Chemistry Definitions Starting With the Letter R

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This chemistry dictionary offers the chemistry definitions starting with the letter R. These glossary terms are commonly used in chemistry and chemical engineering. Click the letter below to find the terms and definitions beginning with that letter.


racemic – Racemic is a mixture of equal parts left- and right-handed enantiomers of a chiral molecule.
Also known as: racemic mixtures, racemate

rad – Rad is a unit of measuring absorbed radiation of matter.
1 rad = 0.01 J of energy per kg of matter.
Rad is also an abbreviation for radian.

radian – A radian is an angle with vertex at the center of a circle with radius r that encompasses an arc of length r. There are 2π radians in a circle.
1 radian = 57.29577951 degrees

radiant energy – Radiant energy is the energy contained in electromagnetic radiation. The SI unit of radiant energy is the Joule.

radiant flux – Radiant flux is the measure of the amount of radiant energy per unit time. The SI unit for radiant flux is the watt.

radiant intensity – Radiant intensity is the measure of the intensity of electromagnetic radiation. Radiant intensity is measured as power per solid angle. The SI unit of radiant intensity is watt per steradian, W·sr-1.
Radiant intensity is the measure of ‘how bright’ a light source is.

radiation – Radiation is the emission and propagation of energy in the form of waves, rays or particles.
Examples: A burning candle emits radiation in the form of heat and light. Uranium-238 decaying into Thorium-234 emits radiation in the form of alpha particles. Electrons dropping from one energy state to a lower state emit radiation in the form of a photon.

radiation absorbed dose – Radiation absorbed dose is a unit of measuring absorbed radiation in matter. The Radiation absorbed dose, or rad is defined as 0.01 joules of energy absorbed per kilogram of matter. The Radiation absorbed dose does not describe any biological effects of the absorbed radiation.

radical – A radical is a molecule containing an unpaired free electron.
Also known as: free radical

radioactivity – Radioactivity is the spontaneous emission of radiation in the form of particles or high energy photons resulting from a nuclear reaction.

radioactive – Radioactive refers to a substance that causes or exhibits behavior due to radioactivity.

radioactive tracer – A radioactive tracer is a radioactive element or compound added to a material to monitor the material’s distribution as it progresses through a system.

radiochemistry – Radiochemistry is the study of the chemistry of radioactive elements.

radioisotope – A radioisotope is an isotope of an element that is radioactive.
Example: Carbon-14 is a beta-emitting radioisotope of carbon.

radionuclide – Radionuclide is another name for radioisotope.

radio waves – Radio waves are electromagnetic waves between 3000 Hz and 3 x 109 Hz. These frequencies correspond to wavelengths from 10 cm. to 100,000 km.

random copolymer – A random copolymer is a polymer comprised of two or more different mer units attached in a random order.

radium – Radium is the name for the alkaline earth element with atomic number 88 and is represented by the symbol Ra. Radium can be used to make surfaces glow in the dark.

radon – Radon is the name for the noble gas element with atomic number 86 and is represented by the symbol Rn.

random – In science and statistics, random refers to a selection process in which each choice or event has an equal probability of being selected or each outcome has an equal probability of occurring. Random events do not follow any pattern, planning, direction, or system.
Synonyms: randomness, chance
Examples: If you flip a coin, it is random whether you will get “heads” or “tails” since there is an equal probability of each event. When you roll a pair of dice, the number each die lands on is random. The outcome for one die is independent of the outcome of the other die.

Rankine – Rankine is an absolute temperature measurement based on the Fahrenheit temperature scale. The Rankine scale is named after William Rankine who first proposed it in 1859. The symbol for degrees Rankine is °R and 1 °R = 1 °F.
Absolute zero is 0 °R or -491.67 °F
Conversion factors:
(°R ↔ °F): °R = °F + 459.67
(°R ↔ °C): °R = (9/5)°C + 491.67
(°R ↔ K): °R = 9/5 K

Rankine Temperature Scale – Same as Rankine. See Rankine definition above.

