1,800+ MCAT terms, defined.

A covalent bond forms when two atoms share one or more pairs of valence electrons, usually between nonmetals. The most testable point: more shared pairs (single, double, triple) make a shorter, stronger bond, and unequal sharing makes th...

Thermodynamic control means a reversible reaction reaches equilibrium, so the major product is the most stable one (lowest Gibbs free energy), not the one that forms fastest. Higher temperature and longer time favor it; the faster-formin...

A reaction is zeroth order in a reactant when its rate does not depend on that reactant's concentration, so rate = k. Most testable point: a plot of [A] versus time is a straight line with slope −k, unlike the curved plots for first and...

A sigma (σ) bond is a covalent bond formed by head-on overlap of orbitals along the line between the two nuclei. Every single bond is one sigma bond, and double and triple bonds contain exactly one sigma bond plus pi bonds. Atoms can usu...

A complex ion forms when a metal cation bonds to ligands (Lewis bases) through coordinate covalent bonds, as in Ag(NH₃)₂⁺. Its formation constant Kf is usually large, so complexing removes free metal ion and raises the solubility of a sp...

The ion product is the product of dissolved ion concentrations, each raised to its coefficient. For water it is Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴ at 25 °C. For a sparingly soluble salt it is Q, compared with Ksp to predict whether a precipita...

A precipitate is an insoluble solid that comes out of solution when dissolved ions combine. The testable point is to compare the ion product Q with Ksp: if Q > Ksp, a precipitate forms; if Q < Ksp, the solution is unsaturated and no soli...

Solvation is the stabilizing attraction between solvent molecules and dissolved solute particles, which surround each particle. In water it is called hydration. Dissolving is favored when solute-solvent attractions are comparable to or s...

An acid-base reaction is usually treated as proton transfer from a Brønsted-Lowry acid to a base, giving a conjugate base and conjugate acid. The Lewis view instead uses electron-pair acceptor and donor. Testable: equilibrium favors the...

Acid-base equilibria are the reversible proton-transfer reactions of acids and bases in water, quantified by Ka, Kb, and pH. The most testable point: a larger Ka means a stronger acid and a weaker conjugate base, and a buffer (HA with A⁻...

Bond enthalpy is the average energy needed to break one mole of a given bond in gaseous molecules. Breaking is endothermic; forming releases that energy. Testable point: ΔH°rxn ≈ Σ(bonds broken) − Σ(bonds formed), an estimate that is neg...

pH calculations use pH = −log[H⁺] to turn an acid or base concentration into a pH. A strong acid gives [H⁺] equal to its concentration; a weak acid needs Ka, often with the approximation [H⁺] ≈ √(Ka·C). Each pH unit is a tenfold change i...

VSEPR (valence shell electron pair repulsion) theory predicts molecular shape by placing electron domains, meaning bonds and lone pairs, around a central atom as far apart as possible. Testable point: the molecular geometry names only th...

A ligand is an ion or molecule that donates a lone pair to a central metal ion, forming a coordinate covalent bond in a complex ion. The ligand is the Lewis base and the metal is the Lewis acid. Adding a ligand that forms a stable comple...

A rate constant, k, is the proportionality constant in a rate law, linking reaction rate to reactant concentrations. It does not change as concentrations change, but it increases with temperature or with a catalyst that lowers activation...

The atm (atmosphere) is a unit of pressure, defined from average air pressure at sea level: 1 atm = 760 mmHg = 760 torr = 1.01325 × 10⁵ Pa. The most testable point is matching units: use pressure in atm when R = 0.0821 L·atm/(mol·K).

A complex ion is a central metal cation bonded to ligands (Lewis bases that donate lone pairs) through coordinate covalent bonds, such as Cu(NH₃)₄²⁺. Its most testable effect: forming a complex pulls the metal out of solution equilibria...

A covalent bond forms when two atoms share one or more pairs of electrons, usually between nonmetals. The most testable point: going from single to double to triple bonds between the same atoms, the bond gets shorter and stronger (higher...

Electrochemical refers to redox chemistry linked to electrical energy. A galvanic (voltaic) cell uses a spontaneous reaction to make current; an electrolytic cell uses an external source to force a nonspontaneous one. In both, oxidation...

Intermolecular attractions are forces between separate molecules, not the bonds inside them. Roughly ranked, London dispersion < dipole–dipole < hydrogen bonding, and stronger attractions generally mean a higher boiling point and lower v...

Specific heat (c) is the heat needed to raise the temperature of one unit mass of a substance by one degree. A higher c means a smaller temperature change for the same heat added. Water's is unusually high, about 4.18 J/(g·K).

A weighted average counts each value in proportion to its share of the sample, not equally. In general chemistry it gives atomic mass from isotope masses and fractional abundances, and the result lies nearer the mass of the most abundant...

A complete ionic equation writes every strong electrolyte in a reaction as separate ions in aqueous solution, while solids, liquids, gases, and weak electrolytes stay as whole formulas. Cancelling the spectator ions, which appear unchang...

Molar solubility is the number of moles of a sparingly soluble salt that dissolve per liter of saturated solution (mol/L). It is calculated from Ksp, and a common ion lowers it. Ksp values can be compared directly only for salts that giv...

Molecular weight is the sum of the average atomic masses of all atoms in a molecular formula, in amu. It is numerically equal to molar mass in g/mol, so it converts grams to moles: n = m/M. Heavier gas molecules also effuse more slowly.

