Chemistry Glossary
142 terms, each with a plain definition, technical definition, example and the confusion it usually causes.
A
Acid
A substance that donates hydrogen ions (protons): sour taste, pH below 7, turns blue litmu…
Activation Energy
The energy hump reactants must get over to become products, like pushing a boulder over a …
Alkali Metal
The super-reactive metals of group 1: soft, shiny, and so eager to react that they are sto…
Alkaline Earth Metal
The group 2 metals: reactive, but calmer than the alkali metals. Calcium in your bones and…
Alkane
Hydrocarbons with only single bonds: the calm, unreactive family including methane, propan…
Alkene
Hydrocarbons with at least one carbon-carbon double bond: more reactive than alkanes, and …
Alkyne
Hydrocarbons with at least one carbon-carbon triple bond: the most unsaturated family, inc…
Alpha Particle
A heavy, slow chunk shot out of decaying nuclei: 2 protons and 2 neutrons, identical to a …
Amino Acid
The building blocks of proteins: small molecules with both an amino group and an acid grou…
Anode
The electrode where oxidation happens. Remember An Ox: Anode = Oxidation.…
Aromatic Compound
Molecules with special ring structures and delocalized electrons, like benzene, that are u…
Atom
The smallest unit of an element that still keeps that element's chemical identity. Everyth…
Atomic Number
The number of protons in an atom's nucleus. It is the atom's ID: it tells you exactly whic…
ATP
The cell's energy currency: a molecule whose high-energy phosphate bonds power nearly ever…
Avogadro's Number
The number of particles in one mole: 6.022 x 10^23. It lets chemists convert between the p…
B
Base
A substance that accepts hydrogen ions (protons): bitter taste, slippery feel, pH above 7.…
Beta Particle
A fast electron (or positron) shot out of a decaying nucleus: more penetrating than alpha,…
Boyle's Law
At constant temperature, squeezing a gas into a smaller volume raises its pressure: pressu…
Buffer
A solution that resists pH changes when small amounts of acid or base are added: made from…
C
Calorimetry
Measuring heat flow in a reaction using an insulated container called a calorimeter: basic…
Carbohydrate
Sugars and starches: the body's quick energy source, built from carbon, hydrogen, and oxyg…
Catalyst
A substance that speeds up a reaction without being used up: it offers an easier path with…
Cathode
The electrode where reduction happens. Remember Red Cat: Reduction = Cathode.…
Charles's Law
At constant pressure, heating a gas makes it expand: volume is directly proportional to ab…
Chemical Equation
A shorthand recipe for a chemical reaction, showing reactants turning into products with f…
Chemical Equilibrium
The state where forward and reverse reactions run at equal speed, so concentrations stop c…
Coefficient
The large number written in front of a formula in a balanced equation. It tells how many m…
Colligative Property
Solution properties that depend only on HOW MANY solute particles are present, not what th…
Common Ion Effect
Adding an ion that is already in a dissolution equilibrium pushes the solid back out: solu…
Compound
A pure substance made of two or more different elements chemically bonded in a fixed ratio…
Conjugate Acid
What a base becomes after it accepts a proton: the acid partner in an acid-base pair.…
Conjugate Base
What an acid becomes after it donates a proton: the base partner in an acid-base pair.…
Covalent Bond
A bond where atoms share electrons instead of trading them. It is the bond holding water, …
D
E
Effusion
Gas escaping through a tiny hole into an empty space. Lighter gases leak out faster, which…
Electrolytic Cell
A device that uses electricity to force a non-spontaneous chemical reaction: like rechargi…
Electron
The tiny negatively charged particle that occupies the space around the nucleus. Electrons…
Electron Configuration
The address book of an atom's electrons: a notation showing which orbitals the electrons o…
Electron Shell
A group of electrons at roughly the same distance from the nucleus, like floors of a build…
Electronegativity
A measure of how strongly an atom pulls shared electrons toward itself in a bond. Fluorine…
Element
A pure substance made of only one kind of atom. Oxygen, iron, and gold are elements: no ch…
Elementary Step
A single, simple collision or rearrangement in a reaction mechanism: the individual frames…
Endothermic
A process that absorbs heat from its surroundings, like an instant cold pack or melting ic…
Enthalpy
A measure of a system's total heat content at constant pressure. Chemists track its change…
