168 



THE POPULAR EDUCATOR. 



LESSONS IN CHEMISTRY. VI. 



HYDEOGEN. 



STMBCL . 



H ATOMIC WEIGHT . 



1 DENSITY 



HYDROGEN is a permanent gas, without colour, taste, or smell. 

 It is never found free in nature, and is the lightest known body, 

 being 14'47 times lighter than the air. 



It was distinguished by Cavendish in 1766 as being one of 

 the constituents of water, from the decomposition of which the 

 gas is generally procured. 



PREPARATION. 



The metals of the alkalies (K, Na, etc.) and of the alkaline earths 

 (Ca, Sr, and Ba) have the power of decomposing water at ordinary 

 temperatures. If a piece of potassium be thrown on water, the 

 potassium takes the oxygen, forming potash, and the chemical 

 action is so vigorous, that sufficient heat is developed to set fire 

 to the escaping hydrogen. Care must be taken not to hold the 

 face too near when the flame has ceased ; for there remains a 

 globule of potash, which is in a melted state, and when it cools 

 down to such a temperature as to permit the water to come in 

 contact with it, steam is generated in a large quantity, and the 

 melted potash blown out of the water. 



The following equation expresses the action : 



H,0 + 2K = K,O + 2H. 



The potash (K Z O) is dissolved in the water, which then pos- 

 sesses the "greasy" feel of an alkaline liquid. It will "blue" 

 red litmus paper, and convert oil poured into it into soap. 



Fig. 22. 



If sodium be used instead of potassium, the metal will melt 

 into a globule, and run about on the water, being pushed by 

 the atoms of hydrogen escaping from its under-snrface ; but 

 the action is not violent enough to set fire to the gas. If, how- 

 ever, the water be thickened with starch, so as to increase the 

 vigour of the action, the hydrogen will be lit. In this case 

 soda (Na0) and hydrogen are the products of the reaction. 



To collect the gas in these cases, fill a test-tube with water, 

 place the thumb over its end, and then invert it in a bowl of 

 water. Wrap a small piece of the metal in paper, and quickly 

 pass it under the mouth of the test-tube, which is, of course, 

 beneath the surface of the water. The gas thus collected will 

 exhibit the properties of hydrogen. To prepare this gas in large 

 quantities, some iron turnings are placed in a porcelain tube, a b 

 (Fig. 22), which passes through a charcoal furnace. Steam is 

 generated in the flask, B, and as it passes over the red-hot iron it 

 is deprived of its oxygen, which forms with the iron the same 

 compound as was produced when the iron was burnt in oxygen 

 (Fe s O 4 , the magnetic oxide) : thus the action will be 



3Fe + 4H a O = 



4 + 8H. 



Zinc, tin, and some other less important metals, also have this 

 power of decomposing water at a high temperature. 



But the property which these metals likewise possess of de- 

 composing water in the presence of an acid is taken advantage 

 of in the laboratory for obtaining the gas. A " Woulffs bottle " 

 with two necks as is shown in Fig. 23 is used, or a bottle 

 with one wide neck, into which a cork is fitted, pierced with 

 two holes. The long tube with a funnel is a " safety tube," 

 by which the acid is added ; the other tube is the " delivery 

 tube," from which the gas is conducted into a receiver, as in the 

 case of oxygen. 



On account of the lightness of the gas, it is frequently collected 



by " displacement," as in this diagram. The hydrogen rises to 

 the top of the jar, and displaces the air. 



To prepare the experiment, some granulated zinc which is 

 obtained by pouring melted zinc into cold water is placed in 

 the bottle. Water is added, and 

 then sulphuric acid (1 part of the 

 acid to 8 of water) gradually. Bub- 

 bles of the gas rapidly rise. Iron 

 may be used ; but it generally con- 

 tains carbon, and the gas comes off 

 mixed with some compound of 

 carbon and hydrogen, and is ren- 

 dered explosive. The action is thus 

 represented : 



Zn + H a S0 4 = ZnSO* + H,. 



Thus zinc sulphate (white vitriol) 

 is formed, which is dissolved in 

 the water. If the water be evapo- 

 rated slowly, this salt crystallises 

 out in white needles. 



The decomposition of the water 

 will be explained to be due to the 

 agency of electricity. Hydrogen 

 may also be set free from hydro- 

 chloric acid (HC1) and zinc : thus 



Zn + 2HC1 = ZnCl, + 2H, 

 forming zinc chloride and hydrogen. 



When hydrogen burns, it of course forms with the oxygen 

 water. If into a jar full of hydrogen a burning taper be intro- 

 duced, it will be extinguished ; but the gas will burn with a 

 pale yellow flame at the place where it meets the oxygen of the 

 air. During the experiment the jar must be held mouth down- 

 wards, and the disc of yellow flame gradually passes up it. 



The gas may be lit as it escapes from the generating jar. For 

 this purpose the delivery tube must be drawn to a point. If 

 over this flame a dry jar be held, the water formed in the com- 

 bustion will be deposited as dew on the sides of the jar. 



A singular property of the hydrogen flame may be shown by 

 allowing it to burn in a tube of glass or metal: a musical 

 note is produced ; the explanation being, that the flame of 

 hydrogen is in reality a series of rapid explosions, which cause 

 the air in the tube to vibrate, thus producing the note. 



In lighting the gas, the greatest care must be taken to allow 

 sufficient time for all the air to be expelled from the bottle. 

 Should there be any oxygen in it, the mixture will explode with 

 great violence ; and in all cases it is a safe preservative to 

 wrap a damp cloth round the bottle, which will prevent the glass 

 from flying, should there be an accident. To show the lightness 

 of the gas, a balloon of gold-beater's skin may be fastened to 

 the delivery tube, which will rise when it becomes filled. If the 

 india-rubber balloons be used, it will be necessary to pass the 



Fig. 23. 



Fig. 24. 



gas into an india-rubber bag, and then into the balloon, for the 

 pressure of the gas escaping from the bottle is not sufficient to 

 distend the balloon. The same experiment may be shown by 

 blowing soap-bubbles with the gas. 



In the laboratory hydrogen is frequently employed as a re- 

 ducing agent. If we heat to redness in an atmosphere of 

 hydrogen metallic oxides, chlorides, iodides, bromides, or sul- 

 phides, the gas combines with the oxygen, chlorine, iodine, 



