Browse All 960 Chemistry Terms A–Z

Abnormal Molar Mass Solutions & Colligative Properties g/mol A molar mass value obtained from colligative measurements that differs from the true value because the solute associates or dissociates. M_normal = i × M_observed

Absolute Zero States of Matter & Gas Laws K The lowest possible temperature, at which particles retain only their irreducible quantum zero-point motion. 0 K = −273.15 °C

Accuracy Foundations of Chemistry Dimensionless The closeness of a measured value to the true or accepted value of the quantity being measured. % error = |measured − true| / true × 100

Acid Acids, Bases & Salts Dimensionless A substance that donates protons, produces hydrogen ions in water, or accepts a pair of electrons. HA → H⁺ + A⁻

Acid Dissociation Constant Acids, Bases & Salts mol/L The equilibrium constant for the ionisation of an acid in water, measuring its strength. Ka = [H⁺][A⁻] / [HA]

Acid Strength and Molecular Structure Acids, Bases & Salts Dimensionless The structural factors that determine how readily a molecule releases a proton.

Acidic Buffer Acids, Bases & Salts Dimensionless A buffer made from a weak acid and one of its salts, giving a pH below 7. pH = pKa + log([salt]/[acid])

Acidic Salt Acids, Bases & Salts Dimensionless A salt whose aqueous solution has a pH below 7, or one that still contains replaceable hydrogen.

Acidic Solution Acids, Bases & Salts mol/L A solution in which the hydrogen ion concentration exceeds the hydroxide ion concentration. [H⁺] > [OH⁻]

Acid–Base Indicator Acids, Bases & Salts Dimensionless A weak acid or base whose conjugate forms have distinctly different colours, used to signal the end point of a titration. HIn ⇌ H⁺ + In⁻

Acid–Base Titration Acids, Bases & Salts mol/L A titration in which an acid is neutralised by a base, or the reverse, monitored by an indicator or a pH meter. M_aV_a / n_a = M_bV_b / n_b

Activated Complex Chemical Kinetics & Catalysis kJ/mol The transient arrangement of atoms present at the transition state, poised between reactants and products.

Activation Energy Chemical Kinetics & Catalysis kJ/mol The minimum energy that colliding particles must possess for a reaction to occur. Ea = E(transition state) − E(reactants)

Active Site Chemical Kinetics & Catalysis Dimensionless The specific region of a catalyst or enzyme at which the reacting molecules bind and are transformed.

Activity Chemical Equilibrium Dimensionless The effective concentration of a species, corrected for non-ideal interactions with its surroundings. a = γ c / c°

Activity Coefficient Chemical Equilibrium Dimensionless The factor by which a species' concentration must be multiplied to give its thermodynamic activity. γ = a / c

Activity Series Stoichiometry & Chemical Reactions V A ranking of metals in order of their tendency to lose electrons and form positive ions.

Actual Yield Stoichiometry & Chemical Reactions g The mass of pure product actually isolated at the end of an experiment.

Addition Polymerisation Biochemistry & Polymer Chemistry Dimensionless Polymerisation in which unsaturated monomers add to one another with no loss of any atoms. n CH₂=CH₂ → –(CH₂–CH₂)ₙ–

Addition Reaction Stoichiometry & Chemical Reactions Dimensionless A reaction in which two molecules combine to form a single larger product, with no atoms left over. A + B → AB

Adenosine Triphosphate Biochemistry & Polymer Chemistry kJ/mol The nucleotide that acts as the universal energy currency of living cells. ATP + H₂O → ADP + Pᵢ

Adiabatic Process Thermochemistry & Thermodynamics J A process in which no heat is exchanged between the system and its surroundings. q = 0, ΔU = w

Adsorption Chemical Kinetics & Catalysis mol/g The accumulation of molecules of a gas or liquid on the surface of a solid.

Aerosol Solutions & Colligative Properties nm A colloidal system in which solid or liquid particles are dispersed in a gas.

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