1,800+ MCAT terms, defined.

Dismutation (disproportionation) is a redox reaction in which a single species is both oxidized and reduced, so one element ends up in a higher and a lower oxidation state, as in 2H₂O₂ → 2H₂O + O₂. Under standard conditions it is spontan...

Phenolphthalein is an acid-base indicator, a weak organic acid that is colorless in acidic solution and pink in basic solution, changing color over roughly pH 8 to 10. It suits titrations whose equivalence point is above pH 7, such as a...

The buffer region is the flat stretch of a weak acid or weak base titration curve where pH changes little because both the weak acid and its conjugate base are present. It is centered on the half-equivalence point, where pH = pKa, and sp...

An excited state is an arrangement of an atom's or molecule's electrons with more energy than the ground state, usually because an electron absorbed energy and moved to a higher level. Testable point: a photon is absorbed when its energy...

The solubility product constant, Ksp, is the equilibrium constant for a slightly soluble ionic solid dissolving into its ions; the solid is left out of the expression. Comparing the ion product Q with Ksp predicts precipitation: a solid...

The water dissociation constant, Kw, is the equilibrium constant for water's autoionization, 2 H₂O ⇌ H₃O⁺ + OH⁻. Kw = [H₃O⁺][OH⁻] = 1.0 × 10⁻¹⁴ at 25 °C, so pH + pOH = 14 at that temperature. Kw changes with temperature, not with added a...

The anode's reduction potential is the tabulated standard potential (in volts) for the anode half-reaction written as a reduction, even though oxidation actually happens there. In a galvanic cell it is the lower (more negative) of the tw...

The mass number (A) of an atom is the number of protons plus neutrons in its nucleus. It is a whole number written as the upper left superscript of an isotope. Isotopes of one element share the atomic number but differ in mass number.

Avogadro's constant, NA, is the number of particles in one mole of a substance, about 6.022 × 10²³ per mole. It converts between moles and a count of atoms, molecules or ions. The testable point: count the right kind of particle, since o...

The cell potential (Ecell) is the voltage between an electrochemical cell's two electrodes, the driving force on electrons. It equals the cathode's reduction potential minus the anode's. A positive E°cell means the reaction is spontaneou...

EMF (electromotive force), or Ecell, is the cell potential: the voltage an electrochemical cell can drive, found as E°cathode − E°anode from reduction potentials. A positive Ecell means a spontaneous (galvanic) reaction, because ΔG = −nF...

Henry's law says that at constant temperature, the concentration of a gas dissolved in a liquid is directly proportional to that gas's partial pressure above the liquid. Doubling the partial pressure doubles the dissolved amount. It fits...