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Lettris is a curious tetris-clone game where all the bricks have the same square shape but different content. Each square carries a letter. To make squares disappear and save space for other squares you have to assemble English words (left, right, up, down) from the falling squares.
Boggle gives you 3 minutes to find as many words (3 letters or more) as you can in a grid of 16 letters. You can also try the grid of 16 letters. Letters must be adjacent and longer words score better. See if you can get into the grid Hall of Fame !
Change the target language to find translations.
Tips: browse the semantic fields (see From ideas to words) in two languages to learn more.
|Jmol-3D images||Image 1
|Molar mass||2.02 g mol−1|
|Density||67.8 kg·m−3 (4.23 lb./cu.ft)|
−259.14 °C (−434.45 °F, 14.01 K)
−252.87 °C (−423.17 °F, 20.28 K) 
|571 °C (1060 °F)|
|Explosive limits||LEL 4.0 %; UEL 74.2 % (in air)|
| (what is: / ?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
To exist as a liquid, H2 must be pressurized above and cooled below hydrogen's critical point. However, for hydrogen to be in a full liquid state without boiling off, it needs to be cooled to 20.28 K (−423.17 °F/−252.87°C) while still pressurized. One common method of obtaining liquid hydrogen involves a compressor resembling a jet engine in both appearance and principle. Liquid hydrogen is typically used as a concentrated form of hydrogen storage. As in any gas, storing it as liquid takes less space than storing it as a gas at normal temperature and pressure, however the liquid density is very low compared to other common fuels. Once liquefied it can be maintained as a liquid in pressurized and thermally insulated containers.
1756 – The first documented public demonstration of artificial refrigeration by William Cullen, Gaspard Monge liquefied the first gas producing liquid sulfur dioxide in 1784. Michael Faraday liquefied ammonia to cause cooling, Oliver Evans designed the first closed circuit refrigeration machine in 1805, Jacob Perkins patented the first refrigerating machine in 1834 and John Gorrie patented his mechanical refrigeration machine in 1851 in the US to make ice to cool the air, Siemens introduced the Regenerative cooling concept in 1857, Carl von Linde patented equipment to liquefy air using tile Joule Thomson expansion process and regenerative cooling in 1876, in 1885 Zygmunt Florenty Wróblewski published hydrogen's critical temperature as 33 K; critical pressure, 13.3 atmospheres; and boiling point, 23 K.
Hydrogen was liquefied for the first time by James Dewar in 1898 by using regenerative cooling and his invention, the vacuum flask. The first synthesis of the stable isomer form of liquid hydrogen, parahydrogen was achieved by Paul Harteck and Karl Friedrich Bonhoeffer in 1929.
Room temperature hydrogen consists mostly of the orthohydrogen form. After production, liquid hydrogen is in a metastable state and must be converted into the parahydrogen isomer form to avoid the exothermic reaction that occurs when it changes at low temperatures, this is usually performed using a catalyst like ferric oxide, activated carbon, platinized asbestos, rare earth metals, uranium compounds, chromic oxide, or some nickel compounds.
It is a common liquid rocket fuel for rocket applications. In most rocket engines fueled by liquid hydrogen, it first cools the nozzle and other parts before being mixed with the oxidizer (usually liquid oxygen (LOX)) and burned to produce water with traces of ozone and hydrogen peroxide. Practical H2/O2 rocket engines run fuel-rich so that the exhaust contains some unburned hydrogen. This reduces combustion chamber and nozzle erosion. It also reduces the molecular weight of the exhaust that can actually increase specific impulse despite the incomplete combustion.
|ACGIH TLV-TWA||Simple asphyxiant|
Liquid hydrogen can be used as the fuel storage in an internal combustion engine or fuel cell. Various submarines (Type 212 submarine, Type 214 submarine) and concept hydrogen vehicles have been built using this form of hydrogen (see DeepC, BMW H2R). Due to its similarity, builders can sometimes modify and share equipment with systems designed for LNG. However, because of the lower volumetric energy, the hydrogen volumes needed for combustion are large. Unless LH2 is injected instead of gas, hydrogen-fueled piston engines usually require larger fuel systems. Unless direct injection is used, a severe gas-displacement effect also hampers maximum breathing and increases pumping losses.
Liquid hydrogen is also used to cool neutrons to be used in neutron scattering. Since neutrons and hydrogen nuclei have similar masses, kinetic energy exchange per interaction is maximum (elastic collision). Finally, superheated liquid hydrogen was used in many bubble chamber experiments.
The byproduct of its combustion with oxygen alone is water vapor (although if its combustion is with oxygen and nitrogen it can form toxic chemicals), which can be cooled with some of the liquid hydrogen. Since water is considered harmless to the environment, an engine burning it can be considered "zero emissions." Liquid hydrogen also has a much higher specific energy than gasoline, natural gas, or diesel.
The density of liquid hydrogen is only 70.99 g/L (for atomic hydrogen at 20 K), a relative density of just 0.07. Although the specific energy is around twice that of other fuels, this gives it a remarkably low volumetric energy density, many fold lower.
Liquid hydrogen requires cryogenic storage technology such as special thermally insulated containers and requires special handling common to all cryogenic fuels. This is similar to, but more severe than liquid oxygen. Even with thermally insulated containers it is difficult to keep such a low temperature, and the hydrogen will gradually leak away (typically at a rate of 1% per day). It also shares many of the same safety issues as other forms of hydrogen, as well as being cold enough to liquefy (and possibly solidify) atmospheric oxygen which can be an explosion hazard.