Science / Chemistry Glossary

Trueness: Trueness is the closeness of an average measurement to a 'true' value, while accuracy is the the closeness of a single measurement to the true value.

Tyndall Effect: Light passing through a colloid is scattered by suspended particles. The light beam becomes clearly visible; this phenomenon is called the Tyndall effect. For example, car headlight beams ca . . . View Full Definition

Ultraviolet Light: Electromagnetic radiation with wavelength longer than that of x-rays but shorter than that of visible light. Ultraviolet light can break some chemical bonds and cause cell damage.

Uncertainty Principle: The exact momentum and exact location of a particle cannot be specified. Werner Heisenberg stated that the product of uncertainties in location and momentum measurements can never be smaller . . . View Full Definition

Unimolecular Reaction: A reaction that involves isomerization or decomposition of a single molecule.

Unit: A standard for comparison in measurements. For example, the meter is a standard length which may be compared to any object to describe its length.

Unit Cell: The simplest arrangement of atoms or molecules that regularly repeats in a crystal structure.

Universal Indicator: A universal indicator is an indicator which undergoes several color changes over a wide range of ph. The color is used to 'indicate' ph directly. Universal indicators are usually mixtures of . . . View Full Definition

Unpaired Spin: A single electron occupying an orbital.

Unsaturated Compound: An organic compound with molecules containing one or more double bonds.

Unsaturated Fat: A lipid containing one or more carbon-carbon double bonds. Unsaturated fats tend to be oily liquids and are obtained from plants.

Unsaturated Solution: A solution with a concentration lower than its equilibrium solubility.

Vacuum: A volume which contains no matter.

Valence: The number of hydrogen atoms that typically bond to an atom of an element. For example, in H2O, oxygen has a valence of 2; carbon in CH4 has a valence of four.

Valence Bond: In the valence bond theory, a valence bond is a chemical bond formed by overlap of half-filled atomic orbitals on two different atoms.

Valence Bond Theory: A theory that explains the shapes of molecules in terms of overlaps between half-filled atomic orbitals, or half filled 'hybridized' orbitals (which are a mixture of atomic orbitals).

Valence Electron: Electrons that can be actively involved in chemical change; usually electrons in the shell with the highest value of n. For example, sodium's ground state electron configuration is 1s2 2s2 2 . . . View Full Definition

Valence Shell: The shell corresponding to the highest value of principal quantum number in the atom. The valence electrons in this shell are on average farther from the nucleus than other electrons; they a . . . View Full Definition

Valence Shell Electron Pair Repulsion Theory: A model that explains the shapes of molecules by assuming that electron pairs arrange themselves around atoms in a way that minimizes electron-electron repulsions.

Van Der Waals Equation: A semiempirical equation that describes the relationship between pressure (P), volume (V), temperature (T), and moles of gas (n) for a real gas. The equation is (P + n2a/V2)(V - nb) = nrt, w . . . View Full Definition

Van Der Waals Force: A force acting between nonbonded atoms or molecules. Includes dipole-dipole, dipole-induced dipole, and London forces.

Van Der Waals Radius: One half the distance between two nonbonded atoms, when attractive and repulsive forces between the atoms are balanced.

Vapor Pressure: The partial pressure of a gas in equilibrium with a condensed form (solid or liquid) of the same substance.

Vapor Pressure Lowering: A colligative property of solutions. The vapor pressure of a solution is always lower than the vapor pressure of the pure solvent; the ratio of solution to pure solvent vapor pressures is ap . . . View Full Definition

Variable: A quantity that can have many possible values. In designing experiments, variables that affect measurements must be identified and controlled. For example, an experiment that measures reacti . . . View Full Definition

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