This is a list of important chemistry vocabulary terms and their definitions. A more comprehensive list of chemistry terms can be found in my alphabetical chemistry glossary. You can use this vocabulary list to look up terms or you can make flashcards from the definitions to help learn them.
absolute zero – Absolute zero is 0K. It is the lowest possible temperature. Theoretically, at absolute zero, atoms stop moving.
accuracy – Accuracy is a measure of how close a measured value is to its true value. For example, if an object is exactly a meter long and you measure it as 1.1 meters long, that is more accurate than if you measured it at 1.5 meters long.
acid – There are several ways to define an acid, but they include any chemical that gives off protons or H+ in water. Acids have a pH less than 7. They turn the pH indicator phenolphthalein colorless and turn litmus paper red.
acid anhydride – An acid anhydride is an oxide that forms an acid when it is reacted with water. For example, when SO3– is added to water, it becomes sulfuric acid, H2SO4.
actual yield – The actual yield is the amount of product you actually obtain from a chemical reaction, as in the amount you can measure or weigh as opposed to a calculated value.
alcohol – An alcohol is any organic molecule that has an -OH group.
aldehyde – An aldehyde is any organic molecule that has a -COH group.
alkali metal – An alkali metal is a metal in Group I of the periodic table. Examples of alkali metals include lithium, sodium, and potassium.
alkane – An alkane is an organic molecule that only contains single carbon-carbon bonds.
alkene – An alkene is an organic molecule that contains at least one C=C or carbon-carbon double bond.
alkyne – An alkyne is an organic molecule that contains at least one carbon-carbon triple bond.
allotrope – Allotropes are different forms of a phase of an element. For example, diamond and graphite are allotropes of carbon.
base – A base is a compound that produces OH– ions or electrons in water or that accepts protons. An example of a common base is sodium hydroxide, NaOH.
bond length – Bond length is the average distance between the nuclei of two atoms that share a bond.
calorimetry – Calorimetry is the study of heat flow. Calorimetry may be used to find the heat of reaction of two compounds or the heat of combustion of a compound, for example.
carboxylic acid – A carboxylic acid is an organic molecule containing a -COOH group. An example of a carboxylic acid is acetic acid.
catalyst – A catalyst is a substance that lowers the activation energy of a reaction or speeds it up without being consumed by the reaction. Enzymes are proteins that act as catalysts for biochemical reactions.
cathode – A cathode is the electrode which gains electrons or is reduced. In other words, it is where reduction occurs in an electrochemical cell.
chemical property – A chemical property is a property that can only be observed when a chemical change occurs. Flammability is an example of a chemical property, since you can’t measure how flammable a substance is without igniting it (making/breaking chemical bonds).
critical mass – Critical mass is the minimum quantity of radioactive material needed to cause a nuclear chain reaction.
critical point – The critical point is the endpoint of the liquid-vapor line in a phase diagram, past which a supercritical liquid forms. At the critical point, the liquid and vapor phases become indistinguishable from one another.
crystal – A crystal is an ordered, repeating three-dimensional pattern of ions, atoms, or molecules. Most crystals are ionic solids, although other forms of crystals exist.
delocalization – Delocalization is when electrons become free to move all over a molecule, such as when double bonds occur on adjacent atoms in a molecule.
denature – There are two common meanings for this in chemistry. First, it can refer to any process used to make ethanol unfit for consumption (denatured alcohol). Second, denaturing can mean breaking down the three-dimensional structure of a molecule, such as a protein is denatured when exposed to heat.
diffusion – Diffusion is the movement of particles from an area of higher concentration to one of lower concentration.
dilution – Dilution is when a solvent is added to a solution, making it less concentrated.
dissociation – Dissociation is when a chemical reaction breaks a compound into two or more parts. For example, NaCl dissociates into Na+ and Cl– in water.
effusion – Effusion is when a gas moves through an opening into a low-pressure container (e.g., is drawn by a vacuum). Effusion occurs more quickly than diffusion because additional molecules aren’t in the way.
electrolysis – Electrolysis is using electricity to break the bonds in a compound to break it apart.
electrolyte – An electrolyte is an ionic compound that dissolves in water to produce ions, which can conduct electricity. Strong electrolytes completely dissociate in water, while weak electrolytes only partially dissociate or break apart in water.
enantiomers – Enantiomers are molecules that are non superimposable mirror images of each other.
endothermic – Endothermic describes a process that absorbs heat. Endothermic reactions feel cold.
endpoint – The endpoint is when a titration is stopped, typically because an indicator has changed color. The endpoint need not be the same as the equivalence point of a titration.
energy level – An energy level is a possible value of energy that an electron can have in an atom.
enthalpy – Enthalpy is a measure of the amount of energy in a system.
entropy – Entropy is a measure of the disorder or randomness in a system.
enzyme – An enzyme is a protein that acts as a catalyst in a biochemical reaction.
equilibrium – Equilibrium occurs in reversible reactions when the forward rate of the reaction is the same as the reverse rate of the reaction.
equivalence point – The equivalence point is when the solution in a titration is completely neutralized. It is not the same as the endpoint of a titration because the indicator may not change colors precisely when the solution is neutral.
ester – An ester is an organic molecule with a R-CO-OR’ function group.
excess reagent – Excess reagent is what you get when there is leftover reagent in a chemical reaction.
exothermic – Exothermic describes a process that gives off heat.
family – A family is a group of elements sharing similar properties. It is not necessarily the same thing as an element group. For example, the chalcogens or oxygen family consists of some different elements from the nonmetal group.
