesis, the production of food by plant cells.
Energy:
The ability to cause an action or for work to be done. Also, power that can be used to perform work, such as solar energy.
Environmental variables:
Nonliving factors such as air temperature, water, pollution, and pH that can affect processes that occur in nature and in an experiment.
Enzymes:
Any of numerous complex proteins produced by living cells that act as catalysts, speeding up the rate of chemical reactions in living organisms.
Enzymology:
The science of studying enzymes.
Ephemerals:
Plants that lie dormant in dry soil for years until major rainstorms occur.
Epicenter:
The location where the seismic waves of an earthquake first appear on the surface, usually almost directly above the focus.
Equilibrium:
A process in which the rates at which various changes take place balance each other, resulting in no overall change.
Erosion:
The process by which topsoil is carried away by water, wind, or ice.
Eutrophic zone:
The upper part of the ocean where sunlight penetrates, supporting plant life such as phytoplankton.
Eutrophication:
Natural process by which a lake or other body of water becomes enriched in dissolved nutrients, spurring aquatic plant growth.
Evaporate/evaporation:
The process by which liquid changes into a gas; also, the escape of water vapor into the air, yielding only the solute.
Exothermic reaction:
A chemical reaction that releases energy, such as the burning of fuel.
Experiment:
A controlled observation.

F

Fat:
A type of lipid, or chemical compound used as a source of energy, to provide insulation, and to protect organs in an animal's body.
Fault:
A crack running through rock that is the result of tectonic forces.
Fault blocks:
Pieces of rock from Earth's crust that overlap and cause earthquakes when they press together and snap from pressure.
Filtration:
The use of a screen or filter to separate larger particles from smaller ones that can slip through the filter's openings.
Fluorescence:
Luminescence (glowing) that stops within 10 nanoseconds after an energy source has been removed.
Focal length:
The distance of a focus from the surface of a lens or concave mirror.
Focal point:
The point at which rays of light converge (come together) or from which they diverge (move apart).
Food web:
An interconnected set of all the food chains in the same ecosystem.
Force:
A physical interaction (pushing or pulling) tending to change the state of motion (velocity) of an object.
Fossil fuel:
A fuel such as coal, oil, or natural gas that is formed over millions of years from the remains of plants and animals.
Fracture:
A mineral's tendency to break into curved, rough, or jagged surfaces.
Frequency:
The rate at which vibrations take place (number of times per second the motion is repeated), given in cycles per second or in hertz (Hz). Also, the number of waves that pass a given point in a given period of time.
Front:
The front edges of moving masses of air.
Fungus:
Kingdom of various single-celled or multicellular organisms, including mushrooms, molds, yeasts, and mildews, that do not contain chlorophyll. (Plural is fungi.)
Fusion:
Combining of nuclei of two or more lighter elements into one nucleus of a heavier element; the process stars use to produce energy to support themselves against their own gravity.

G

Galaxy:
A large collection of stars and clusters of stars containing anywhere from a few million to a few trillion stars.
Gene:
A segment of a DNA (deoxyribonucleic acid) molecule contained in the nucleus of a cell that acts as a kind of code for the production of some specific protein. Genes carry instructions for the formation, functioning, and transmission of specific traits from one generation to another.
Genetic material:
Material that transfers characteristics from a parent to its offspring.
Geology:
The study of the origin, history, and structure of Earth.
Geotropism:
The tendency of roots to bend toward Earth.
Germ theory of disease:
The belief that disease is caused by germs.
Germination:
The beginning of growth of a seed.
Gibbous moon:
A phase of the Moon when more than half of its surface is lighted.
Glacier:
A large mass of ice formed from snow that has packed together and which moves slowly down a slope under its own weight.
Global warming:
Warming of Earth's atmosphere that results from an increase in the concentration of gases that store heat such as carbon dioxide.
Glucose:
Also known as blood sugar; a simple sugar broken down in cells to produce energy.
Golgi body:
Organelle that sorts, modifies, and packages molecules.
Gravity:
Force of attraction between objects, the strength of which depends on the mass of each object and the distance between them.
Greenhouse effect:
The warming of Earth's atmosphere due to water vapor, carbon dioxide, and other gases in the atmosphere that trap heat radiated from Earth's surface.
Greenhouse gases:
Gases that absorb infrared radiation and warm air before it escapes into space.
Groundwater:
Water that soaks into the ground and is stored in the small spaces between the rocks and soil.