Raoult’s Law – Raoult’s Law is a law that relates the vapor pressure of a solution is dependent on the mole fraction of a solute added to the solution. Raoult’s Law is expressed by
Psolution = ΧsolventP0solvent
Psolution is the vapor pressure of the solution
Χsolvent is mole fraction of the solvent
P0solvent is the vapor pressure of the pure solvent
If more than one solute is added to the solution, each individual solvent’s component is added to the total pressure.

rare earth – Rare earth is an oxide of a rare earth element.

rare earth element – Rare earth elements are a group of elements including the lanthanides, scandium, and yttrium. There are 17 rare earth elements. Rare earth elements are often abbreviated as REE.
Synonyms: Rare Earth Metals

rate determining step – A rate determining step is the step in a multi-step chemical reaction with the slowest rate of reaction. This step’s rate will set the overall rate of reaction for the multi-step reaction.

rayon – Rayon is a commercial polymer fiber made from cellulose mer units.

reactant – Reactants are the starting material in a chemical reaction.
Examples: H2 and O2 are reactants in the reaction H2(g) + ½ O2(g) → H2O(l).

reaction – A reaction or chemical reaction is a chemical change which forms new substances.
Example: The chemical reaction H2(g) + ½ O2(g) → H2O(l) describes the formation of water from its elements.
Also known as: chemical reaction, chemical change

reaction coordinate – A reaction coordinate is an abstract coordinate used to measure the progress of a chemical reaction. As a reaction progresses, reactants become products. To measure this progress, a reaction coordinate is chosen from a measurable coordinate such as bond length, bond angle, a combination of bond length and bond angle, or bond order.
Reaction coordinates are often plotted against energy to show an energy profile of a reaction. This profile will show the initial energy of the reactants, the activation energy of the reaction, and the final energy of the products.
Also known as: transition coordinate

reaction quotient – Q – Reaction quotient is the ratio of the concentrations of the products of a reaction to the concentrations of the reactants. Each concentration is raised to the power of the stoichiometric coefficient in the chemical formula. In general, for the reaction:
aA + bB → cC + dD
The reaction quotient, Q is
Q = [C]c[D]d/[A]a[B]b

reaction rate – The reaction rate is the rate at which the reactants of a chemical reaction form the products. Reaction rates are expressed as concentration per unit time.

reagent – A reagent is a compound or mixture added to a system to cause a chemical reaction or to test if a reaction occurs.
The term reagent is often used in place of reactant, but a reagent may not necessarily be consumed in a reaction with a reactant. For example, a catalyst is a reagent but is not consumed in the reaction.

real gas – A real gas is a gas that does not behave as an ideal gas due to interactions between gas molecules.
Also known as: nonideal gas

rearrangement reaction – A rearrangement reaction is a type of reaction where the atoms of a molecule are rearranged to form a new isomer of the original molecule.

receptor – A receptor is a molecule or surface on a cell that binds to or reacts with a messenger molecule to initiate a biological response.

reciprocal rule – The reciprocal rule is the relationship between equilibrium constants for forward (Kf) and reverse (Kr) reactions such that
Kr = 1/Kf.

redox indicator – A redox indicator is an indicator compound that changes color at specific potential differences. A redox indicator compound must have a reduced and oxidized form with different colors and the redox process must be reversible.
Example: The molecule 2,2′-Bipyridine is a redox indicator. In solution, it changes from light blue to red at an electrode potential of 0.97 V.

redox reaction – A redox reaction is any chemical reaction which involves the processes of oxidation and reduction.

redox titration – Redox titration is titration of a reducing agent by an oxidizing agent or titration of an oxidizing agent by a reducing agent.

reducing agent – A chemical species which provides electrons for another chemical species.
Also known as: reduction agent, reductant, reducer
Example: Metallic zinc, Zn(s), is the reducing agent in the reaction: Zn(s) + 2 H+(aq) → Zn2+(aq) + H2(g).

reductant – Reductant is another term for reducing agent. See definition above.