Normality (N) is concentration in equivalents per liter of solution, where one equivalent is one mole of H⁺ or OH⁻ (acid-base) or of electrons (redox). It equals molarity × equivalents per mole, so it depends on the reaction: 1 M H₂SO₄ i...

The heat of reaction is the heat absorbed or released when a reaction proceeds as written; at constant pressure it equals the enthalpy change, ΔH. Negative ΔH is exothermic, positive is endothermic, and ΔH can be found from formation ent...

Kw is the ion-product constant of water, Kw = [H₃O⁺][OH⁻], which equals 1.0 × 10⁻¹⁴ at 25 °C. It depends on temperature, not on added acid or base, so pH + pOH = 14 holds at 25 °C; when Kw rises with heating, neutral pH falls below 7.

Percent yield is the actual yield of a reaction (what is really isolated) divided by the theoretical yield (the maximum predicted from the limiting reagent), times 100. The testable point: find the limiting reagent first, since it sets t...

The triple point is the single temperature and pressure at which a pure substance's solid, liquid and gas phases coexist in equilibrium. On a phase diagram it is where the three boundary lines meet. For water it is 273.16 K (0.01 °C) at...

A salt bridge links the two half-cells of an electrochemical cell with an inert electrolyte, letting ions move to keep each solution neutral and complete the circuit. Anions migrate toward the anode and cations toward the cathode; electr...

A dipole is a separation of partial positive and partial negative charge, which arises when bonded atoms differ in electronegativity. A molecule is polar only if its bond dipoles do not cancel, so molecular shape matters as much as the b...

The melting point is the temperature at which a pure solid and its liquid are in equilibrium at a given pressure; it equals the freezing point. Stronger intermolecular forces or bonding generally mean a higher value, and impurities lower...

Avogadro's number, about 6.02 × 10²³ mol⁻¹, is the number of particles (atoms, molecules, ions) in one mole of a substance. It links the atomic scale to the lab scale: a substance's molar mass in g/mol is the mass of 6.02 × 10²³ of its p...

An electrolyte is a substance that releases mobile ions in water, so the solution conducts electricity. Strong electrolytes dissociate essentially completely, weak ones only partly, and nonelectrolytes like glucose do not form ions. More...

The gas constant R is the proportionality constant in the ideal gas law, with units of energy per mole per kelvin. Its value is 8.314 J/(mol·K), or 0.0821 L·atm/(mol·K); the number depends only on the units, so choose the version that ma...

Hybridisation is the mixing of an atom's valence s and p orbitals into equivalent hybrid orbitals that form sigma bonds and hold lone pairs. Count sigma bonds plus lone pairs on the atom: 2 is sp, 3 is sp², 4 is sp³. Pi bonds are not cou...

Gas molecules are the particles described by kinetic molecular theory: far apart, in constant random motion, with negligible volume and no intermolecular forces in an ideal gas. Average kinetic energy depends only on temperature, so at t...

Qsp is the ion product: the solubility product expression evaluated with the ion concentrations present right now, not necessarily at equilibrium. Compare it with Ksp. If Qsp > Ksp, a precipitate forms; if Qsp < Ksp, more solid can disso...

Dalton's law says the total pressure of a mixture of ideal gases equals the sum of the partial pressures of its components, each acting as if it alone filled the container. Most testable: partial pressure = mole fraction × total pressure.

A concentration gradient is a difference in a species' concentration between two regions, such as across a membrane. Net diffusion of an uncharged species runs down the gradient, from high to low, until it levels out. In a concentration...

pOH is the negative base-10 logarithm of the hydroxide ion concentration, pOH = −log[OH⁻]. At 25 °C, pH + pOH = 14, so a pOH below 7 means a basic solution, and each 1-unit drop in pOH means a tenfold rise in [OH⁻].

Metalloids are elements with properties intermediate between metals and nonmetals, found along the diagonal staircase of the periodic table (B, Si, Ge, As, Sb, Te are the usual list). Most testable: many are semiconductors, whose conduct...

ΔG is the change in Gibbs free energy, the energy available to do useful work at constant temperature and pressure. A negative ΔG means a process is spontaneous (thermodynamically favorable) as written, ΔG = 0 means equilibrium, and ΔG s...

Bond energy is the energy needed to break one mole of a particular bond in the gas phase, usually in kJ/mol. A higher value means a stronger bond, which is usually shorter. Most testable: ΔH ≈ energy of bonds broken − energy of bonds for...

A spontaneous process proceeds in the forward direction on its own, without continuous outside energy input. At constant T and P it has ΔG < 0, equivalently the entropy of the universe increases. Spontaneous says nothing about speed, whi...

Disproportionation is a redox reaction in which one species is both oxidized and reduced, so an element in an intermediate oxidation state forms products with a higher and a lower oxidation number. Example: 2H₂O₂ → 2H₂O + O₂. Spot it by...

Peroxides are compounds containing an O–O single bond, such as H₂O₂ and Na₂O₂, with oxygen in the −1 oxidation state instead of the usual −2. The most testable point is that this is the standard exception when assigning oxidation numbers.

A real gas is a gas whose behavior departs from PV = nRT because its particles take up volume and attract one another. The deviation is generally largest at high pressure and low temperature, and smallest for small, nonpolar gases such a...

A coordination complex is a central metal ion bonded to ligands, each of which donates a lone pair to the metal in a coordinate covalent bond (Lewis base to Lewis acid). Testable point: forming a complex with a large Kf lowers free metal...