Kelvin – Kelvin is a unit of temperature. A Kelvin is equal in size to a degree Celsius, although Kelvin starts from absolute zero. Add 273.15 to a Celsius temperature to get the Kelvin value. Kelvin is not reported with a ° symbol. For example, you would simply write 300K not 300°K.
ketone – A ketone is a molecule that contains a R-CO-R’ functional group. An example of a common ketone is acetone (dimethyl ketone).
lanthanide contraction – The lanthanide contraction refers to the trend in which lanthanide atoms become smaller as you move left to right across the periodic table, even though they increase in atomic number.
lattice energy – Lattice energy is the amount of energy released when one mole of a crystal forms from its gaseous ions.
ligand – A ligand is a molecule or ion stuck to the central atom in a complex. Examples of common ligands include water, carbon monoxide, and ammonia.
mass – Mass is the amount of matter in a substance. It is commonly reported in units of grams.
mole – Avogadro’s number (6.02 x 1023) of anything.
node – A node is a location in an orbital with no probability of containing an electron.
nucleon – A nucleon is a particle in the nucleus of an atom (proton or neutron).
oxidation number The oxidation number is the apparent charge on an atom. For example, the oxidation number of an oxygen atom is -2.
period – A period is a row (left to right) of the periodic table.
pressure – Pressure is force per area.
product – A product is something made as a result of a chemical reaction.
quantum theory – Quantum theory is the description of energy levels and the predictions about the behavior of atoms at specific energy levels.
radioactivity – Radioactivity occurs when the atomic nucleus is unstable and breaks apart, releasing energy or radiation.
Raoult’s Law – Raoult’s Law states that the vapor pressure of a solution is directly proportional to the mole fraction of solvent.
rate determining step – The rate determining step is the slowest step in any chemical reaction.
rate law – A rate law is a mathematical expression relating the speed of a chemical reaction as a function of concentration.
redox reaction – A redox reaction is a chemical reaction that involves oxidation and reduction.
resonance structure – Resonance structures are the set of Lewis structures that can be drawn for a molecule when it has delocalized electrons.
RMS velocity – The RMS or root mean square velocity is the square root of the average of the squares of individual velocities of gas particles, which is a way of describing the average speed of gas particles.
salt – An ionic compound formed from reacting an acid and a base.
solute – The solute is the substance that gets dissolved in a solvent. Usually, it refers to a solid that is dissolved in a liquid. If you are mixing two liquids, the solute is the one that is present in a smaller amount.
solvent – This is the liquid that dissolves a solute in solution. Technically, you can dissolve gases into liquids or into other gases, too. When making a solution where both substances are in the same phase (e.g., liquid-liquid), the solvent is the largest component of the solution.
STP – STP means standard temperature and pressure, which is 273K and 1 atmosphere.
strong nuclear force – The strong nuclear force is the force that holds the protons and neutrons in an atomic nucleus together.
sublimation – Sublimation is when a solid changes directly into a gas. At atmospheric pressure, dry ice or solid carbon dioxide goes directly into carbon dioxide vapor, never becoming liquid carbon dioxide.
synthesis – Synthesis is making a larger molecule from two or more atoms or smaller molecules.
system – A system includes everything you are evaluating in a situation.
temperature – Temperature is a measure of the average kinetic energy of particles.
theoretical yield – Theoretical yield is the amount of product which would result if a chemical reaction proceeded perfectly, to completion, with no loss.
thermodynamics – Thermodynamics is the study of energy.
titration – Titration is a procedure in which the concentration of an acid or base is determined by measuring how much base or acid is required to neutralize it.
unit cell – A unit cell is the simplest repeating structure of a crystal.
unsaturated – There are two common meanings for unsaturated in chemistry. The first refers to a chemical solution that does not contain all of the solute that can be dissolved in it. Unsaturated also refers to an organic compound which contains one or more double or triple carbon-carbon bonds.
unshared electron pair – An unshared electron pair or lone pair refers to two electrons that aren’t participating in chemical bonding.
valence electron – The valence electrons are the atom’s outermost electrons.
volatile – Volatile refers to a substance that has a high vapor pressure.
VSEPR – VSEPR stands for Valence Shell Electron Pair Repulsion. This is a theory used that predicts molecular shapes based on the assumption that electrons stay as far as possible from each other.