Chemistry

Radium: properties, history, uses and health effects Radium Radium (Ra, atomic number 88) is a radioactive alkaline earth metal discovered by the Curies in 1898. This article surveys its properties, history, applications, health risks and i…

Radon (element Rn) — properties, occurrence, health effects and uses Radon Radon (Rn, atomic number 86) is a colorless, odorless noble gas produced by uranium and radium decay. It is radioactive, can accumulate indoors, and is a known lung-cancer risk; mi…

Rare earth element (REE): overview, properties, uses and supply Rare earth element Comprehensive overview of rare earth elements: classification, chemical properties, history, uses in technology and clean energy, geological distribution, supply chains, and enviro…

Rate equation (rate law) in chemical kinetics Rate equation A concise account of the rate equation (rate law): its form, interpretation, relation to mechanism, common integrated laws, and practical uses in chemistry and engineering.

Rate-Determining Step (Chemical Kinetics) Rate-determining step The rate-determining step is the slowest, highest-energy step in a multi-step chemical reaction; it controls overall rate, informs rate laws, and is central to catalyst and mechani…

Reaction An explanation of 'reaction' across disciplines: chemical, physical, biological, psychological and social meanings, with characteristics, examples, history, and distinctions.

Reaction inhibitor (chemical and biochemical) Reaction inhibitor A reaction inhibitor is a substance that reduces the rate of a chemical or biochemical reaction without becoming a permanent reactant. Common in industry, medicine and materials pr…

Chemical Reaction Intermediates: Transient Species in Multistep Reaction Mechanisms Reaction intermediate Concise overview of chemical reaction intermediates—transient molecules or ions formed between elementary steps. Covers lifetimes, detection and isolation methods, common types, an…

Reaction mechanism: stepwise pathways of chemical reactions Reaction mechanism Overview of how chemical reactions proceed through elementary steps: intermediates, transition states, electron flow, common organic mechanisms, experimental and computational meth…

Reactivity: general concept and applications across science and society Reactivity Reactivity describes how systems respond or change when exposed to stimuli. It spans chemistry, materials, biology, psychology, computing and markets, with different measures, risk…

Reactivity (chemistry) Reactivity in chemistry: how readily substances undergo chemical change, what controls it (thermodynamics, kinetics, conditions), common measures and examples, and why it matters i…

Reactivity series (activity series) of metals Reactivity series Ordered list of metals ranked by chemical reactivity. Explains principles, typical ordering, uses in predicting reactions and metal extraction, and limitations of the series.

Reagent — chemical substances used for detection, transformation and analysis Reagent A reagent is a substance used to provoke, detect or quantify chemical changes in analysis, synthesis and diagnostics. Types include indicators, titrants, precipitating agents and b…

Realgar — the red arsenic sulfide mineral Realgar Realgar (As4S4) is a soft, red arsenic sulfide mineral known for its photochemical alteration to yellow pararealgar, historical uses as a pigment and in fireworks, and its toxicity…

Rearrangement reaction (organic chemistry) Rearrangement reaction An organic process where atoms or groups relocate within a molecule to form structural isomers. Covers 1,2-shifts, pericyclic moves, Wagner–Meerwein shifts, allylic rearrangements…

Red (color) Red Red is the long-wavelength visible color, associated with light roughly 620–750 nm, used in art, signals and cultural symbolism, and distinct in additive vs subtractive color syste…

Redox (Reduction–Oxidation) Reactions: Principles, Examples, and Applications Redox Overview of redox reactions: definitions, oxidation states, electron transfer, half‑reactions, examples (rusting, combustion, respiration), electrochemistry, biological roles, and…

Reducing agent (reductant) Reducing agent A reducing agent (reductant) is a chemical species that donates electrons to another species in a redox reaction. This article explains behavior, common examples, uses, and how to…

Reduction: concepts, types and interdisciplinary applications Reduction Reduction is the process of making something smaller, simpler or more basic. The term appears across chemistry, cooking, philosophy, linguistics, medicine, history, environment, ec…

Reduction (chemistry) Reduction is the chemical process in which an atom, ion or molecule gains electrons and decreases its oxidation state; central to redox chemistry, metallurgy, corrosion and batteri…

Refining: Principles, Methods, Uses and Impacts Refining Refining is the set of industrial and food-industry processes that purify, separate or concentrate materials such as minerals, petroleum and edible oils; methods and impacts vary b…

Refrigerant — properties, refrigeration cycle, uses and environmental issues Refrigerant A refrigerant is a working fluid used in cooling systems. This article explains its physical properties, how it operates in the refrigeration cycle, common types, safety and enviro…

Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH) Registration, Evaluation, Authorisation and Restriction of Chemicals REACH is the EU regulation that manages chemical safety by requiring companies to register substances, assess risks, and follow evaluation, authorisation and restriction procedures…

Relative atomic mass: definition, calculation, and significance Relative atomic mass Relative atomic mass (Ar) is the dimensionless ratio of the average mass of atoms in a sample to 1/12 of a carbon‑12 atom; it depends on isotopic composition and underpins chemical…

