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accuracy

the closeness of a measurement to the actual value.

base unit

a unit that defines the standard for one of the seven physical quantities in the International System of Units (SI).

calibration

the process of correcting for systematic error of a measuring device by comparing it to a known standard.

celsius scale

a temperature scale in which the freezing and boiling points of water are defined as 0°C and 100°C

chemical change

A change in which one or more substances are converted into one or more substances with different composition and properties.

chemical property

A characteristic of a substance that appears as it interacts with, or transforms into, other substances.

chemistry

The scientific study of matter and its properties, the changes it undergoes, and the energy associated with those changes.

combustion

The process of burning in air, often with release of heat and light.

composition

The types and amounts of simpler substances that make up a sample of matter.

controlled experiment

An experiment that measures the effect of one variable at a time by keeping other variables constant.

conversion factor

A ratio of equivalent quantities that is equal to 1 and used to express a quantity in different units.

cubic meter (m³)

The derived SI unit of volume.

data

Pieces of quantitative information obtained by observation.

density (d)

An intensive physical property of a substance at a given temperature and pressure, defined as the ratio of the mass to the volume:d=m/V.

derived unit

Any of various combinations of the seven SI base units.

dimensional analysis

A calculation method in which arithmetic steps are accompanied by canceling units that represent physical dimensions.

energy

The ability to do work, that is, to move matter. [See alsokinetic energy(E_k) andpotential energy(E_p).]

exact number

A quantity, usually obtained by counting or based on a unit definition, that has no uncertainty associated with it and, therefore, contains as many significant figures as a calculation requires.

experiment

A set of procedural steps that tests a hypothesis.

extensive property

A property, such as mass, that depends on the quantity of substance present.

gas

One of the three states of matter. A gas fills its container regardless of the shape because its particles are far apart.

heat

The energy transferred between objects because of a difference in their temperatures only.

hypothesis

A testable proposal made to explain an observation. If inconsistent with experimental results, a hypothesis is revised or discarded.

intensive property

A property, such as density, that does not depend on the quantity of substance present.

Kelvin scale

The preferred temperature scale in scientific work, which has absolute zero (0 K, or −273.15°C) as the lowest temperature.

kelvin (K)

The SI base unit of temperature. The kelvin is the same size as the Celsius degree.

kilogram (kg)

The SI base unit of mass.

kinetic energy (E_k)

The energy an object has because of its motion.

liquid

One of the three states of matter. A liquid fills a container to the extent of its own volume and thus forms a surface.

liter (L)

A non-SI unit of volume equivalent to 1 cubic decimeter (0.001 m³).

mass

The quantity of matter an object contains. Balances are designed to measure mass.

matter

Anything that possesses mass and occupies volume.

natural law

A summary, often in mathematical form, of a universal observation.

observation

A fact obtained with the senses, often with the aid of instruments. Quantitative observations provide data that can be compared.

physical change

A change in which the physical form (or state) of a substance, but not its composition, is altered.

physical property

A characteristic shown by a substance itself, without interacting with or changing into other substances.

potential energy

The energy an object has as a result of its position relative to other objects or because of its composition.

precision

The closeness of a measurement to other measurements of the same phenomenon in a series of experiments.

property

A characteristic that gives a substance its unique identity.

random error

Error that occurs in all measurements (with its size depending on the measurer’s skill and the instrument’s precision) and results in valuesbothhigher and lower than the actual value.

round off

The process of removing digits based on a series of rules to obtain an answer with the proper number of significant figures (or decimal places).

scientific method

A process of creative proposals and testing aimed at objective, verifiable discoveries of the causes of natural events.

SI unit

A unit composed of one or more of the base units of the Système International d’Unités, a revised form of the metric system.

significant figures

The digits obtained in a measurement. The greater the number of significant figures, the greater the certainty of the measurement.

solid

One of the three states of matter. A solid has a fixed shape that does not conform to the container shape.

state of matter

One of the three physical forms of matter: solid, liquid, or gas.

systematic error

A type of error producing values that are all either higher or lower than the actual value, often caused by faulty equipment or a consistent flaw in technique.

temperature (T)

A measure of how hot or cold a substance is relative to another substance. A measure of the average kinetic energy of the particles in a sample.

thermometer

A device for measuring temperature that contains a fluid that expands or contracts within a graduated tube.

uncertainty

A characteristic of every measurement that results from the inexactness of the measuring device and the need to estimate when taking a reading.

variable

A quantity that can have more than a single value. (See alsocontrolled experiment.)

weight

The force that is exerted by a gravitational field on an object and is directly proportional to the object’s mass.

anion

A negatively charged ion.

atom

The smallest unit of an element that retains the chemical nature of the element. A neutral, spherical entity composed of a positively charged central nucleus surrounded by one or more negatively charged electrons.

