Combination of Hydrogen With Oxygen

Until the end of the 18th century, water and air were considered chemical elements in their own right.

Around 1781, Priestley, Watt, and Cavendish recognized that hydrogen produces water when it burns in air. In 1789, Lavoisier demonstrated that water is composed of hydrogen and oxygen, and that these two gases, when combined, form an amount of water equal to the sum of their weights.

Analysis and synthesis of water

Water is proven to consist of oxygen and hydrogen:

1. By igniting a jet of dry hydrogen beneath a bell jar, you can see water droplets forming on its inner surface, increasing in number as long as combustion continues.
2. By bringing water into contact with metals that decompose it, either at low temperatures, such as potassium, or at high temperatures, such as iron, tin, etc. During this decomposition, the oxygen in the water combines with the metals and the hydrogen is released.
3. By decomposing water using a battery, oxygen moves to the positive pole and hydrogen to the negative pole. The volume of the first gas is approximately half that of the second.

A represents a battery, B a glass vessel with two platinum wires passing through its bottom, covered by small graduated bell jars filled with water.

This liquid is made into a good conductor of electricity by adding a little sulfuric acid. As soon as each platinum wire is brought into contact with the battery's rheophores (particularly flexible metal cords covered with silk), the water decomposes; oxygen moves to the positive pole C and hydrogen to the negative pole D.

These experiments having demonstrated that water consists of hydrogen and oxygen, the exact ratio of its two elements remains to be established.

Eudiometric analyses (a method of analyzing gases through combustion) carried out with the utmost care by Gay-Lussac and de Humboldt demonstrate that water consists of two volumes of hydrogen and one volume of oxygen.

Indeed, if you introduce 200 volumes of hydrogen and 200 volumes of oxygen into a eudiometer and pass an electric spark through the mixture, the gases combine immediately, a layer of moisture forms on the instrument's inner surface, and 100 volumes of pure oxygen remain in the eudiometer, which can be completely absorbed by phosphorus. This experiment therefore shows that 200 volumes of hydrogen combined with 100 volumes of oxygen to form water.

The composition of water by weight is precisely established through synthesis by reducing a known weight of an oxide (copper oxide, for example) using pure, dry hydrogen; the composition of water is reduced from the weight of the water formed during the operation.

Let P be a known weight of copper oxide and P’ its weight after reduction (that is, the weight of the copper). P - P’ gives the weight of the oxygen contained in the oxide. By weighing the water produced precisely and subtracting the weight of the oxygen P - P’ from the weight of this water, the remainder gives the amount of hydrogen combined with the oxygen to form water.

The weight of the two elements that make up water has thus been determined.

The preceding method was first applied to the synthesis of water by Berélius and Dulong. These chemists found that 100 g of water consisted of 88.91 g of oxygen and 11.09 g of hydrogen. The method used by Berélius and Dulong was later refined by Dumas. Here we will provide some details about the apparatus Dumas used to determine the composition of water through synthesis. Dumas focused primarily on completely purifying the hydrogen and producing, in each experiment, a quality of water that generally amounted to 50 or 60 grams.

Hydrogen obtained by the ordinary method, using zinc, water, and commercial sulfuric acid, may contain nitrous vapors, sulfurous acid, carbureted hydrogen, sulfuric acid, and arsenical hydrogen. To obtain hydrogen in a state of absolute purity, it is essential to use sulfuric acid free of sulfurous acid and nitric acid.

The other foreign substances come from the carbon, sulfur, and arsenic often present in small quantities in commercial zinc. The gas is stripped of them as it passes through three U-shaped tubes filled with glass fragments moistened with various solutions: carbureted hydrogen is retained by caustic potash; hydrogen sulfide by lead nitrate; arsenical hydrogen by silver sulfate or mercuric chloride.

The hydrogen is dried as it passes through a resin of tubes containing sulfuric acid and anhydrous phosphoric acid separated by fragments of pumice stone. The copper oxide is placed in a flask made of very hard glass, capable of withstanding red heat for an entire day without deforming. The flask is heated using a powerful alcohol lamp.

The flask containing the oxide is first subjected to prolonged dehydration; it is evacuated before the operation begins. The water formed during the experiment condenses in a flask, then in a series of drying tubes designed to retain the water vapor carried along by the excess hydrogen.

The synthesis of water takes place in several successive steps:

1. Releasing hydrogen into the apparatus to flush out the air
2. Weighing the flask filled with copper oxide and free of air
3. Weighing the apparatus intended to retain the water
4. Adjusting the apparatus
5. Reduction
6. Cooling the flask while maintaining the flow of hydrogen
7. Weighing the cold flask free of hydrogen
8. Flushing the hydrogen out of the apparatus containing the water using a stream of dry air.

Dumas's numerous experiments show that water consists of 100 g of oxygen for every 12.50 g of hydrogen.

The composition of water in hundredths then becomes: Oxygen: 88.888; Hydrogen: 11.112: WATER: 100.000.

The composition of water can also be deduced from the densities of oxygen and hydrogen.

We have seen that water consists of two volumes of hydrogen and one volume of oxygen: 0.13852 = 2 times the density of hydrogen; 1.10563 = 1 times the density of oxygen. These numbers have the same ratio as 12.50 and 100. To determine the relationship between the volume of water and the volumes of the gases that make it up, you simply need to compare the density of water vapor with the sum of the density of oxygen and twice the density of hydrogen.

The density of water vapor determined by Gay-Lussac is 0.624, and according to Mr. Regnault, 0.622.

Adding twice the density of hydrogen = 0.43582 to the density of oxygen = 1.10563 gives a sum of 1.24445, which is exactly equal to twice the density of water vapor.

We therefore see that one volume of oxygen and two volumes of hydrogen represent two volumes of water vapor, or that one volume of water vapor consists of one volume of hydrogen and half a volume of oxygen.

In summary, water contains, in hundredths, 88.888 oxygen; 11.112 hydrogen.

In equivalents, it consists of one equivalent of oxygen = 100 and one equivalent of hydrogen = 12.50.

In volumes, it contains two volumes of hydrogen and one volume of oxygen, representing two volumes of water vapor.