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Academia - Collins - Pocket Scientific Dictionaries - Chemistry - English - French - Arabic, letter U


ultraviolet
rayonnement m ultraviolet

ultraviolet. Invisible radiation with a frequency (energy) slightly higher than that of violet light. This radiation is produced in large quantities by stars, such as the Sun. It is harmful to humans if exposed to large quantities of it, and may lead to skin cancer. Not much of the solar ultraviolet radiation reaches the Earth, as the ozone layer in the upper atmosphere absorbs most of it.


units
units fpl

Units. Quantities or measurements used as standards for measuring other things. See SI units.

Measurement ---- Unit

Distance----meters
Time----seconds
Mass----Kilogram


universal indicator
undicated universe

Comprehensive index. A mixture of indicators. Because it is a mixture, it changes color many times as the pH of the solution changes. The approximate pH of the solution can be determined by adding a few drops of the universal indicator to it and reading the pH (AH) from a list such as the one shown below.

There are also paper tapes for the comprehensive indicator:
Color <- red orange yellow green blue purple -->

(Ah) 14 13 12 11 10 9 8 7 6 5 43210


unsaturated compounds
Composés mpl non-saturés

Unsaturated compounds. Carbon compounds that have a double bond or triple bond between two carbon atoms. Examples include:

Alkenes
Daughters-in-law

These compounds contain more electrons in their bond than in a normal single bond, and therefore they are reactive and tend to react through addition reactions, forming new bonds with the electrons they possess, so alkenes become alkanes, for example.

etan (saturated)
Aten (unsaturated)


uranium
uranium m

Uranium. A metallic element that has three naturally occurring isotopes (masses: 234 at a very small percentage, 235 at 0.7%, and 238 at 99.3%). All of these isotopes are radioactive, and they eventually decay to give stable isotopes of lead.

Uranium is used as fuel in nuclear stations to generate electricity. U308 oxide is used in some types of these stations, and uranium containing a higher percentage of 295-U (enriched uranium) is used in others. Uranium 238 is converted into plutonium 239 in the fast breeder reactor, which can then be used as fuel.