reduction – Reduction is a half-reaction in which a chemical species decreases its oxidation number, usually by gaining electrons.
Example: The H+ ions, with an oxidation number of +1, are reduced to H2, with an oxidation number of 0, in the reaction: Zn(s) + 2 H+(aq) → Zn2+(aq) + H2(g).

reduction agent – A reduction agent is a reactant that donates electrons to other reactants during a redox reaction. The reduction agent is said to be oxidized when it gives its electrons to the oxidizing reactant.
Also known as: reductant, reducer, reducing agent
Example: Hydrogen is a reducing agent when reacted with oxygen to form water.

reduction half reaction – A reduction half-reaction is the part of a redox reaction that involves a gain of electrons.
Example: For the redox reaction Zn(s) + 2 H+(aq) → Zn2+(aq) + H2(g), the reduction half-reaction is 2 H+(aq) + 2 e → H2(g).

reductone – A reductone is an enediol compound with a carbonyl group (RC(=O)R’) attached to the enediol group (HO-C=C-OH).

refrigerant – A refrigerant is a compound that can readily absorb heat at one temperature, then compressed by a heat pump to a higher temperature and pressure where it changes phase and discharges the absorbed heat.
Example: Nearly all chlorofluorocarbons are refrigerants.

regelation – Regelation is a phenomenon where a substance melts under pressure and freezes again when the pressure is reduced.

relative density – Relative density is the dimensionless ratio of the densities of two materials. It is similar to specific gravity, in which the reference material is water.

relative error – Relative error is a measure of the uncertainty of measurement compared to the size of the measurement.
Also known as: relative uncertainty
Example: Three weights are measured at 5.05 g, 5.00 g, and 4.95 g. The mean mass is 5.00 g. The absolute error is ± 0.05 g.
The relative error is 0.05 g/5.00 g = 0.01 or 1%.

relative humidity – A measure of the amount of water in air divided by the amount of water that the air could hold, expressed as 100 x P/P0, where P is the pressure of the water vapor in the air and P0 is the equilibrium vapor pressure of water at the same temperature.
Example: A relative humidity of 95% indicates that there is 95/100ths as much water in the air as that air is capable of holding at that temperature.

relative standard deviation – Relative standard deviation is a measure of precision in data analysis. Relative standard deviation is calculated by dividing the standard deviation of a series of values by the average of the values.

relative uncertainty – Relative uncertainty is another name for relative error. See definition above.

rem – A unit of absorbed radiation in biological tissue. Rem is equal to n times the number of rads, where the factor n is dependent on the type of radiation which is being absorbed.
Rem is also an acronym for rare earth metal.

rennet – Rennet is any preparation containing the enzyme rennin.
Rennet is also the name for the curdled milk from the fourth stomach or dried extract made from the stomach lining of an unweaned ruminant. Rennet is used as a coagulant in cheese production. Rennet consists mainly of hydrochloric acid, mucin and the enzymes pepsin, rennin and lipase.

rennin – Rennin is an endopeptidase enzyme in the gastric juice of the fourth stomach of unweaned ruminants that coagulates milk by converting caseinogen to casein to paracasein. Rennin from calves was used as a coagulating agent to make cheese, although modern cheese-making uses artificially produced microbial rennet. Rennin is a component of rennet.
Although not used in the production of cheese, rennin is produced by non-ruminant mammals, too, such as human babies.
Also known as: rennet, chymosin
Common misspellings: renin – also an enzyme, but produced by the kidney and used to regulate blood pressure.

residue – Residue can mean many different things in chemistry.