H

Heat:
A form of energy produced by the motion of molecules that make up a substance.
Heat energy:
The energy produced when two substances that have different temperatures are combined.
Herbivore:
Plant-eating organism.
Hertz (Hz):
The unit of frequency; a measure of the number of waves that pass a given point per second of time.
Heterotrophs:
Organisms that cannot make their own food and that must, therefore, obtain their food from other organisms.
High air pressure:
An area where the air molecules are more dense.
Hormone:
A chemical produced in living cells that regulates the functions of the organism.
Humidity:
The amount of water vapor (moisture) contained in the air.
Humus:
Fragrant, spongy, nutrient-rich decayed plant or animal matter.
Hydrologic cycle:
Continual movement of water from the atmosphere to Earth's surface through precipitation and back to the atmosphere through evaporation and transpiration.
Hydrologists:
Scientists who study water and its cycle.
Hydrology:
The study of water and its cycle.
Hydrometer:
An instrument that determines the specific gravity of a liquid.
Hydrophilic:
A substance that is attracted to and readily mixes with water.
Hydrophobic:
A substance that is repelled by and does not mix with water.
Hydrotropism:
The tendency of roots to grow toward a water source.
Hypertonic solution:
A solution with a higher osmotic pressure (solute concentration) than another solution.
Hypothesis:
An idea in the form of a statement that can be tested by observation and/or experiment.
Hypotonic solution:
A solution with a lower osmotic pressure (solute concentration) than another solution.

I

Igneous rock:
Rock formed from the cooling and hardening of magma.
Immiscible:
Incapable of being mixed.
Impermeable:
Not allowing substances to pass through.
Impurities:
Chemicals or other pollutants in water.
Incomplete metamorphosis:
Metamorphosis in which a nymph form gradually becomes an adult through molting.
Independent variable:
The variable in a function that determines the final value of the function.
Indicator:
Pigments that change color when they come into contact with acidic or basic solutions.
Inertia:
The tendency of an object to continue in its state of motion.
Infrared radiation:
Electromagnetic radiation of a wavelength shorter than radio waves but longer than visible light that takes the form of heat.
Inner core:
Very dense, solid center of Earth.
Inorganic:
Not made of or coming from living things.
Insulated wire:
Electrical wire coated with a nonconducting material such as plastic.
Insulation/insulator:
A material that does not conduct heat or electricity.
Interference fringes:
Bands of color that fan around an object.
Ion:
An atom or group of atoms that carries an electrical charge—either positive or negative—as a result of losing or gaining one or more electrons.
Ionic conduction:
The flow of an electrical current by the movement of charged particles, or ions.
Isobars:
Continuous lines on a map that connect areas with the same air pressure.
Isotonic solutions:
Two solutions that have the same concentration of solute particles and therefore the same osmotic pressure.

K

Kinetic energy:
Energy of an object or system due to its motion.

L

Lactobacilli:
A strain of bacteria.
Larva:
Immature form (wormlike in insects; fishlike in amphibians) of an organism capable of surviving on its own. A larva does not resemble the parent and must go through metamorphosis, or change, to reach its adult stage.
Lava:
Molten rock that occurs at the surface of Earth, usually through volcanic eruptions.
Lens:
A piece of transparent material with two curved surfaces that bring together and focus rays of light passing through it.
Lichen:
An organism composed of a fungus and a photosynthetic organism in a symbiotic relationship.
Lift:
Upper force on the wings of an aircraft created by differences in air pressure on top of and underneath the wings.
Light-year:
Distance light travels in one year in the vacuum of space, roughly 5.9 trillion miles (9.5 trillion km).
The Local Group:
A cluster of 30 galaxies, including the Milky Way, pulled together gravitationally.
Low air pressure:
An area where the air molecules are less dense.
Lunar eclipse:
Eclipse that occurs when Earth passes between the Sun and the Moon, casting a shadow on the Moon.
Luster:
A glow of reflected light; a sheen.

