1,800+ MCAT terms, defined.

Temperature measures the average kinetic energy of the particles in a sample, and it sets the direction of heat flow (hot to cold). For an ideal gas, average translational kinetic energy is proportional to absolute temperature, so gas pr...

Electrons are negatively charged subatomic particles that occupy quantized orbitals around the nucleus. Their arrangement, especially the valence electrons, sets bonding, periodic trends and redox behavior. Transition metal cations lose...

The equivalence point of a titration is where the moles of added titrant exactly equal the moles of analyte, as the reaction stoichiometry requires. Its pH is 7 for a strong acid with a strong base, above 7 for a weak acid with a strong...

A base is a species that accepts a proton (Brønsted-Lowry), raises [OH⁻] in water (Arrhenius), or donates an electron pair (Lewis). Most questions use the Brønsted-Lowry view: a stronger base has a larger Kb and a weaker conjugate acid.

Entropy (S) measures how many microscopic arrangements are available to a system, often described as disorder or energy dispersal. In a spontaneous process the entropy of the universe increases, but the system's own entropy can fall if t...

An acid is a species that donates a proton (Brønsted-Lowry), raises H⁺ in water (Arrhenius), or accepts an electron pair (Lewis). The MCAT most often uses Brønsted-Lowry: a stronger acid has a larger Ka, a lower pKa and a more stable con...

Osmotic pressure is the pressure that must be applied to a solution to stop net flow of solvent into it across a semipermeable membrane. It is colligative (Π = iMRT), so it depends on particle concentration, not identity. Water flows tow...

pH is the negative base-10 logarithm of the hydrogen ion concentration: pH = −log[H⁺]. Lower pH means more acidic, and each pH unit is a tenfold change in [H⁺]. At 25 °C, pH 7 is neutral.

An orbital is a wavefunction describing an electron in an atom, whose square gives the probability of finding the electron at a location. Each orbital holds at most two electrons, and they must have opposite spins. A p subshell has 3 orb...

pKa is −log Ka, where Ka is the acid dissociation constant, so a lower pKa means a stronger acid. The most testable point: when pH = pKa, the acid and its conjugate base are present in equal concentrations, as at the half-equivalence poi...

Raoult's law says that in an ideal solution, the partial vapor pressure of a component equals its mole fraction in the liquid times its pure vapor pressure. A nonvolatile solute lowers the solvent's vapor pressure. Real solutions deviate...

The anode is the electrode where oxidation occurs, in every type of electrochemical cell. Its sign differs by cell type: negative in a galvanic cell, positive in an electrolytic cell. Electrons always leave the anode through the external...

The cathode is the electrode where reduction occurs, so species gain electrons there. This is true in every electrochemical cell. Its sign is what changes: positive in a galvanic cell, negative in an electrolytic cell.

Concentration is the amount of solute per unit of solution (or solvent), most often molarity: moles of solute per liter of solution. Diluting a solution lowers its concentration while the moles of solute stay the same, which is the idea...

Solubility is the maximum amount of solute that dissolves in a given amount of solvent at a set temperature, giving a saturated solution at equilibrium. Most testable: for a salt, solubility is linked to Ksp, and a common ion lowers it.

The Heisenberg uncertainty principle says a particle's position and momentum cannot both be known with unlimited precision at the same time. The product of their uncertainties has a minimum value of h/4π. This is why electrons are descri...

Gibbs free energy (G = H − TS) is the energy available to do useful work at constant temperature and pressure. Its change, ΔG = ΔH − TΔS, predicts direction: ΔG < 0 is spontaneous (exergonic), ΔG > 0 is nonspontaneous, and ΔG = 0 is equi...

A titration curve plots the pH of the analyte against the volume of titrant added. The steep rise marks the equivalence point, which is at pH 7 only for a strong acid with a strong base. For a weak acid, pH = pKa at the half-equivalence...

A quantum number is one of four values (n, l, ml, ms) that together specify an electron's state in an atom: its shell, subshell shape, orbital orientation, and spin. The most testable point is that no two electrons in an atom can have th...

The Aufbau principle says that in an atom's ground state, electrons generally fill orbitals from lowest to highest energy (1s, 2s, 2p, 3s, 3p, 4s, 3d...). The most testable point is that 4s fills before 3d. A few elements, such as Cr and...

Enthalpy (H) is the heat content of a system, H = U + PV. At constant pressure, the heat absorbed or released equals ΔH: negative is exothermic, positive is endothermic. It is a state function, so ΔH depends only on initial and final sta...

The Nernst equation gives a cell's potential under nonstandard conditions: E = E° − (RT/nF) ln Q. A higher product-to-reactant ratio Q lowers E, and at equilibrium Q = K and E = 0. In a concentration cell E° = 0.

A catalyst speeds up a reaction by providing an alternative pathway with a lower activation energy, and it is regenerated rather than consumed in the net reaction. It does not change ΔG, ΔH, or Keq, so it speeds the forward and reverse r...

A phase change is a transition of a substance between solid, liquid and gas, such as melting or boiling. For a pure substance at constant pressure, temperature stays constant during the change because the heat goes into intermolecular at...