  1. The matter left in a container after evaporation or distillation has occurred.
  2. The undesirable byproduct of a chemical reaction.
  3. A recognizable molecular part of a larger molecule. For example, an amino acid is the residue of a larger protein chain.

resin – Resin is secretion of plants containing hydrocarbons and terpenes. Resin also applies to any chemical additive that sets into a hard enamel or lacquer finish.

resonance – Resonance in Chemistry is used to explain properties of the octet rule when a single Lewis structure is inadequate. Resonance structure is an average of two of more Lewis structures which differ only in the position of their electrons. These species are said to exhibit resonance.

reverse osmosis – Reverse osmosis is a process in which purified water is obtained from a salt solution. Water is moved across the membrane against the concentration gradient, from lower concentration to higher concentration. To illustrate, imagine a semipermeable membrane with fresh water on one side and a concentrated aqueous solution on the other side. If normal osmosis takes place, the fresh water will cross the membrane to dilute the concentrated solution. In reverse osmosis, pressure is exerted on the side with the concentrated solution to force the water molecules across the membrane to the fresh water side.
Reverse osmosis is often used in commercial and residential water filtration. It is also one of the methods used to desalinate seawater. Sometimes reverse osmosis is used to purify liquids in which water is an undesirable impurity (e.g., ethanol).

reversible reactions – A reversible reaction is a chemical reaction where the reactants form products that, in turn, react together to give the reactants back. Reversible reactions will reach an equilibrium point where the concentrations of the reactants and products will no longer change.
A reversible reaction is denoted by a double arrow pointing both directions in a chemical equation. For example, a two reagent, two product equation would be written as
A + B ↔ C + D

rhenium – Rhenium is the name for the transition metal element with atomic number 75 and is represented by the symbol Re.

rhodium – Rhodium is the name for the transition metal element with atomic number 45 and is represented by the symbol Rh.

ring molecule – A ring molecule is a molecule made up of a series of atoms bonded together to form a ring or circle.
Examples: Benzene, cyclopentene, and imidazole are all ring molecules.

RNA – RNA is the acronym for ribonucleic acid. Forms of RNA include messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA). RNA codes for amino acid sequences, which may be combined to form proteins. Where DNA is used, RNA acts as an intermediary, transcribing the DNA code so that it can be translated into proteins.

roasting – Roasting is a process in metallurgy in which a sulfide ore is heated in air. The process may convert a metal sulfide to a metal oxide or to a free metal.
Examples: Roasting ZnS may yield ZnO; roasting HgS may yield free Hg metal.

rodenticide – A rodenticide is a compound designed to kill rodents.
Also known as: rat poison, rat killer

roentgen – Roentgen is a unit of measurement for the amount of ionizing radiation that produces 2.1 x 109 coulombs of charge in 1 cm3 of air at STP. In SI units, 1 Roentgen (R) = 2.58 x 10-4 C/kg.
Also known as: röntgen

Roentgen Equivalent Man (rem) – Roentgen Equivalent Man, or REM for short, is a measure of absorbed dose of radiation in biological tissue.
rem = n·rad where rad is the radiation absorbed dose and n is a factor unique to the type of incident radiation being absorbed.
Also known as: Roentgen Equivalent Mammal

roentgenium – Roentgenium is the name for the transition metal element with atomic number 111 and is represented by the symbol Rg. The roentgenium name was approved in 2004 to replace the placeholder name unununium (Uuu).

room temperatureRoom temperature is a range of temperatures that denote comfortable habitation for humans. There is no ‘exact’ room temperature. Room temperature can be any temperature between 20 °C and 29 °C. 300 K can also be used as room temperature for easy calculations when using absolute temperature.

rosin – Rosin is a natural resin boiled down from pine trees and other conifers.
Also known as: colophony

RT – RT stands for room temperature. The abbreviations RT, rt, or r.t. are commonly used in chemical equations to denote the reaction can be run at room temperature.

rubidium – Rubidium is the name for the alkali metal element with atomic number 37 and is represented by the symbol Rb.

RuBisCO – RuBisCO is the shorthand name for ribulose-1,5-bisphosphate carboxylase oxygenase, the enzyme used in the first step of carbon fixation. RuBisCO catalyzes the carboxylation of ribulose-1,5-bisphosphate (RuBP). Many consider RuBisCO to be the most abundant protein in the world.

rust – Rust is a general term for the oxides of iron.

ruthenium – Ruthenium is the name for the transition metal element with atomic number 44 and is represented by the symbol Ru.

rutherfordium – Rutherfordium is the name for the transition metal element with atomic number 104 and is represented by the symbol Rf.