hydrogen

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A colorless, highly flammable gaseous element, the lightest of all gases and the most abundant element in the universe, used in the production of synthetic ammonia and methanol, in petroleum refining, in the hydrogenation of organic materials, as a reducing atmosphere, in oxyhydrogen torches, and in rocket fuels. Atomic number 1; atomic weight 1.00794; melting point -259.14°C; boiling point -252.8°C; density at 0°C 0.08987 gram per liter; valence 1. See Table at element.

Century Dictionary and Cyclopedia

1. n. Chemical symbol, H. One of the elementary substances, existing as a colorless, tasteless, and inodorous gas. It is the lightest substance known, and for that reason its specific gravity has been taken as the unit for comparing the specific gravity of gases, though air is the more commonly accepted standard. Under like conditions of temperature and pressure, hydrogen is approximately 14.4 times as light as an equal volume of air. Its combining weight is also less than that of any other element, and is therefore called unity, all the other atomic weights being expressed as multiples of it. It is but slightly soluble in water or any other liquid. Hydrogen refracts light strongly, is extremely diffusible, and is absorbed or occluded in a remarkable manner by certain metals when they are heated, as though it formed a kind of alloy with them. Hydrogen burns in air with a very pale blue flame and intense heat, the sole product of combustion being water, H₂O, which is the protoxid of hydrogen. A mixture of two volumes of hydrogen and six of air or one of oxygen explodes violently when brought in contact with a flame or the electric spark. Hydrogen is not specifically poisonous when inhaled, but is fatal to life by preventing or hindering access of oxygen to the blood. It is prepared by the action of dilute sulphuric acid on zinc or iron, by passing steam through a red-hot tube filled with iron turnings, by the electrolysis of water, and in a variety of other ways. Hydrogen occurs free in nature in small quantity in the emanations of volcanoes and of some oil-wells, but generally it is found only in its combinations, which are universally distributed. One ninth of the weight of water consists of hydrogen, and it is an indispensable element of every animal or vegetable structure. It is a component of all acids, and its replacement in them by bases produces salts. In December, 1877, and January, 1878, the French chemists Cailletet and Pictet succeeded in liquefying hydrogen, and the latter in solidifying it, by means of extreme pressure and cold produced in special forms of apparatus independently invented by them.
2. n. H₂S, a colorless inflammable gas having a sweetish taste and an exceedingly fetid smell resembling rotten eggs. It is extremely poisonous when inhaled. It has feeble acid properties, and its compounds with bases are called sulphids. It occurs in the emanations of volcanoes, and is evolved when animal or vegetable tissue containing sulphur decays. It also occurs in mineral springs, being liberated by the reduction of gypsum or other sulphates through the action of a microbe.
3. n. Hydrogen compounds with strongly electronegative elements or radicals, easily exchanging hydrogen for strongly electropositive elements or radicals to form salts, are the same as acids: as hydrogen chlorid (hydrochloric acid), hydrogen sulphate (sulphuric acid), etc.

Wiktionary

1. n. The lightest chemical element (symbol H) with an atomic number of 1 and atomic weight of 1.00794.
2. n. Molecular hydrogen (H2), a colourless, odourless and flammable gas at room temperature.
3. n. An atom of the element.
4. n. A sample of the element.

GNU Webster's 1913

1. n. (Chem.) A gaseous element, colorless, tasteless, and odorless, the lightest known substance, being fourteen and a half times lighter than air (hence its use in filling balloons), and over eleven thousand times lighter than water. It is very abundant, being an ingredient of water and of many other substances, especially those of animal or vegetable origin. It may by produced in many ways, but is chiefly obtained by the action of acids (as sulphuric) on metals, as zinc, iron, etc. It is very inflammable, and is an ingredient of coal gas and water gas. It is standard of chemical equivalents or combining weights, and also of valence, being the typical monad. Symbol H. Atomic weight 1.

WordNet 3.0

1. n. a nonmetallic univalent element that is normally a colorless and odorless highly flammable diatomic gas; the simplest and lightest and most abundant element in the universe