Glossary of chemical terms

racemate
	Racemate is a mixture of equal quantities of the d- and l-forms of an optically active compound. A racemic mixture is not optically active.
Raman effect
	Raman effect is a type of scattering of electromagnetic radiation in which light suffers a change in frequency and a change in phase as it passes through a material medium. Is named according to the Indian physicist C. V. Raman (1889-1970). The intensity of Raman scattering is about one-thousandth of that in Rayleigh scattering in liquids.
Rankine cycle
	Rankine cycle is a thermodynamic cycle which can be used to calculate the ideal performance of a heat engine that uses a condensable vapor as the working fluid.
Raoult's law
	Raoult's law is the expression for the vapor pressure pA of component A in an ideal solution, viz., pA = pAo xA where xA is the mole fraction of component A and pAo the vapor pressure of the pure substance A.
rare earth elements
	Rare earth elements are the elements Sc, Y, and the lanthanides (La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu). These elements got their name from the fact that chemists first isolated them in their oxide forms. These oxides somewhat resemble calcium, magnesium and aluminum oxides, sometimes called common earths.
Rayleigh scattering
	Rayleigh scattering is the scattering of light by particles which are much smaller than the wavelength of the light.
relative atomic mass
	Relative atomic mass (Ar) is the ratio of the average mass per atom of the naturally occurring form of an element to 1/12 of the mass of nuclide 12C. The term atomic weight is synonymous with relative atomic mass.
relative density
	Relative density is the ratio of the density of a substance to the density of some reference substance. For liquids or solids it is the ratio of the density (usualy at 20 °C) to the density of water at 4 °C. This quantity was formerly called specific gravity.
relative humidity
	Relative humidity is the ratio of the partial pressure of water vapour in air to the saturation vapour pressure of water at the same temperature, expressed as a percentage.
relative molecular mass
	Relative molecular mass (Mr) is the ratio of the average mass per molecule or specified entity of a substance to 1/12 of the mass of nuclide 12C. Also called molecular weight. It is equal to the sum of the relative atomic masses of all the atoms that comprise a molecule. For example Mr(H2SO4) = 2×Ar(H) + Ar(S) + 4×Ar(O) = 2×1.0079 + 32.066 + 4×15.999 = 2.0158 + 32.066 + 63.996 = 98.078
rem
	Rem (rem) (roentgen equivalent man) is a non-SI unit of dose equivalent employed in radioprotection (rem = 10-2 Sv).
resistance
	Resistance (R) is electric potential difference divided by current when there is no electromotive force in the conductor. This definition applies to direct current. More generally, resistance is defined as the real part of impedance.
Reynolds number
	Reynolds number (Re) is a dimensionless quantity used in fluid mechanics, defined by Re = ρ vl/η where ρ is density, v is velocity, l is length, and η is viscosity.
rheology
	Rheology is the study of the deformation and flow of materials. Has important bearing on the behavior of viscous liquids in plastic molding.
ribonucleic acids
	Ribonucleic acids (RNA) is naturally occurring polyribonucleotides that is concerned with protein synthesis. Four types are recognized: messenger RNA (mRNA), ribosomal RNA (rRNA), transfer RNA (tRNA), and viral RNA.
RNA
	RNA is abbreviation for ribonucleic acid.

 

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