Browse All 960 Chemistry Terms A–Z

Period Atomic Structure & Periodicity Dimensionless A horizontal row of the periodic table, corresponding to one principal quantum number of the valence shell.

Periodic Law Atomic Structure & Periodicity Dimensionless The principle that the properties of the elements are periodic functions of their atomic numbers.

Periodic Table Atomic Structure & Periodicity Dimensionless The tabular arrangement of the chemical elements by increasing atomic number, grouping those with similar properties in vertical columns.

Perturbation and Re-equilibration Chemical Equilibrium s The disturbance of an equilibrium followed by its relaxation to a new equilibrium state.

pH Acids, Bases & Salts Dimensionless The negative logarithm to base ten of the hydrogen ion concentration, measuring the acidity of a solution. pH = −log[H⁺]

pH Scale Acids, Bases & Salts Dimensionless The conventional scale, usually running from 0 to 14, on which the acidity or alkalinity of aqueous solutions is expressed. 0 (acidic) – 7 (neutral) – 14 (alkaline)

Phase Diagram States of Matter & Gas Laws Pa, K A pressure–temperature plot showing which phase of a substance is thermodynamically stable under any given conditions.

Phase Equilibrium Chemical Equilibrium Dimensionless The condition in which two or more phases of a substance coexist with no net transfer between them. μ_phase1 = μ_phase2

Phenol Organic Chemistry Dimensionless A compound in which a hydroxyl group is bonded directly to an aromatic ring. C₆H₅–OH

Phenolphthalein Acids, Bases & Salts Dimensionless An acid–base indicator that is colourless in acid and pink in alkaline solution, changing between pH 8.3 and 10.0. C₂₀H₁₄O₄

Phospholipid Biochemistry & Polymer Chemistry Dimensionless A lipid in which one fatty acid of a triglyceride is replaced by a phosphate-containing group.

Photochemical Quantum Yield Chemical Kinetics & Catalysis Dimensionless The number of molecules reacting for each photon of light absorbed by the system. Φ = molecules reacted / photons absorbed

Photochemical Reaction Stoichiometry & Chemical Reactions nm A reaction initiated by the absorption of light, which promotes a molecule to an electronically excited state. A + hν → A*

Photosynthesis Biochemistry & Polymer Chemistry kJ/mol The process by which plants use light energy to convert carbon dioxide and water into carbohydrate and oxygen. 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Physical Change Foundations of Chemistry Dimensionless A change that alters the form or state of a substance while leaving its chemical composition intact.

Physical Equilibrium Chemical Equilibrium Dimensionless An equilibrium between two physical states or phases of the same substance. H₂O(l) ⇌ H₂O(g)

Physical Property Foundations of Chemistry Various A characteristic of a substance that can be observed or measured without altering its chemical identity.

Physisorption Chemical Kinetics & Catalysis kJ/mol Weak adsorption in which molecules are held to a surface by van der Waals forces alone. ΔH ≈ −20 to −40 kJ/mol

Pi Bond Chemical Bonding & Molecular Structure kJ/mol A covalent bond formed by the sideways overlap of parallel p orbitals, with electron density above and below the internuclear axis.

pKa Acids, Bases & Salts Dimensionless The negative logarithm of the acid dissociation constant, giving a convenient scale of acid strength. pKa = −log Ka

pKb Acids, Bases & Salts Dimensionless The negative logarithm of the base dissociation constant. pKb = −log Kb

Plasma States of Matter & Gas Laws K A state of matter consisting of a hot, electrically conducting mixture of ions and free electrons.

Plum Pudding Model Atomic Structure & Periodicity Dimensionless J. J. Thomson's 1904 model picturing the atom as a sphere of diffuse positive charge with electrons embedded within it.

pOH Acids, Bases & Salts Dimensionless The negative logarithm of the hydroxide ion concentration. pOH = −log[OH⁻]; pH + pOH = 14

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