sists of 2 atoms), then the density will always equal one-half the molecular weight. All elements, excepting P, As, Hg, and Cd, contain 2 atoms to a molecule; hence their atomic weights represent the density of elements in a gaseous state. In case of compounds like CO, :— C=12, 0=16 .'. CO2 = 12+16 +16=44, and the density is 22 times as heavy as hydrogen. 1 litre of H at standard temperature and pressure weighs 0.0896 gm. Hence to find the weight of any volume of any gas, multiply one-half its molecular weight by 0.0896 by the number of litres contained in a given volume; result weight in gms. = It is derived from hudor, water, and genesis, generation, meaning "water-former." Hydrogen was named by Lavoisier in 1775, although discovered by Cavendish in 1766. How does hydrogen occur? It is found FREE in volcanic gases, in natural gas from gas-wells, in meteoric iron (occluded), in the atmosphere of the sun as determined by the spectroscope, and in other self-luminous bodies. It is found COMBINED in water and in an infinite number of organic compounds, such as coal, petroleum, the tissues of plants and animals, etc. How may hydrogen be prepared? (1) By decomposing water with metallic sodium; 66 an electric current; metals. Give the formula for each. (1) 2H,O+2Na=2NaHO+H2; (2) 2H,O dissociated by electricity into 2H,+01; (4) Zn+H,SO=ZnSO+H. What are the physical properties of hydrogen? In a pure state it is a gas, tasteless, odorless, and colorless. Being the lightest substance known, it is taken as the standard of weight for gases. One litre at 0° C. and 760 mm. pressure weighs 0.0896 gram. This weight is called a crith. At a temperature of -140° C. and a pressure of 650 atmospheres, to which hydrogen was subjected by M. Pictet, it becomes liquefied. What are the chemical properties of hydrogen? It is an inflammable gas, burning with intense heat and a pale blue flame. One gram of burning hydrogen produces heat sufficient to raise 34,462 grams of water from 0° to 1° C. It is not a poisonous gas, but it does not support respiration. When inhaled it causes the voice to become shrill. Hydrogen burned in the air, thus uniting with the oxygen, forms water: H2+0=H2O. It is an effective reducing_agent, capable of withdrawing oxygen from its heated compounds. Hydrogen passed over red-hot copper oxide unites with the oxygen, leaving the copper: CuO+H2 = Cu+H2O. What are the compounds of hydrogen with oxygen? There are but two compounds of hydrogen and oxygen. The first is water, H2O, or hydrogen monoxide; the second is the dioxide of hydrogen, H2O2, used for bleaching purposes. All hydrates contain both oxygen and hydrogen; as, calcium hydrate, Ca(OH)2; sodium hydrate, NaOH. The compound radical (OH) is called hydroxyl. Every acid contains hydrogen. Its important compounds with other elements are with nitrogen, carbon, sulphur, phosphorus, and numerous others. Give the derivation of its name and its history. Atomic weight 35.5. Chlorine was discovered by Scheele in 1774, and named from chloros, green, on account of its color. How does it occur? It is an abundant element, and is found combined as chlorides in common salt or sodium chloride, chlorides of the alkalies and alkaline earths, in sea-water, and rarer compounds as mineral cerargyrite or silver chloride. How is it prepared ? By the action of sulphuric acid on common salt and manganese dioxide: : 2H2SO4+2NaCl + MnO, = 2(H2O) + MnSO4 + Na„SO + Cl2; or, by the action of hydrochloric acid upon manganese dioxide :— 4(HCl) + MnO2 = MnCl2+2(H2O) + Cl2. What are the physical properties of chlorine? It is a greenish-yellow gas of intensely suffocating odor, condensing by cold and pressure to a heavy, yellow liquid. What are its chemical properties? Under the influence of light it unites directly and violently with hydrogen, producing hydrochloric acid gas. Cold water absorbs two and one-half times its volume of this gas, forming chlorine-water. When moist it acts as a bleaching agent by seizing the hydrogen of water and liberating the oxygen, which bleaches by direct oxidation. Finely divided copper, antimony, or arsenic placed in the gas combines, evolving heat and light, to produce a chloride. Hydrogen burns freely in chlorine and chlorine in hydrogen. What are its uses? It is used as a bleaching agent for linen, cotton, and paper, and as a disinfectant. What are its important compounds? With hydrogen it forms hydrochloric acid, HCl; with oxygen it forms oxides, as hypochlorous oxide, Cl,O; chlorous oxide, Cl,O,; and perchloric oxide, Cl2O.. : With oxygen and hydrogen it gives rise to several oxy-acids : Hypochlorous acid, HCIO; Chlorous acid, HCIO2; Chloric acid, HCIÓ,; Perchloric acid, HCIO. With nitrogen: NC; and with carbon: C2C1, C2C1, C2Cl。. HYDROCHLORIC ACID, HCl. What other names are given to hydrochloric acid? It is often called muriatic acid or chlorhydric acid. How is it prepared? It is produced for commerce by treating a chloride with sulphuric acid. Chloride of sodium is generally employed :— 2NaCl + H2SO, Na,SO, + 2HC1. = What are the physical properties of this acid ? It is a gas very soluble in water, one volume of which at 15° C. will dissolve about 450 volumes of gas, resulting in the liquid acid as it appears in commerce. The pure acid is colorless; the yellowish color often seen is due to impurities of iron. What are its chemical properties? It is one of the strongest and most important acids, dissolving many metals with the evolution of hydrogen : Zn + 2HCl = ZnCl2 + H2 ; Fe+ 2HCl= FeCl2 + H2. What is aqua regia? It is a mixture of hydrochloric and nitric acids, sometimes termed royal water. It is important on account of its power to dissolve gold and platinum, two metals which resist the action of any single acid. The acids unite, liberating chlorine : : HNO,+3HCl=2H2O + NOCl (nitrosyl chloride) + Cl.. How is chlorine applied as a bleaching agent? Chlorine passed over slaked lime produces chloride of lime or hypochlorite of calcium, one of the most applicable compounds of chlorine for bleaching and disinfecting : 2CaH,O,+2C1, CaCl,O,+ 2H2O+ CaCl,. = Bleaching powder is a mixture of calcium chloride and hypochlorite of calcium. Exposed to heat, air, and moisture it yields hypochlorous acid (HCIO), and soon breaks up into water, calcium chlorate, and free chlorine. How may hydrochloric acid be detected? Any soluble chloride put with silver nitrate gives a curdy precipi tate: HCl + AgNO,HNO, +AgCl; The oxides of chlorine are all dangerously explosive. How is hypochlorous acid (HClO) prepared? It is obtained by passing chlorine gas through water containing calcium carbonate (chalk) in suspension: Cl + H2O+CaCO3 = CaCl,+CO2+ 2HCIO. What are its properties? An aqueous solution has a yellow color, acid taste, and sweet smell. It is a powerful oxidizing and bleaching agent. Give the preparation and properties of chlorous acid, HC1O,. It is prepared by reducing chloric acid with arsenic trioxide and nitric acid. The result is a greenish-yellow liquid having strong bleaching and oxidizing properties. Give the method and preparation of chloric acid, HClO. It is prepared from barium chlorate and sulphuric acid, or by passing chlorine through a solution of potassium hydrate : - 3(Cl)+6KHO=KCIO,+5KCl+3(H,O). |