M

Macroorganisms:
Visible organisms that aid in breaking down organic matter.
Magma:
Molten rock deep within Earth that consists of liquids, gases, and particles of rocks and crystals. Magma underlies areas of volcanic activity and at Earth's surface is called lava.
Magma chambers:
Pools of bubbling liquid rock that are the energy sources causing volcanoes to be active.
Magma surge:
A swell or rising wave of magma caused by the movement and friction of tectonic plates; the surge heats and melts rock, adding to the magma and its force.
Magnet:
A material that attracts other like material, especially metals.
Magnetic circuit:
A series of magnetic domains aligned in the same direction.
Magnetic field:
The space around an electric current or a magnet in which a magnetic force can be observed.
Magnetism:
A fundamental force of nature caused by the motion of electrons in an atom. Magnetism is manifested by the attraction of certain materials for iron.
Mantle:
Thick, dense layer of rock that underlies Earth's crust and overlies the core.
Manure:
The waste matter of animals.
Mass:
Measure of the total amount of matter in an object. Also, an object's quantity of matter as shown by its gravitational pull on another object.
Matter:
Anything that has mass and takes up space.
Meandering river:
A lowland river that twists and turns along its route to the sea.
Medium:
A material that carries the acoustic vibrations away from the body producing them.
Meniscus:
The curved surface of a column of liquid.
Metamorphic rock:
Rock formed by transformation of pre-existing rock through changes in temperature and pressure.
Metamorphosis:
Transformation of an immature animal into an adult.
Meteorologists:
Scientists who study weather and weather forecasting.
Microbiology:
Branch of biology dealing with microscopic forms of life.
Microclimate:
A local climate.
Microorganisms:
Living organisms so small that they can be seen only with the aid of a microscope.
Micropyle:
Seed opening that enables water to enter easily.
Milky Way:
The galaxy in which our solar system is located.
Mineral:
An inorganic substance found in nature with a definite chemical composition and structure. As a nutrient, helps build bones and soft tissues and regulates body functions.
Mixtures:
Combinations of two or more substances that are not chemically combined with each other and can exist in any proportion.
Molecule:
The smallest particle of a substance that retains all the properties of the substance and is composed of one or more atoms.
Molting:
Shedding of the outer layer of an animal, as occurs during growth of insect larvae.
Monocot:
Plants with a single embryonic seed at germination.
Moraine:
Mass of boulders, stones, and other rock debris carried along and deposited by a glacier.
Multicellular:
Living things with many cells joined together.

N

Nanometer:
A unit of length; this measurement is equal to one-billionth of a meter.
Nansen bottles:
Self-closing containers with thermometers that draw in water at different depths.
Nebula:
Bright or dark cloud, often composed of gases and dust, hovering in the space between the stars.
Neutralization:
A chemical process in which the mixing of an acidic solution with a basic (alkaline) solution results in a solution that has the properties of neither an acid nor a base.
Neutron:
A subatomic particle with a mass of about one atomic mass unit and no electrical charge that is found in the nucleus of an atom.
Niche:
The specific role that an organism carries out in its ecosystem.
Nonpoint source:
An unidentified source of pollution; may actually be a number of sources.
Nucleus:
The central core of an atom, consisting of protons and (usually) neutrons.
Nutrient:
A substance needed by an organism in order for it to survive, grow, and develop.
Nutrition:
The study of the food nutrients an organism needs in order to maintain well-being.
Nymph:
An immature form in the life cycle of insects that go through an incomplete metamorphosis.

O

Oceanography:
The study of the chemistry of the oceans, as well as their currents, marine life, and the ocean bed.
Optics:
The study of the nature of light and its properties.
Organelles:
Membrane-bounded cellular "organs" performing a specific set of functions within a eukaryotic cell.
Organic:
Made of or coming from living things.
Osmosis:
The movement of fluids and substances dissolved in liquids across a semipermeable membrane from an area of its greater concentration to an area of its lesser concentration until all substances involved reach a balance.
Outer core:
A liquid core that surrounds Earth's solid inner core; made mostly of iron.
Oxidation:
A chemical reaction in which oxygen reacts with some other substance and in which ions, atoms, or molecules lose electrons.
Oxidation-reduction reaction:
A chemical reaction in which one substance loses one or more electrons and the other substance gains one or more electrons.
Oxidation state:
The sum of an atom's positive and negative charges.
Oxidizing agent:
A chemical substance that gives up oxygen or takes on electrons from another substance.
Ozone layer:
The atmospheric layer of approximately 15 to 30 miles (24 to 48 km) above Earth's surface in which the concentration of ozone is significantly higher than in other parts of the atmosphere and that protects the lower atmosphere from harmful solar radiation.

