hard

hard water

Water that contains mineral salts of calcium and magnesium, principally as bicarbonates, chlorides, and sulfates, and sometimes iron. Hardness caused by calcium bicarbonate is known as temporary, because boiling converts the bicarbonate to the insoluble carbonate; hardness from the other salts is called permanent. The calcium and magnesium in hard waters form a hard, adherent scale on boiler plates, increasing fuel consumption and leading to deterioration through overheating. Home water softeners consist of tanks containing zeolite minerals or ion-exchange resins, which contain sodium ions that change places with the calcium and magnesium.

Learn more about hard water with a free trial on Britannica.com.

or hard coal

Coal containing more fixed carbon than any other form of coal and the lowest amount of volatile (quickly evaporating) material, giving it the greatest heat value. The most valuable of the coals, it is also the least plentiful, making up less than 2percnt of all coal reserves in the U.S., with most of the known deposits occurring in the East. Anthracites are black and have a brilliant, almost metallic lustre. Hard and brittle, they can be polished and used for decorative purposes. They are difficult to ignite but burn with a pale-blue flame and require little attention to sustain combustion. In the past they were used for domestic heating, but today they have given way to other sources of energy (e.g., natural gas and electricity).

Learn more about anthracite with a free trial on Britannica.com.

(born July 28, 1915, Greenville, S.C., U.S.) U.S. physicist. He studied at Furman University, Duke University, and the California Institute of Technology and worked for Bell Labs before joining the faculty of Columbia University (1948). In the early 1950s he and his students constructed the first maser and showed that a similar device producing visible light was also possible. For his role in the invention of the maser and later the laser, he shared a 1964 Nobel Prize with Aleksandr M. Prokhorov (b. 1916) and Nikolay G. Basov (b. 1922).

Learn more about Townes, Charles H(ard) with a free trial on Britannica.com.

See NP-hard
Search another word or see hardon Dictionary | Thesaurus |Spanish
Copyright © 2014 Dictionary.com, LLC. All rights reserved.
  • Please Login or Sign Up to use the Recent Searches feature
FAVORITES
RECENT

;