Helium boiling point Table

The most important source of helium is the natural gas from certain petroleum wells in the United States and Canada. This gas may contain as much as 8 % of helium. Because helium has a lower boiling point Table 12.1) than any other gas, it is readily obtained by cooling natural gas to a temperature at which all the other gases are liquid (77 K) almost pure helium can then be pumped off. The yearly production in this way may be many millions of m of gas. but something like 10 m per year is still wasted. 

When we look at the critical states and triple points of other gases, we find the situation shown in table 4.34. The liquid phase exists only when the pressure is between the critical and the triple-point pressures. If we cool down hydrogen, helium or water at room temperature and pressure, we will get liquids before we get solids. But if we cool down CO2 from room temperature and pressure, we get dry ice rather than liquid carbonic to obtain liquid carbon dioxide we have to raise the pressure to at least 5.1 atm to exceed the triple-point pressure. The melting point is not as sensitive to the pressure as the boiling point, which is stated usually for a room pressure of 1 atm, which prevails at sea level on Earth and not in Colorado or the Himalayas. 

The closed-shell electronic structures of the noble gas atoms are extremely stable, as shown by the high ionization enthalpies, especially of the lighter members (Table 14-1). The elements are all low-boiling gases whose physical properties vary systematically with atomic number. The boiling point of helium is the lowest of any known substance. The boiling points and heats of vaporization increase monotonically with increasing atomic number. 

There are a vast number of different sorts of superconducting compound. Examples of these are summarized in Table 7.2. They include organic polymers and intercalation compounds (such as RbjCgjj), as well as ceramic oxides and sulfides. In all these compounds the critical temperature is still below the boiling point of liquid nitrogen. Hence, these materials would be very expensive to use in everyday life (as a litre of liquid helium costs 20 times as much as liquid nitrogen). 

Air is a mixture of primarily nitrogen (78 percent) and oxygen (21 percent), with trace amounts of argon, neon, helium, and krypton. These pure elements have many applications. They are separated and purified by cooling air in a process known as fractionation. As air cools, the different elements liquefy based on their boiling points. In what order do the elements liquefy Use Table D.4. 

Figure 1.21. The Lennard-Jones potential for helium He, hydrogen H2 and argon Ar, calculated 6 0m the parameters in table 1.6. An increasing bond energy in the series He < H2 < At is also reflected in the increasing values of the boiling points of the substances, which are 4 K, 20 K and 87 K, respectively.

Helium-4 is by far the more common of the two isotopes. Ordinary helium gas contains about 1.3 x 10 " percent helium-3, so that when we speak of helium or liquid helium, we normally are referring to helium-4 (molecular weight 4.0026). Liquid helium-4 has a normal boiling point of 4.224 K and a density at the normal boiling point of 124.96 kg/m, or about one-eighth that of water. Liquid helium has no solidification point at normal atmospheric pressure. In fact, liquid helium does not solidify under its own vapor pressure even if the temperature is reduced to absolute zero. Saturated liquid helium must be compressed to a pressure of 2.53 MPa before it will solidify. Liquid helium-4 is odorless and colorless and somewhat difficult to see in a container, since its index of refraction is so near that of the gas ( = 1.02 for liquid He). The heat of vaporization of liquid He at the normal boiling point is 20.73 kJ/kg, which is only 1/110 that of water. Table 2.7, prepared by McCarty, presents densities for helium-4 at the critical point, normal boiling point, lower lambda point, and upper lambda point. 

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