Chemistry -- Terms and Definitions

chemical reaction

a chemical change resulting from a collision between atoms or molecules

endothermic reaction

a chemical reaction in which the products have a higher energy content than the reactants

exothermic reaction

a chemical reaction in which the products have a lower energy content than the reactants

scientific notation

a form for conveniently expressing very large or small numbers

substance

a form of matter that has its own unique properties which make it different from every other substance

formula

a grouping of symbols which tells what kinds of atoms compose a compound as well as the number of each kind of atom in one molecule of the compound

phase

a homogeneous part of a system which is in contact with but physically distinct from other parts of the system

plasma

a hot gas in which atoms are partially broken down to form ions

heterogeneous mixture

a mixture which is not the same throughout

the planetary model of the atom

a model for the atom consisting of a dense positively charged nucleus and mostly empty space occupied by the light electrons

exact number

a number which results from counting the number of something instead of measuring it

mass spectrum

a record of the distribution of particles in a sample according to mass

homogeneous mixture

a solution

compound

a substance which can be decomposed by chemical means into simpler substances

pure substance

a substance which has the same properties throughout and is of a definite composition

chromatography

a technique used to separate components of mixtures based on their differing solubilities or interactions with a stationary liquid or solid

kelvin

a unit of measuring temperature in which each unit kelvin is equal to one unit Celsius, but 0 K is the absolute zero

Celsius

a unit of measuring temperature in which the freezing point of water is 0 degrees and the boiling point of water is 100 degrees

significant figures

all digits that are certain in a measurement, as well as one uncertain digit

anion

an atom or molecule which has an overall negative charge

ion

an atom or molecule which has an overall negative or positive charge

cation

an atom or molecule which has an overall positive charge

mass spectrometer

an instrument used to determine the isotopes present in an element, their abundances, and their masses

quarks

any of a class of particles of which neutrons and protons and certain other particles are thought to be composed

second law of thermodynamics

any system left to itself will tend towards a condition of minimum potential energy and maximum entropy

matter

anything which occupies space and has mass

isotopes

atoms of the same element that differ in their mass numbers

physical changes

change in the physical appearance of matter but not in the identity/composition of a substance

chemical change

change in which a substance becomes a different substance with different composition/properties

molecular formula

chemical formula which states how many of each type of atom are found in a molecule of a substance

empirical formula

chemical formula which states the ratio of the various types of atoms in a compound

system of measurement

collection of compatible, related units that can be used to measure quantities such as length, mass, or volume

formula unit

empirical formula

potential energy

energy of an object due to its position

molecule

group of 2+ atoms which are chemically combine

law of multiple proportions

law, derived from Dalton's theories, which states that when two elements can combine to form more than one compound, the masses of one element that combine with a fixed amount of the other element are in a ratio of small whole numbers

law of definite composition

law, derived from John Dalton's theories, stating that the ratios of the masses of each element in a given compound are constant

density

mass per unit volume of a substance

the law of conservation of mass-energy

matter and energy can neither be created nor destroyed, only changed from one form to another (also known as the 1st law of thermodynamics)

homogeneous matter

matter which has the same composition throughout

distillation

means by which homogeneous mixtures in liquid form may be separated

weight

measure of the force of gravity upon an object

mass

measure of the quantity of matter in an object

nano-

metric prefix meaning "billionth"

centi-

metric prefix meaning "hundredth"

mega-

metric prefix meaning "million"

micro-

metric prefix meaning "millionth"

kilo-

metric prefix meaning "thousand"

milli

metric prefix meaning "thousandth"

meter

metric unit of length

second

metric unit of time

liter

metric unit of volume

physical properties

properties which describe a substance's appearance

chemical properties

properties which describe how matter reacts to change into other chemically different substances having different properties

nucleons

protons and neutrons

stoichiometry

quantitative relationships between substances involved in chemical changes

atom

smallest particle of an individual element

solid

state of matter in which objects have a definite shape and volume

liquid

state of matter in which objects have a definite volume but not a definite shape

gas

state of matter in which objects have neither a definite shape or volume

mixture

substance containing 2+ elements or compounds which are physically, and not chemically, combined

element

substance which cannot be broken down into simpler substances by chemical means

chemical formulas

symbolic representations which specify the elements present in a compound as well as the relative numbers of atoms of each element present

metric system

system of measurement based on units of ten, officially known as SI

physical chemistry

the branch of chemistry which deals with applying the theories of physics to the study of reactions and properties of matter

analytical chemistry

the branch of chemistry which deals with identifying which substances are present in materials and how much of each is present

organic chemistry

the branch of chemistry which deals with studying carbon compounds

inorganic chemistry

the branch of chemistry which deals with studying non-carbon compounds

biochemistry

the branch of chemistry which deals with the study of living organisms

Chemistry

the branch of science that deals with the study of the composition, structure, and properties of matter and the changes which matter undergoes

properties

the characteristics by which one type of matter my be described, identified, or distinguished from another

precision

the consistency or reproducability of an experiment

kinetic energy

the energy possessed by matter due to its motion

absolute zero

the lowest possible temperature; the temperature at which molecules cease to vibrate; -273.15 C

accuracy

the measure of how close a measurement is to the actual, exact value

entropy

the measure of the disorder of a system

cathode rays

the negatively charged rays produced in a cathode ray tube or gas discharge tube, composed of a beam of electrons

atomic mass number

the number of protons and neutrons in an atom's nucleus

atomic number

the number of protons in an atom's nucleus

nucleus

the small, dense, central part of an atom containing all the positive charge and virtually all of the atom's mass

atomic mass units

the standard by which the mass of an atom is measured, one unit of which being 1/12 of the mass of an atom of C-12

thermodynamics

the study of the flow of heat energy

neutrons

the subatomic, electrically neutral particles in the nucleus of an atom

electrons

the subatomic, negatively charged particles which surround the nucleus of an atom

protons

the subatomic, positively charged particles in the nucleus of an atom

boiling point

the temperature at which a liquid may be evaporated

melting point

the temperature at which a solid will change into a liquid