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When electric sparks are passed through steam, it is decomposed to a slight extent into hydrogen and oxygen: 2H2O <=> 2H2 + O2. The dissociation of steam by heat increases with the temperature. The following table gives the percentage dissociation i.e., the number of molecules decomposed out of every 100 molecules of steam.
| T° abs. | 10 atm. | 1 atm. | 0.1 atm. | 0.01 atm. | | 1000 | 1.39x10-5 | 3.00x10-5 | 6.46x10-5 | 1.39x10-4 | | 1500 | 1.03x10-2 | 2.21x10-2 | 4.76x10-2 | 0.103 | | 2000 | 0.273 | 0.588 | 1.26 | 2.70 | | 2500 | 1.98 | 3.98 | 8.16 | 16.6 |
At the melting point of platinum (1755°) and 760 mm. pressure, about 6 molecules of steam in every thousand are dissociated. At 7.6 mm. pressure this number increases to 27.
Water is also decomposed when the liquid is exposed to short-wave ultra-violet light. At first only hydrogen is evolved, the oxygen probably forming hydrogen peroxide: 2H2O = H2 + H2O2. After a time oxygen is also evolved: 2H2O2 = 2H2O + O2. Water is also decomposed by the α-rays from radium emanation.
The thermal dissociation of steam was discovered by Grove (1847) who heated a platinum wire electrically in steam, passed sparks through steam, and plunged the fused end of a platinum wire into water. In 1863, Deville poured more than a kilogram of fused platinum into water, and found that detonating gas was freely evolved. By passing a stream of moist carbon dioxide through a porcelain tube heated at 1300° and absorbing the gas in potash, he obtained 25 c.c. of detonating gas in two hours.
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