P

Papain:
An enzyme obtained from the fruit of the papaya used as a meat tenderizer, as a drug to clean cuts and wounds, and as a digestive aid for stomach disorders.
Passive solar energy system:
A solar energy system in which the heat of the Sun is captured, used, and stored by means of the design of a building and the materials from which it is made.
Pasteurization:
The process of slow heating that kills bacteria and other microorganisms.
Penicillin:
A mold from the fungi group of microorganisms used as an antibiotic.
Pepsin:
Digestive enzyme that breaks down protein.
Percolate:
To pass through a permeable substance.
Permeable:
Having pores that permit a liquid or a gas to pass through.
pH:
Abbreviation for potential hydrogen. A measure of the acidity or alkalinity of a solution determined by the concentration of hydrogen ions present in a liter of a given fluid. The pH scale ranges from 0 (greatest concentration of hydrogen ions and therefore most acidic) to 14 (least concentration of hydrogen ions and therefore most alkaline), with 7 representing a neutral solution, such as pure water.
Pharmacology:
The science dealing with the properties, reactions, and therapeutic values of drugs.
Phases:
Changes in the illuminated Moon surfaces as the Moon revolves around Earth.
Phloem:
Plant tissue consisting of elongated cells that transport carbohydrates and other nutrients.
Phosphorescence:
Luminescence (glowing) that stops within 10 nanoseconds after an energy source has been removed.
Photoelectric effect:
The phenomenon in which light falling upon certain metals stimulates the emission of electrons and changes light into electricity.
Photosynthesis:
Chemical process by which plants containing chlorophyll use sunlight to manufacture their own food by converting carbon dioxide and water to carbohydrates, releasing oxygen as a by-product.
Phototropism:
The tendency of a plant to grow toward a source of light.
Photovoltaic cells:
A device made of silicon that converts sunlight into electricity.
Physical change:
A change in which the substance keeps its identity, such as a piece of chalk that has been ground up.
Physical property:
A characteristic that you can detect with your senses, such as color and shape.
Phytoplankton:
Microscopic aquatic plants that live suspended in the water.
Pigment:
A substance that displays a color because of the wavelengths of light that it reflects.
Pitch:
A property of a sound, determined by its frequency; the highness or lowness of a sound.
Plates:
Large regions of Earth's surface, composed of the crust and uppermost mantle, which move about, forming many of Earth's major geologic surface features.
Pnematocysts:
Stinging cells.
Point source:
An identified source of pollution.
Pollination:
The transfer of pollen from the male reproductive organs to the female reproductive organs of plants.
Pore:
An opening or space.
Potential energy:
The energy possessed by a body as a result of its position.
Precipitation:
Water in its liquid or frozen form when it falls from clouds as rain, snow, sleet, or hail.
Probe:
The terminal of a voltmeter, used to connect the voltmeter to a circuit.
Producer:
An organism that can manufacture its own food from nonliving materials and an external energy source, usually by photosynthesis.
Product:
A compound that is formed as a result of a chemical reaction.
Prominences:
Masses of glowing gas, mainly hydrogen, that rise from the Sun's surface like flames.
Propeller:
Radiating blades mounted on a quickly rotating shaft that are used to move aircraft forward.
Protein:
A complex chemical compound that consists of many amino acids attached to each other that are essential to the structure and functioning of all living cells.
Protists:
Members of the kingdom Protista, primarily single-celled organisms that are not plants or animals.
Proton:
A subatomic particle with a mass of about one atomic mass unit and a single negative electrical change that is found in the nucleus of an atom.
Protozoan:
Single-celled animal-like microscopic organisms that live by taking in food rather than making it by photosynthesis and must live in the presence of water. (Plural is protozoa.)
Pupa:
A stage in the metamorphosis of an insect during which its tissues are completely reorganized to take on their adult shape.

