CHEMICAL LECTURE EXPERIMENTS

HYDROGEN

MODES OF PREPARATION

1. By the action of sodium or potassium upon water. Na+H2O=NaHO+H. A fragment of sodium or potassium, about the size of a pea, is thrown upon cold water contained in a soup plate. The plate should be instantly covered by a glass bell-jar when potassium is employed, as the molten globule of potassium hydrate which remains floating, in a spheroidal state, upon the water for a few seconds, will, as it cools, be scattered with some violence out of the plate.

If a fragment of sodium, instead of being thrown upon water, be placed upon a piece of wet filter paper in a plate, the heat of the reaction will rise sufficiently high to inflame the hydrogen, and in this case it must also be covered with a bell-jar.

2. To collect hydrogen from the action of sodium upon water, the following is the best method, and the only one which is free from danger. A piece of ordinary lead or composition pipe, about 25 millimetres long and about 4 millimetres bore, has one end closed by squeezing in a vice, or by a few taps with a hammer. A pellet of sodium is rolled between the fingers and pushed into this tube. It may be well forced in by pressing the mouth of the tube down upon the table, the excess of the metal being afterwards trimmed off with a knife. This tube with its

B

contents is then dropped into the pneumatic trough, in which it will sink, and hydrogen will be evolved in a steady and gentle stream. The mouth of an inverted glass cylinder can be brought

FIG. 1.

over the stream of uprising bubbles, and the gas so collected. If the little tubes are filled as described, it is impossible for any explosion to take place in the performance of this experiment. The filled tubes may be preserved in naphtha.

3. By the action of zinc-copper couple on water. Zn + H2O= ZnO+H. About 30 grams of zinc-dust is placed in a small flask of about 150 c.c. capa

The contents of coppered zinc is

city, and the flask filled up with a solution of copper sulphate (containing 40 grams of the salt to the litre). the flask are then briskly shaken together, the allowed to settle and the water poured off. The flask is then filled to the neck with distilled water, and a cork carrying a delivery tube is inserted; on heating the contents of the flask a slow stream of hydrogen is evolved. Zinc filings may be substituted for zinc-dust; but in this case, the action will be slower.

=

4. By the action of magnesium on steam. Mg + H2O MgO+H2. A strip of magnesium ribbon about 12 centimetres long is folded up and placed in a bulb, which is blown in the middle of a piece of combustion tube about 24 centimetres long. Each end of the tube is fitted with a cork carrying a short piece

of narrow tube. A current of steam is passed through the tube, which should be held in a clamp and inclined slightly downwards towards the incoming steam (fig. 2), and it is well to pass the steam through a small empty flask immediately before

it

enters the bulb-tube. While the steam is passing, the bulb-tube should be

warmed

throughout its whole length by means of a Bunsen flame, until it is hot enough to prevent condensation of the steam, and then

FIG. 2.

A

31

the lamp directed entirely upon the fragment of magnesium within the bulb. As the metal approaches to a red heat it will burst into flame, and if the supply of steam be regulated the hydrogen can be ignited as it issues from the extremity of the tube. The supply of steam may be easily regulated by partially withdrawing the lamp from the vessel in which the steam is being generated.

Instead of fitting corks to the bulb-tube, a most convenient plan is to use pieces of glass tube, so drawn out as to taper at

FIG. 3.

each end. A piece of caoutchouc slipped over such a tube will enable it to fit into tubes of various bores.

These connectors' are so useful for a variety of purposes that it will be found convenient to have a number of them, varying in size, ready for use.

A simple form of boiler for supplying steam for lecture