An electrolytic cell uses an external power source to drive a nonspontaneous redox reaction (E°cell < 0, ΔG > 0). Oxidation is still at the anode and reduction at the cathode, but the anode is positive and the cathode negative, the rever...

A buffer is a solution containing a weak acid and its conjugate base (or a weak base and its conjugate acid) that resists large pH changes when small amounts of strong acid or base are added. It works best when pH is within about 1 unit...

An indicator is a weak acid or base whose protonated and deprotonated forms have different colors, so its color signals pH in a titration. It changes color near pH = its pKa, so the best choice has a pKa close to the pH at the equivalenc...

Intermolecular forces are attractions between separate molecules (or between ions and molecules), much weaker than the covalent bonds within a molecule. For molecules of similar size, strength usually rises from London dispersion to dipo...

The solvent is the component of a solution that dissolves the others (the solutes), usually the one present in the largest amount. By "like dissolves like," polar solvents such as water dissolve ionic and polar solutes, and nonpolar solv...

Boiling point elevation is the rise in a solvent's boiling point when a nonvolatile solute dissolves in it. It is a colligative property, so it depends on the number of dissolved particles (i × molality), not their identity. At equal mol...

Ionization energy is the energy required to remove an electron from a gaseous atom or ion in its ground state. The first ionization energy generally increases across a period and decreases down a group, with exceptions such as B below Be...

A strong acid dissociates essentially completely in water, so Ka is much greater than 1 and, for a monoprotic acid, [H₃O⁺] equals the acid concentration. Examples are HCl, HBr, HI, HNO₃, HClO₄ and H₂SO₄ (first proton). Strong describes d...

The ideal gas law, PV = nRT, links the pressure, volume, amount and absolute temperature of a gas whose particles have negligible volume and no attractions. Most testable: T must be in kelvin. Real gases deviate most at high pressure and...

The Arrhenius equation, k = Ae^(−Ea/RT), gives a reaction's rate constant in terms of temperature and activation energy. Raising T or lowering Ea increases k, and k depends exponentially on T, not linearly.

An electrochemical cell links a redox reaction to electron flow through an external circuit. Galvanic cells run spontaneously (E°cell > 0) and make current; electrolytic cells use a power source to drive a nonspontaneous reaction. Oxidat...

Insoluble means a substance dissolves so little in a solvent, usually water, that it is treated as not dissolving. It is a relative label for a very small Ksp, not zero solubility. The MCAT tests it through solubility rules and by compar...

Ksp is the solubility product constant: the equilibrium constant for a slightly soluble ionic solid dissolving into its ions. Comparing the ion product Q with Ksp predicts whether a precipitate forms (Q > Ksp) or more solid can dissolve...

Reduction is the gain of electrons by a species, which lowers its oxidation number. It always occurs at the cathode, in galvanic and electrolytic cells alike, and the species reduced is the oxidizing agent. In organic chemistry it often...

A titration finds an unknown concentration by adding a solution of known concentration until the reaction is stoichiometrically complete (the equivalence point). Testable point: equivalence means equal moles of H⁺ and OH⁻ have reacted, b...

Isotopes are atoms of the same element (same number of protons, same atomic number Z) with different numbers of neutrons, so different mass numbers A. They behave almost identically in chemistry but differ in mass, and some are radioacti...

Hess's law says the enthalpy change of an overall reaction equals the sum of the enthalpy changes of its steps, because enthalpy is a state function. Most testable: reversing a step flips the sign of ΔH, and scaling the coefficients scal...

Chemical equilibrium is the state of a reversible reaction where forward and reverse rates are equal, so concentrations stay constant (a dynamic state, not a stopped reaction). Most testable: Le Châtelier's principle predicts the directi...

The equilibrium constant, Keq, is the ratio of product to reactant concentrations, each raised to its coefficient, once a reversible reaction reaches equilibrium. Most testable: temperature changes Keq, but added reactant, pressure chang...

The common ion effect is the decrease in solubility of a sparingly soluble salt, or in ionization of a weak acid or base, when the solution already contains an ion the species produces. Le Châtelier's principle explains it: the added ion...

Colligative properties are solution properties that depend on the number of dissolved solute particles, not their identity: vapor pressure lowering, boiling point elevation, freezing point depression, and osmotic pressure. Most testable:...

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

A mole is the SI amount of substance containing 6.022 × 10²³ elementary entities (atoms, molecules, ions). It links the atomic scale to the lab scale: the molar mass in g/mol is numerically equal to the mass of one atom or molecule in amu.

A solute is the substance dissolved in a solvent to form a solution, and it is usually the component present in the smaller amount. Most testable: colligative properties depend on the number of dissolved solute particles, so a dissociati...

Activation energy (Ea) is the minimum energy reactants must have to reach the transition state and react. A catalyst lowers Ea without changing ΔG, ΔH, or Keq, so it speeds up the forward and reverse reactions.

Valence electrons are the electrons in an atom's outermost occupied shell, and they do the bonding. For main-group elements the count equals the group's ones digit (group 1 has 1, group 17 has 7; helium has 2). Counting them correctly is...