CHAPTER III

HYDROGEN

20. Preparation of hydrogen from water. Fill a test tube with water and invert it in a beaker of water. Wrap a piece of sodium, the size of a small pea, in a bit of filter paper previously moistened with coal oil. Raise the inverted test tube until its mouth dips just below the surface of the water in the beaker, and quickly insert the sodium. Stand at arm's length, as a slight explosion sometimes occurs. Notice that the sodium decomposes the water, liberating a gas which is caught in the tube. Holding the tube mouth downward, test the gas by quickly inserting a burning splint. Does the gas act like oxygen? What is the source of it? What other methods may be employed for obtaining it from the same source?

21. Preparation from sodium hydroxide. Provide a hardglass test tube with a stopper and a delivery tube, as shown in Fig. 16. Finely powder about 2 g. of solid sodium hydroxide in a mortar and mix it with a little more than an equal weight of zinc dust. Place the mixture in the bottom of the test tube, clamping the tube in a nearly horizontal position, the bottom being slightly higher than the mouth (why?). Heat the mixture carefully, collecting the gas evolved in a test tube by displacement of water. Test it with a lighted splint as in § 20. What is the source of the gas?

22. Preparation from acids. a. In clean test tubes place samples of the following metals: iron, tin, lead, magnesium, aluminium, zinc, and copper. Prepare some dilute hydrochloric acid by adding about 20 cc. of the concentrated reagent to an equal volume of water. Pour 4 or 5 cc. of the dilute acid in each of the test tubes, gently warming the solution

if no action is noticed. In which cases do you observe the formation of gas bubbles? Where do they form? Do not mistake bubbles of air or steam for the gas. How can you tell the difference? Does the condition of the metal make any difference (compare the action of the acid on a tack with its action on iron filings)? Do all of the metals yield hydrogen with acids ?

b. Repeat the experiment, replacing the hydrochloric acid with nitric acid diluted with an equal volume of water. In what cases is a gas evolved? What differences do you note as compared with a? In each case test the gas with a lighted

splint. Is it hydrogen? Do the metals which give hydrogen with hydrochloric acid give it with all acids?

c. Select two test tubes and in each place a few pieces of mossy zinc, adding enough water to cover the metal. To one test tube add a few drops of a solution of copper sulfate. What change do you notice? Now add to each about 1 cc. of hydrochloric acid. Which evolves hydrogen the more rapidly? What is the function of the copper sulfate (§ 14)?

23. Usual laboratory method. Arrange a hydrogen generator according to Fig. 18, in which D represents a wide-mouthed bottle of about 250-cc. capacity. The gas delivery tube B, C is the same as that used in the preparation of oxygen (Fig. 16). The funnel tube A must extend nearly to the bottom of the bottle (why ?). Put 10 g. of mossy zinc (why mossy

zinc ?) into D and add a few drops of a solution of copper sulfate. Pour just enough water through the funnel tube to cover the zinc. Prove that the apparatus is air-tight by blowing into the delivery tube until the water is forced nearly to the top of the funnel tube; then quickly close the rubber tube either by tightly pinching it or by placing the tongue firmly against its end. If the apparatus is air-tight, the water in the funnel tube will not fall (why?). Prepare some dilute sulfuric acid by slowly pouring 15 cc. of concentrated acid into a beaker containing 50 cc. of water. Stir the water with a glass rod while the acid is being added. Notice that the acid is poured into the water· never the reverse. Cool the mixture and pour a few drops of it through the funnel tube. Hydrogen is at once evolved. Enough of the acid must be added from time to time to cause a gentle and continuous evolution of the gas. It is evident that the first gas which passes over is a mixture of hydrogen and air. The student must remember that such a confined mixture of hydrogen and air or hydrogen and oxygen, if ignited, explodes with great violence. On this account see that the end of the delivery tube is not brought near any flame. Determine when the hydrogen is free from air by collecting test tubes full of gas and igniting it, holding the tube mouth downward. If pure, the gas burns quietly, otherwise there is a slight explosion. After all the air has been expelled from the generator, collect four bottles (250-cc., wide-mouthed) of the gas. What is the source of the hydrogen? What is the use of the zinc? Why is the copper sulfate solution added? Remove the stopper from the generator, add a few more fragments of zinc, and set the generator aside until the gas ceases to be evolved. Sufficient zinc should be used so that at least a small portion of it remains undissolved. Filter the liquid from the undissolved zinc into an evaporating-dish and evaporate it to dryness. What is left in the dish? After the dish is cool, dissolve the residue in as little hot water as possible and set it aside until crystals are deposited. Compare these with crystals of zinc sulfate.