atomic mass

The average of the masses of the naturally occurring isotopes of an element weighted according to their abundances.

binary covalent compound

A compound that consists of atoms of two elements, typically nonmetals, in which bonding occurs primarily through electron sharing.

binary ionic compound

A compound that consists of the oppositely charged ions of two elements.

cathode ray

The ray of light emitted by the cathode (negative electrode) in a gas discharge tube; travels in a straight line, unless deflected by magnetic or electric fields.

cation

A positively charged ion.

chemical bond

The force that holds two atoms together in a molecule (or formula unit).

chemical formula

A notation of atomic symbols and numerical subscripts that shows the type and number of each atom in a molecule or formula unit of a substance.

chromatography

A separation technique in which a mixture is dissolved in a fluid (gas or liquid) and the components are separated through differences in adsorption to (or solubility in) a solid surface (or viscous liquid).

compound

A substance composed of two or more elements that are chemically combined in fixed proportions.

covalent bond

A type of bond in which atoms are bonded through the sharing of electrons; the mutual attraction of the nuclei and an electron pair that holds atoms together in a molecule.

covalent compound

A compound that consists of atoms bonded together by shared electron pairs.

crystallization

A technique used to separate and purify the components of a mixture through differences in solubility, resulting in a component coming out of solution as crystals.

distillation

A separation technique in which a more volatile component of a mixture vaporizes and condenses separately from the less volatile components.

electron (e⁻)

A subatomic particle that possesses a unit negative charge (− 1.60218 × 10⁻¹⁹ C) and occupies the space around the atomic nucleus.

element

The simplest type of substance with unique physical and chemical properties. An element consists of only one kind of atom, so it cannot be broken down into simpler substances.

filtration

A method of separating the components of a mixture on the basis of differences in particle size.

formula mass

The sum (in amu) of the atomic masses of a formula unit of a (usually ionic) compound.

formula unit

The chemical unit of a compound that contains the relative numbers of the types of atoms or ions expressed in the chemical formula.

fraction by mass

The portion of a compound’s mass contributed by an element; the mass of an element in a compound divided by the mass of the compound.

heterogeneous mixture

A mixture that has one or more visible boundaries among its components.

homogeneous mixture

(alsosolution) A mixture that has no visible boundaries among its components.

hydrate

A compound in which a specific number of water molecules are associated with each formula unit.

ion

A charged particle that forms from an atom (or covalently bonded group of atoms) when it gains or loses one or more electrons.

ionic compound

A compound that consists of oppositely charged ions.

isotopes

Atoms of a given atomic number (that is, of a specific element) that have different numbers of neutrons and therefore different mass numbers.

isotopic mass

The mass (in amu) of an isotope relative to the mass of carbon-12.

law of definite (or constant) composition

A mass law stating that, no matter what its source, a particular compound is composed of the same elements in the same parts (fractions) by mass.

law of mass conservation

A mass law stating that the total mass of substances does not change during a chemical reaction.

law of multiple proportions

A mass law stating that, if elements A and B react to form two or more compounds, the different masses of B that combine with a fixed mass of A can be expressed as a ratio of small whole numbers.

mass number (A)

The total number of protons and neutrons in the nucleus of an atom.

mass spectrometry

An instrumental method for measuring the relative masses of particles in a sample by creating charged particles and separating them according to their mass/charge ratios.

metal

A substance or mixture that is relatively shiny and malleable and is a good conductor of heat and electricity. In reactions, metals tend to transfer electrons to nonmetals and form ionic compounds.

metalloid

(alsosemimetal) An element with properties between those of metals and nonmetals.

mixture

Two or more elements and/or compounds that are physically intermingled but not chemically combined.

molecular mass

(alsomolecular weight) The sum (in amu) of the atomic masses of the elements in a molecule (or formula unit) of a compound.

molecule

A structure consisting of two or more atoms that are bound chemically and behave as an independent unit.

monatomic ion

An ion derived from a single atom.

nonmetal

An element that lacks metallic properties. In reactions, nonmetals tend to share electrons with each other to form covalent compounds or accept electrons from metals to form ionic compounds.

polyatomic ion

An ion in which two or more atoms are bonded covalently.

substance

A type of matter, either an element or a compound, that has a fixed composition.

volatility

The tendency of a substance to become a gas.

actual yield

The amount of product actually obtained in a chemical reaction.

Avogadro’s number

A number (6.022 × 10²³ to four significant figures) equal to the number of atoms in exactly 12 g of carbon-12; the number of atoms, molecules, or formula units in 1 mole of an element or a compound.

empirical formula

A chemical formula that shows the lowest relative number of atoms of each element in a compound.

isomer

One of two or more compounds with the same molecular formula but different properties, often as a result of different arrangements of atoms.