R

Radiation:
Energy transmitted in the form of electromagnetic waves or subatomic particles.
Radicule:
A seed's root system.
Radio wave:
Longest form of electromagnetic radiation, measuring up to 6 miles (9.6 km) from peak to peak.
Radiosonde balloons:
Instruments for collecting data in the atmosphere and then transmitting that data back to Earth by means of radio waves.
Reactant:
A compound present at the beginning of a chemical reaction.
Reaction:
Response to an action prompted by a stimulus.
Reduction:
A process in which a chemical substance gives off oxygen or takes on electrons.
Reflection:
The bouncing of light rays in a regular pattern off the surface of an object.
Refraction:
The bending of light rays as they pass at an angle from one transparent or clear medium into a second one of different density.
Rennin:
Enzyme used in making cheese.
Resistance:
A partial or complete limiting of the flow of electrical current through a material.
Respiration:
The physical process that supplies oxygen to living cells and the chemical reactions that take place inside the cells.
Resultant:
A force that results from the combined action of two other forces.
Retina:
The light-sensitive part of the eyeball that receives images and transmits visual impulses through the optic nerve to the brain.
River:
A main course of water into which many other smaller bodies of water flow.
Rock:
Naturally occurring solid mixture of minerals.
Runoff:
Water in excess of what can be absorbed by the ground.

S

Salinity:
The amount of salts dissolved in seawater.
Saturated:
Containing the maximum amount of a solute for a given amount of solvent at a certain temperature.
Scientific method:
Collecting evidence meticulously and then theorizing from it.
Scribes:
Ancient scholars.
Scurvy:
A disease caused by a deficiency of vitamin C, which causes a weakening of connective tissue in bone and muscle.
Sediment:
Sand, silt, clay, rock, gravel, mud, or other matter that has been transported by flowing water.
Sedimentary rock:
Rock formed from the compressed and solidified layers of organic or inorganic matter.
Sedimentation:
A process during which gravity pulls particles out of a liquid.
Seismic belt:
Boundaries where Earth's plates meet.
Seismic waves:
Classified as body waves or surface waves, vibrations in rock and soil that transfer the force of the earthquake from the focus (center) into the surrounding area.
Seismograph:
A device that records vibrations of the ground and within Earth.
Seismology:
The study and measurement of earthquakes.
Seismometer:
A seismograph that measures the movement of the ground.
Semipermeable membrane:
A thin barrier between two solutions that permits only certain components of the solutions, usually the solvent, to pass through.
Sexual reproduction:
A reproductive process that involves the union of two individuals in the exchange of genetic material.
Silt:
Medium-sized soil particles.
Solar collector:
A device that absorbs sunlight and collects solar heat.
Solar eclipse:
Eclipse that occurs when the Moon passes between Earth and the Sun, casting a shadow on Earth.
Solar energy:
Any form of electromagnetic radiation that is emitted by the Sun.
Solute:
The substance that is dissolved to make a solution and exists in the least amount in a solution, for example sugar in sugar water.
Solution:
A mixture of two or more substances that appears to be uniform throughout except on a molecular level.
Solvent:
The major component of a solution or the liquid in which some other component is dissolved, for example water in sugar water.
Specific gravity:
The ratio of the density of a substance to the density of another substance.
Spectrum:
Range of individual wavelengths of radiation produced when white light is broken down into its component colors when it passes through a prism or is broken apart by some other means.
Standard:
A base for comparison.
Star:
A vast clump of hydrogen gas and dust that produces great energy through fusion reactions at its core.
Static electricity:
A form of electricity produced by friction in which the electric charge does not flow in a current but stays in one place.
Streak:
The color of the dust left when a mineral is rubbed across a surface.
Substrate:
The substance on which an enzyme operates in a chemical reaction.
Succulent:
Plants that live in dry environments and have water storage tissue.
Surface water:
Water in lakes, rivers, ponds, and streams.
Suspension:
A temporary mixture of a solid in a gas or liquid from which the solid will eventually settle out.
Symbiosis:
A pattern in which two or more organisms live in close connection with each other, often to the benefit of both or all organisms.
Synthesis reaction:
A chemical reaction in which two or more substances combine to form a new substance.