Rhenium (element Re) Rhenium Rhenium is a rare, silvery-white transition metal (Re, atomic number 75) with an exceptionally high melting point. It is used mainly in jet-engine superalloys and as a catalyst in…

Rhodium: properties, occurrence, uses, and market significance Rhodium Rhodium (Rh, atomic number 45) is a rare, silvery transition metal of the platinum group used chiefly as a catalytic material, for plating and alloys, and valued for its scarcity a…

Rieke metals — highly activated, finely divided reactive metal powders Rieke metals Rieke metals are highly reactive, high-surface-area metal powders prepared by chemical reduction. They enable formation of organometallic reagents and challenging bond-forming reac…

Risk and Safety Statements (R‑ and S‑phrases) Risk and Safety Statements Codes used historically in chemical hazard communication in the EU and elsewhere: R (risk) phrases and S (safety) phrases that summarize key hazards and recommended precautions.

Rocket fuel (rocket propellant) Rocket fuel Rocket fuel, or propellant, is the chemical or gas used to produce thrust in rockets. Types include solid, liquid, hybrid and pressurized gas systems; choice affects performance, h…

Roentgenium (element 111) Roentgenium Roentgenium (Rg, Z=111) is a synthetic, superheavy transition metal named for Wilhelm Röntgen. It has no stable isotopes, is produced in accelerators, and is studied only in trace…

Room temperature Typical indoor temperature considered normal for human-occupied spaces; a context-dependent, informal term used across everyday life, science, and industry to indicate ambient cond…

Rotary evaporator (rotavap) Rotary evaporator Laboratory apparatus that removes solvents by controlled evaporation under reduced pressure; used to concentrate solutions, recover solvents and gently dry heat-sensitive materials…

Round-bottom flask: design, uses, and laboratory practice Round-bottom flask Spherical laboratory flasks with one or more necks used for heating, distillation, and synthesis. Typically borosilicate glass with ground joints; designed for even heating and vac…

Rubber: natural and synthetic elastomers Rubber Flexible, elastic materials derived from plant latex or synthesized polymers; used in tires, seals, gloves and many industrial products. Covers properties, production, uses, histor…

Rubidium — properties, history, occurrence and uses Rubidium Rubidium (Rb) is a soft, highly reactive alkali metal (atomic number 37) used in research, atomic clocks and specialty chemical processes. Discovered in 1861 by Bunsen and Kirchhof…

Rubidium chloride (RbCl) Rubidium chloride Rubidium chloride is an inorganic ionic salt of rubidium and chloride. It is an alkali metal halide used in research, some medical applications, and industrial chemistry; handling…

Rubidium hydride Rubidium hydride (RbH) is an ionic binary hydride of rubidium and hydrogen. It is a white crystalline solid, a strong base and reducing agent, prepared from rubidium metal and hydr…

Rubidium hydroxide (RbOH) Rubidium hydroxide An overview of rubidium hydroxide, its composition, physical and chemical properties, preparation methods, laboratory uses, hazards and how it differs from other alkali metal hydro…

Rubidium nitrate (RbNO3): properties, preparation, uses and safety Rubidium nitrate Rubidium nitrate is a colorless crystalline oxidizing salt (RbNO3) used as a laboratory reagent and precursor to other rubidium compounds; it is prepared by neutralizing rubidium b…

Rubidium oxide (Rb2O) Rubidium oxide Rubidium oxide (Rb2O) is an ionic alkali metal oxide: a yellow solid that reacts vigorously with water to form rubidium hydroxide. It is mainly a laboratory reagent and must be han…

Rust (iron oxide): causes, chemistry, prevention, and impacts Rust Rust is the common name for the corrosion of iron and its alloys. This article explains how rust forms, what affects it, common protections, and its practical and historical signif…

Ruthenium — properties, occurrence, uses, and history Ruthenium Ruthenium (Ru, atomic number 44) is a rare, silvery transition metal of the platinum group. Overview of its properties, occurrence in ores, main applications, history, and safety c…

Rutherfordium (element 104) Rutherfordium Rutherfordium (Rf, Z=104) is a synthetic, highly radioactive transactinide metal. It has short-lived isotopes, no commercial uses, and its observed chemistry is consistent with gro…

S-block elements of the periodic table s-block Overview of the s-block: definition, members (groups 1–2 plus hydrogen/helium issues), electron configuration, chemical and physical properties, common uses, history and notable ex…

Saccharide: types, structure, and biological roles Saccharide Saccharides, commonly called sugars, are organic compounds from simple monosaccharides to complex polysaccharides. They serve as energy sources, structural materials, and molecular…

Safety data sheet (SDS): purpose, content, and practical uses Safety data sheet A Safety Data Sheet (SDS) describes hazards, handling, storage, emergency measures and regulatory information for chemical products, helping workers and responders manage chemical…

Salicylic acid: properties, history, and common uses Salicylic acid Overview of salicylic acid: chemistry, natural sources, medical and cosmetic uses, formulations, mechanism, safety, and regulatory notes.

Saline Relating to salt or saltwater: covers saline solutions in medicine and labs, saline water and soils in nature, place names, uses, and distinctions.