1,800+ MCAT terms, defined.

A polyvalent (polyprotic) acid can donate more than one proton per molecule, and a polyvalent base can accept more than one. Protons transfer stepwise, and each later step generally has a smaller Ka (Ka1 > Ka2 > Ka3), giving several buff...

Post-transition metals (poor metals) are the p-block metals right of the transition metals, such as Al, Ga, In, Sn, Tl, Pb and Bi. They are generally softer and lower-melting than transition metals, and the heavier ones often form ions w...

The Rutherford model pictures the atom as a tiny, dense, positively charged nucleus holding nearly all the mass, with electrons in the mostly empty space around it. It came from the gold foil experiment, where most alpha particles passed...

A solvation layer is the shell of solvent molecules oriented around a dissolved ion or polar solute by intermolecular attraction (a hydration shell in water). Key point: favorable solute–solvent attractions such as ion–dipole forces driv...

A titration indicator is a weak acid or base whose acid and conjugate-base forms differ in color, so its color change signals the endpoint of a titration. It works best when its pKa is near the pH of the equivalence point, which sits on...

A double-displacement (metathesis) reaction swaps the partner ions of two compounds: AB + CD → AD + CB. It typically proceeds when a product leaves solution as a precipitate, a gas, or a weak electrolyte such as water. No oxidation numbe...

Square planar is a molecular geometry in which a central atom has four bonded atoms in one flat plane at 90° to each other. It arises from six electron domains, four bonds and two lone pairs on opposite sides (AX₄E₂), as in XeF₄.

Effusion is the escape of a gas through a tiny hole into a vacuum or a region of much lower pressure. At the same temperature, a lighter gas effuses faster: rate is inversely proportional to the square root of molar mass (Graham's law).

Emf (electromotive force) is the voltage a cell can produce when no net current flows. It is energy per unit charge, not a force. A positive E°cell = E°cathode − E°anode means a spontaneous galvanic cell, and ΔG = −nFE links emf to free...

The ideal gas constant R is the proportionality constant in PV = nRT. It is 8.314 J/(mol·K), or 0.0821 L·atm/(mol·K). It is the same for every ideal gas, and only its numerical value changes with the units, so match R to the units of P a...

Molecular geometry is the 3D shape formed by a molecule's atoms around a central atom, predicted by VSEPR from the repulsion of electron domains. Lone pairs shape it but are not named in it, so H₂O has tetrahedral electron geometry but b...

PO₄³⁻ is the phosphate ion, the fully deprotonated form of triprotic phosphoric acid (H₃PO₄). It is tetrahedral, with P at +5. Most testable: near pH 7.4 the buffer pair is H₂PO₄⁻/HPO₄²⁻ (pKa ≈ 7.2), not PO₄³⁻.

The frequency factor (A, the pre-exponential factor) in the Arrhenius equation reflects how often reactant collisions occur with a suitable orientation. It multiplies e^(−Ea/RT), the fraction of collisions with enough energy, to give k....

Latent heat is the heat absorbed or released when a substance changes phase at constant temperature, calculated with q = mL. On a heating curve it is the flat plateau, where added energy changes intermolecular attractions instead of temp...

Bromine (Br, Z = 35) is a group 17 halogen that exists as diatomic Br₂, a reddish-brown liquid at room temperature. Its most testable points are the halogen trends: it is a weaker oxidizing agent and less electronegative than chlorine, b...

An s orbital is a spherically symmetric electron probability distribution around the nucleus, with angular momentum quantum number ℓ = 0. Each s subshell has one orbital holding at most 2 electrons, and it grows larger and higher in ener...

The equilibrium point is the state of a reversible reaction in which forward and reverse rates are equal, so concentrations stay constant at a given temperature while both reactions continue. The most testable point: at equilibrium Q = K...

STP means standard temperature and pressure: 0 °C (273 K) and 1 atm. At STP, one mole of any ideal gas occupies 22.4 L, so you can convert directly between gas volume and moles without using the full ideal gas law.

The ideal gas equation, PV = nRT, links the pressure, volume, moles and absolute temperature of a gas modeled as having negligible particle volume and no intermolecular forces. The most testable point is that T must be in kelvins. Real g...

An exponent tells how many times a base is multiplied by itself. In general chemistry it appears in powers of ten, equilibrium expressions (coefficients become exponents) and rate laws (reaction orders). Most testable: exponents add when...

SO₄²⁻ is the sulfate ion, a tetrahedral polyatomic anion with sulfur in the +6 oxidation state. It is the conjugate base of HSO₄⁻ and a very weak base. Most sulfate salts dissolve, but BaSO₄ and PbSO₄ are the classic insoluble exceptions.

A reaction is zero order in a reactant when its rate does not depend on that reactant's concentration, so rate = k. The most testable points: [A] versus time is a straight line with slope −k, and k has units of M/s.

The standard state is the agreed reference condition for thermodynamic values: pure solids and liquids, gases at 1 bar (older texts use 1 atm), solutes at 1 M, usually tabulated at 298 K. Values there carry a ° sign (ΔG°, E°); ΔG° relate...

A d orbital belongs to a subshell with angular momentum quantum number l = 2. Each d subshell has five orbitals holding up to 10 electrons and first appears at n = 3. Most testable: transition metals fill (n−1)d, and as cations they lose...

ΔH is the enthalpy change, the heat absorbed or released by a system at constant pressure. ΔH < 0 is exothermic (heat released) and ΔH > 0 is endothermic (heat absorbed). It can be estimated from bonds broken minus bonds formed.