T

Taiga:
A large land biome mostly dominated by coniferous trees.
Tectonic plates:
Huge flat rocks that form Earth's crust.
Temperate:
Mild or moderate weather conditions.
Temperature:
The measure of the average energy of the molecules in a substance.
Terminal:
A connection in an electric circuit; usually a connection on a source of electric energy such as a battery.
Terracing:
A series of horizontal ridges made in a hillside to reduce erosion.
Testa:
A tough outer layer that protects the embryo and endosperm of a seed from damage.
Thermal conductivity:
A number representing a material's ability to conduct heat.
Thermal energy:
Energy caused by the movement of molecules due to the transfer of heat.
Thiamine:
A vitamin of the B complex that is essential to normal metabolism and nerve function.
Thigmotropism:
The tendency for a plant to grow toward a surface it touches.
Titration:
A procedure in which an acid and a base are slowly mixed to achieve a neutral substance.
Toxic:
Poisonous.
Trace element:
A chemical element present in minute quantities.
Translucent:
Permits the passage of light.
Tropism:
The growth or movement of a plant toward or away from a stimulus.
Troposphere:
The lowest layer of Earth's atmosphere, ranging to an altitude of about 9 miles (15 km) above Earth's surface.
Tsunami:
A tidal wave caused by an earthquake.
Tuber:
An underground, starch-storing stem, such as a potato.
Tundra:
A treeless, frozen biome with low-lying plants.
Turbulence:
Air disturbance or unrest that affects an aircraft's flight.
Tyndall effect:
The effect achieved when colloidal particles reflect a beam of light, making it visible when shined through such a mixture.

U

Ultraviolet:
Electromagnetic radiation (energy) of a wavelength just shorter than the violet (shortest wavelength) end of the visible light spectrum and thus with higher energy than the visible light.
Unconfined aquifer:
An aquifer under a layer of permeable rock and soil.
Unicellular:
Living things that have one cell. Protozoans are unicellular.
Universal gravitation:
The notion of the constancy of the force of gravity between two bodies.

V

Vacuole:
A space-filling organelle of plant cells.
Variable:
Something that can change the results of an experiment.
Vegetative propagation:
A form of asexual reproduction in which plants are produced that are genetically identical to the parent.
Viable:
The capability of developing or growing under favorable conditions.
Vibration:
A regular, back-and-forth motion of molecules in the air.
Visible spectrum:
Light waves visible to the eye.
Vitamin:
A complex organic compound found naturally in plants and animals that the body needs in small amounts for normal growth and activity.
Volcano:
A conical mountain or dome of lava, ash, and cinders that forms around a vent leading to molten rock deep within Earth.
Voltage:
Also called potential difference; the amount of electric energy stored in a mass of electric charges compared to the energy stored in some other mass of charges.
Voltmeter:
An instrument for measuring the conductivity or resistance in a circuit or the voltage produced by an electric source.
Volume:
The amount of space occupied by a three-dimensional object; the amplitude or loudness of a sound.

W

Water (hydrologic) cycle:
The constant movement of water molecules on Earth as they rise into the atmosphere as water vapor, condense into droplets and fall to land or bodies of water, evaporate, and rise again.
Waterline:
The highest point to which water rises on the hull of a ship. The portion of the hull below the waterline is under water.
Water table:
The upper surface of groundwater.
Water vapor:
Water in its gaseous state.
Wave:
A motion in which energy and momentum is carried away from some source.
Wavelength:
The distance between the peak of a wave of light, heat, or energy and the next corresponding peak.
Weather:
The state of the troposphere at a particular time and place.
Weather forecasting:
The scientific predictions of future weather patterns.
Weight:
The gravitational attraction of Earth on an object; the measure of the heaviness of an object.
Wetlands:
Areas that are wet or covered with water for at least part of the year.

X

Xanthophyll:
Yellow pigment in plants.
Xerophytes:
Plants that require little water to survive.
Xylem:
Plant tissue consisting of elongated, thick-walled cells that transport water and mineral nutrients.