Entropy
A measure of disorder or energy dispersal in a system. Nature favors higher entropy: it is…
Enzyme
A protein that speeds up biological reactions: the catalysts of life, each specialized for…
Equilibrium Constant (Kc)
A number describing where equilibrium lies: products over reactants, each raised to its co…
Equilibrium Constant (Kp)
The equilibrium constant written with gas partial pressures instead of concentrations: use…
Equivalence Point
The moment in a titration when exactly enough titrant has been added to react with all the…
Excited State
When an atom has absorbed energy and one of its electrons has jumped to a higher orbital: …
Exothermic
A process that releases heat to its surroundings, like burning wood or a hand warmer activ…
F
G
Galvanic Cell
A battery: a device that turns a spontaneous chemical reaction into electric current.…
Gamma Ray
Pure high-energy light from the nucleus: the most penetrating common radiation, needing le…
Gibbs Free Energy
The energy available to do useful work, and the decider of spontaneity. If Gibbs energy dr…
Ground State
An atom's lowest-energy, most stable arrangement of electrons: its normal resting conditio…
Group
A vertical column of the periodic table. Elements in the same group behave similarly becau…
H
Half-Life
The time for half the reactant to be used up. For first-order reactions it is constant: th…
Halogen
The group 17 elements: fluorine, chlorine, bromine, iodine. They are aggressive nonmetals …
Heat Capacity
How much heat a whole object needs to warm up by 1 degree Celsius. A swimming pool has eno…
Hydrocarbon
A compound of only hydrogen and carbon: the family behind fuels, plastics, and natural gas…
Hydrogen Bond
A strong attraction between a hydrogen attached to nitrogen, oxygen, or fluorine and a nea…
I
ICE Table
A bookkeeping chart for equilibrium problems: write Initial concentrations, the Change, an…
Ideal Gas
A simplified model of a gas where particles are tiny points that never attract or repel ea…
Indicator
A dye that changes color at a certain pH: the visual signal telling you a titration has re…
Ion
An atom or molecule that carries an electric charge because it has gained or lost electron…
Ionic Bond
A bond formed when one atom gives electrons to another, creating oppositely charged ions t…
Ionization Energy
The energy needed to pull an electron completely off an atom. Atoms that hold electrons lo…
Isomer
Molecules with the same formula but different arrangements: same atoms, different structur…
Isotope
Atoms of the same element with different numbers of neutrons. They behave the same chemica…
K
L
Lattice Energy
The energy released when separated gas-phase ions come together to form a solid crystal. B…
Le Chatelier's Principle
If you disturb a system at equilibrium, it shifts to fight the disturbance: add reactant a…
Lewis Structure
A diagram showing how atoms are bonded and where the electrons sit: dots for lone pairs, l…
Lipid
Fats, oils, and membranes: energy-dense molecules that do not mix with water, built mostly…
M
Mass Number
The total count of protons plus neutrons in an atom's nucleus. It tells you which isotope …
Matter
Anything that has mass and occupies space. Your desk, the air in the room, and a drop of w…
Metallic Bond
The bond in metals: positive metal ions sitting in a sea of freely moving electrons. It ex…
Metalloid
Elements that behave like a mix of metals and nonmetals: the staircase elements such as si…
Mixture
Two or more substances physically combined without chemical bonding. Air, salt water, and …
Molality
Concentration as moles of solute per kilogram of solvent: the temperature-proof alternativ…
Molar Mass
The mass of one mole of a substance, in grams per mole. Add up the atomic weights of every…
Molarity
Concentration as moles of solute per liter of solution: the most common concentration unit…
Mole
The chemist's counting unit for enormous numbers of particles. One mole contains 6.022 x 1…
Molecule
Two or more atoms joined by chemical bonds, acting as a single particle. Water is made of …
Monomer
The small molecule that links up with others to build a polymer: like a single Lego brick …
N
Neutralization
An acid and a base reacting to cancel each other out, making water and a salt.…
Neutron
The neutral particle in the nucleus. It adds mass and helps hold the nucleus together, but…
Noble Gas
The group 18 elements: helium, neon, argon, and friends. They almost never react because t…
Nonpolar Bond
A covalent bond where electrons are shared equally, or nearly so: neither atom hogs the el…
Nuclear Binding Energy
