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- ultraviolet light. ultraviolet; ultraviolet radiation; ultraviolet region; UV.
- 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. Heisenberg's uncertainty principle; Heisenberg principle; indeterminancy; indeterminancy 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 than h/4, where h is Planck's constant.
- unimolecular reaction.
- A reaction that involves isomerization or decomposition of a single molecule.
- 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 several indicators.
- unpaired spin. () unpaired electron. Compare with paired spin.
- A single electron occupying an orbital.
- unsaturated compound.
- An organic compound with molecules containing one or more double bonds.
- unsaturated fat. Compare with saturated 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. Compare with saturated solution.
- A solution with a concentration lower than its equilibrium solubility.