There was a piece of satire online: “According to incomplete statistics, the mortality rate caused by inhaling liquid dihydrogen monoxide approaches one in ten thousand each year, ranking among the top ten causes of unnatural death in humans; prolonged exposure to an environment with solid dihydrogen monoxide without protective gear can lead to tissue blackening, necrosis, and physiological dysfunction.” In reality, this refers to drowning and frostbite. The "dihydrogen monoxide" referred to here is water.
According to IUPAC regulations, there are only two official names: Water and Oxidane. Alternative names such as dihydrogen monoxide, hydrogen oxide, and hydroxic acid that appear online cannot be used in formal scientific contexts.
For a long time, water was considered an element by people. For example, ancient Greek philosophers regarded it as one of the "four elements" (water, fire, air, and earth). It wasn’t until the late 18th century that scientists gradually unveiled the mysteries of water and deciphered its composition.
The British scientist Joseph Priestley (1733–1804), often performed magic tricks for his friends. Once, he took a bottle containing “inflammable air” (hydrogen), shook it in front of his friends, and then quickly moved a lit candle close to the bottle. “Pop!” A long flame shot out from the bottle’s mouth but quickly extinguished, leaving many tiny droplets on the inner wall. In 1781, Henry Cavendish (1731–1810) learned about Priestley’s trick. Cavendish conducted experiments with mixtures of “inflammable air” and air in varying proportions and found that approximately two volumes of “inflammable air” combined completely with one volume of oxygen, confirming that the resulting droplets were water. However, both scientists were constrained by traditional beliefs and mistakenly explained the observed phenomena by suggesting that both gases contained water, an “element.” Later, the French scientist Antoine Lavoisier (1743–1794) conducted an experiment where he passed steam through a red-hot gun barrel and observed the production of “inflammable air.” From this, he concluded that water was not an element but a compound of “inflammable air” and oxygen, naming “inflammable air” as hydrogen. In 1800, the British scientists William Nicholson (1753–1815) and Anthony Carlisle (1768–1840) successfully electrolyzed water using a voltaic pile, further proving Lavoisier’s conclusion.
In middle school chemistry, the composition of water is studied by simulating Carlisle’s electrolysis experiment. A water electrolyzer’s glass tubes are filled with water, and direct current is applied. Bubbles can be observed forming at the electrodes, and after some time, the volume ratio of gases collected in the tubes connected to the positive and negative terminals of the power supply is approximately 1:2. The gas collected in the tube connected to the positive terminal can make a burning wooden stick burn more vigorously, proving it is oxygen. The gas collected in the tube connected to the negative terminal can be ignited by a burning wooden stick, producing a pale blue flame during combustion, proving it is hydrogen.
The electrolysis of water experiment proves that water is a compound composed of hydrogen and oxygen elements.
Image: Water Electrolyzer
Image: Testing the Products of Water Electrolysis
The volume ratio of hydrogen and oxygen produced by electrolyzing water is 2:1. Given that at 10.1 kPa and 0°C, the density of hydrogen is approximately 0.0899 g/L and that of oxygen is approximately 1.429 g/L, the mass ratio of hydrogen to oxygen produced can be calculated as 1:8. According to the law of conservation of mass, the mass ratio of hydrogen to oxygen in water is also 1:8. With the relative atomic mass of hydrogen being approximately 1 and that of oxygen approximately 16, the atomic ratio of hydrogen to oxygen in a water molecule can be calculated as 2:1—meaning one water molecule consists of two hydrogen atoms and one oxygen atom. Therefore, the chemical formula for water is H₂O.
Image: Water Molecule Model
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Reviewed by: Li Jiliang
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