The energy that holds a nucleus together: equivalently, the missing mass when you weigh a …
Nuclear Fission
Splitting a heavy nucleus into lighter ones, releasing enormous energy: the process poweri…
Nuclear Fusion
Joining light nuclei into heavier ones, releasing energy: the process powering the Sun and…
O
Octet Rule
The guideline that atoms tend to gain, lose, or share electrons to end up with eight elect…
Orbital
A region around the nucleus where an electron is most likely to be found. Different orbita…
Order of Reaction
The exponents in the rate law, showing how strongly each reactant's concentration affects …
Osmosis
Water moving across a semipermeable membrane from the less-concentrated side to the more-c…
Oxidation
Losing electrons. Remember OIL: Oxidation Is Loss. When iron rusts, iron atoms are oxidize…
Oxidizing Agent
The substance that causes oxidation by taking electrons, and gets reduced itself in the pr…
P
Partial Pressure
The pressure one gas in a mixture would exert all by itself. The total pressure is simply …
Period
A horizontal row of the periodic table. The period number tells you how many electron shel…
pH
The 0 to 14 scale measuring acidity: below 7 is acidic, 7 is neutral, above 7 is basic. Ea…
pOH
pH's partner scale for hydroxide ions: pOH = -log[OH-]. Low pOH means strongly basic.…
Polar Bond
A covalent bond where electrons are shared unequally, giving one end a slight positive cha…
Polymer
A giant molecule built by linking many small repeating units: plastics, rubber, DNA, and p…
Product
A substance formed by a chemical reaction: what you get at the end.…
Protein
Large molecules built from amino acid chains, folded into precise shapes. They do most of …
Proton
The positively charged particle in an atom's nucleus. The number of protons decides which …
Q
R
Radioactivity
The spontaneous breakup of unstable atomic nuclei, shooting out particles or energy: a nuc…
Rate Constant
The k in the rate law: a number capturing how inherently fast a reaction is at a given tem…
Rate Law
An equation showing how reaction speed depends on reactant concentrations: determined by e…
Rate-Determining Step
The slowest step in a reaction mechanism: it acts like a bottleneck and sets the speed of …
Reactant
A starting substance in a chemical reaction: what gets used up as the reaction proceeds.…
Reaction Mechanism
The step-by-step story of how a reaction actually happens: which bonds break when, in what…
Reducing Agent
The substance that causes reduction by giving away electrons, and gets oxidized itself in …
Reduction
Gaining electrons. Remember RIG: Reduction Is Gain. When copper ions become copper metal, …
S
Salt Bridge
The tube of salt solution connecting a battery's two halves: it lets ions flow to balance …
Saturated Solution
A solution holding the maximum dissolved solute it can at a given temperature: add more an…
Solubility
How much of a substance can dissolve in a given amount of solvent: the limit that defines …
Solubility Product (Ksp)
The equilibrium constant for a solid dissolving: a tiny Ksp means the salt barely dissolve…
Solute
The substance that gets dissolved: the salt in salt water, the sugar in tea.…
Solvent
The substance doing the dissolving: usually the liquid present in the largest amount. Wate…
Specific Heat
How much heat it takes to raise 1 gram of a substance by 1 degree Celsius. Water's unusual…
Standard Reduction Potential
A ranking of how badly a species wants electrons, measured in volts against hydrogen's zer…
Stoichiometry
The mathematics of chemical reactions: using a balanced equation to predict how much react…
Strong Acid
An acid that dissociates completely in water: every molecule donates its proton. The famou…
Subscript
The small number written after an element symbol in a formula. It counts how many atoms of…
Substituent
An atom or group that replaces a hydrogen on a parent molecule: the branches and attachmen…
Supersaturated Solution
A solution holding MORE dissolved solute than should be possible: an unstable state that c…
Surroundings
Everything outside the system you are studying. Heat lost by the system is gained by the s…
System
The part of the universe you are studying: the reaction flask, the beaker, whatever you ha…
T
U
V
Valence Electron
The electrons in an atom's outermost shell: the ones that actually do the chemistry of bon…
van der Waals Forces
The weak, universal attractions between all molecules: even noble gases feel them. They ar…
Vapor Pressure
The pressure exerted by a liquid's vapor when evaporation and condensation balance out. Hi…