H₂SO₄ is sulfuric acid, a diprotic strong acid. Its first proton dissociates essentially completely, but the second, from HSO₄⁻, only partly (Ka2 near 10⁻²). Each mole can still neutralize 2 mol of OH⁻ in a titration.

The thermodynamic product is the most stable product, the one with the lowest Gibbs free energy. It dominates when a reaction is reversible and reaches equilibrium, usually at higher temperature or longer time. The faster-forming kinetic...

Vaporization is the change from liquid to gas. It is endothermic because energy is absorbed to overcome intermolecular attractions. On a heating curve the temperature stays constant during the change, and the heat absorbed equals n × ΔHvap.

An electrode is a conductor, usually a metal or graphite, where a redox half-reaction occurs in an electrochemical cell. The anode is always where oxidation occurs and the cathode is always where reduction occurs, in both galvanic and el...

Molar mass is the mass of one mole of a substance, usually in g/mol. It equals the sum of the atomic masses in the formula and is numerically the same as the formula mass in amu. It is the conversion factor between grams and moles: n = m/M.

A molecular orbital is a wavefunction for an electron spread over a whole molecule, formed by combining atomic orbitals. Bonding MOs lie lower in energy and antibonding MOs higher, so bond order = ½(bonding − antibonding electrons) predi...

A lattice is the regular, repeating three-dimensional arrangement of particles in a crystalline solid, described by its unit cell. Most testable: shared corner and face particles give 1, 2 and 4 particles per cell for simple, body-center...

To neutralize is to react an acid with a base until the moles of H⁺ supplied equal the moles of OH⁻ (or other base) supplied. The result is pH 7 at 25 °C only for a strong acid with a strong base. With a weak acid or weak base, the equiv...

Charles' law says that for a fixed amount of an ideal gas at constant pressure, volume is directly proportional to absolute temperature: V₁/T₁ = V₂/T₂. Heating makes the gas expand. The testable trap is that T must be in kelvins, not °C.

A photon's energy is the energy carried by one quantum of light, E = hf = hc/λ. It rises with frequency and falls with wavelength, so a UV photon carries more energy than a red or infrared one. Brightness changes the number of photons, n...

Standard free energy (ΔG°) is the Gibbs free energy change for a reaction when all species are in their standard states (1 M solutes, 1 atm gases, usually 298 K). It fixes Keq: ΔG° < 0 means Keq > 1. Direction in real conditions depends...

The principal quantum number, n, is a positive integer (1, 2, 3, …) that labels an electron's shell and sets the orbital's size and main energy level. Higher n means higher energy and a larger average distance from the nucleus. (Standard...

Deposition is the phase change in which a gas becomes a solid directly, skipping the liquid phase. It is the reverse of sublimation, so it is exothermic and lowers entropy. On a phase diagram it is the gas-to-solid crossing of the solid–...

A half reaction is the oxidation or the reduction step of a redox reaction, written alone with the electrons shown. Oxidation occurs at the anode and reduction at the cathode, and the two add, with electrons cancelled, to give the overal...

A nonvolatile substance has negligible vapor pressure at the temperature of interest, so it stays in the liquid or solid rather than evaporating. The key point: a nonvolatile solute lowers the solvent's vapor pressure, which raises the b...

pKw is −log Kw, where Kw is the ion-product constant for the autoionization of water. At 25 °C, Kw = 1.0 × 10⁻¹⁴ and pKw = 14, so pH + pOH = 14. Autoionization is endothermic, so Kw rises and pKw falls as temperature increases.

Rate order is the exponent of a reactant's concentration in the experimentally determined rate law. It shows how strongly the rate depends on that concentration, and the overall order is the sum of the exponents. Orders come from data, n...

Dipole-dipole forces are attractions between the partial positive end of one polar molecule and the partial negative end of another. They are stronger than London dispersion forces for molecules of similar size, so they usually raise boi...

Chelation is the binding of one metal ion by a single ligand through two or more donor atoms, forming a ring. Chelates are usually more stable than complexes of comparable single-donor ligands, mainly because of a favorable entropy gain...

Homogeneous means uniform in composition and properties throughout, as in a solution, which is a single phase. The MCAT also applies it to equilibria and catalysts in which everything is in one phase. In a heterogeneous equilibrium, pure...

Read as SO₃²⁻, the sulfite ion: sulfur bonded to three oxygens with one lone pair, giving a trigonal pyramidal shape and a sulfur oxidation state of +4. Testable point: it is pyramidal, unlike planar SO₃, and it can be oxidized to sulfat...

A supercritical fluid is a substance above both its critical temperature and critical pressure, where it is no longer distinctly liquid or gas. The testable point: above the critical temperature, compression alone cannot liquefy a gas, b...

Heat transfer is the movement of thermal energy between objects at different temperatures, spontaneously from hotter to colder, by conduction, convection, or radiation. Most testable: q = mcΔT, and in an isolated calorimeter the heat los...

Avogadro's law says that at constant temperature and pressure, the volume of a gas is directly proportional to its moles (V/n is constant). Equal volumes of different ideal gases at the same T and P hold equal numbers of particles, whate...

A chelate is a coordination complex in which one polydentate ligand binds a central metal ion through two or more donor atoms, forming ring structures. Chelates are typically more stable than similar complexes of monodentate ligands (the...