Helium belongs to the group of noble gases. Liquid helium is the coldest liquid in the world. In this state of aggregation, it has a number of unique features, such as superfluidity and superconductivity. We learn more about its properties later.
Helium gas
Helium is a simple substance widely distributed in the Universe in a gaseous state. He is second in the periodic table and stands immediately after hydrogen. It refers to inert or noble gases.
The element is designated as "He". From the ancient Greek language its name means "Sun." At first it was supposed to be metal. However, it turned out to be a monatomic gas. Helium is the second chemical in terms of lightness; it does not have a taste, color or smell. It has the lowest boiling point.
Under normal conditions, it is an ideal gas. In addition to gaseous, it is capable of being in solid and liquid state. Its inertness is manifested in inactive interaction with other substances. It practically does not dissolve in water. For industrial purposes, it is extracted from natural gas, separated from impurities by strong cooling.
Gas can be dangerous to humans. An increase in its concentration in the air leads to a lack of oxygen in the blood, which in medicine is called oxygen starvation. When ingested in large quantities, it causes vomiting, loss of consciousness, and sometimes death.
Helium liquefaction
Any gas can go into a liquid state of aggregation under certain conditions. Liquefaction is commonly used in industry as well as in research. For some substances, it is enough to simply increase the pressure. Others, such as helium, become liquid only after cooling.
If the gas temperature is above a critical point, then it will not condense, whatever the pressure. For helium, the critical point is the temperature of 5, 19 Kelvin, for its 3He isotope it is 3.35 K.
Liquid helium is an almost perfect liquid. It is characterized by the absence of surface tension, viscosity. After changes in pressure and temperature, its volume remains the same. Liquid helium has an extremely low tension. The substance is colorless and highly fluid.
Properties of liquid helium
In the liquid state, helium is poorly distinguishable because it weakly refracts light rays. Under certain conditions, it has the properties of a quantum fluid. Due to this, at normal pressure, it does not crystallize even at a temperature of −273.15 Celsius (absolute zero). All other known substances solidify under such conditions.
The temperature of liquid helium at which it begins to boil is -268.9 degrees Celsius. The physical properties of its isotopes vary widely. So, helium-4 boils at a temperature of 4.215 K.
It is a Bose liquid, which is characterized by phase transitions at a temperature of 2.172 Kelvin and below. The He II phase is characterized by superfluidity and superconductivity. At temperatures below the phase, He I and He II occur simultaneously, due to which two speeds of sound appear in the liquid.
Helium-3 is a Fermi liquid. It boils at a temperature of 3.19 Kelvin. An isotope is capable of achieving superfluidity only at very low temperatures (several millikelvins), when sufficient attraction appears between its particles.
Superfluidity of helium
The science is obliged to study the concept of superfluidity by academicians Kapitsa S.P. and Landau L.D. Studying the properties of liquid helium in 1938, Sergey Kapitsa noted that, approaching absolute zero, the liquid loses viscosity, instead of hardening.
The academician concluded that after the temperature of helium drops below 2.172 K, the substance passes from the phase of the normal state to a completely new one, called helium-II. In this phase, the substance passes through the capillaries and narrow openings without the slightest friction. This condition is called "superfluidity".
In 1941, Landau L. D. continued to study the properties of liquid helium and developed a theory of superfluidity. He undertook to explain it by quantum methods, applying the concept of the energy spectrum of excitations.
Helium application
The helium element was discovered in the spectrum of the Sun in 1868. On Earth, it was discovered by William Ramsey in 1895, after which it was studied for a long time and was not used in the economic sphere. In industrial activity, it began to be used as fuel for airships during the First World War.
Gas is actively used for packaging in the food industry, in the smelting of metals. Geologists use it to detect faults in the earth's crust. Liquid helium is mainly used as a refrigerant capable of maintaining ultra-low temperatures. This property of him is necessary when conducting scientific research.
The liquid cooler is used in cryogenic electric machines, in scanning tunneling microscopes, in medical NMR tomograph devices, and in charged particle accelerators.
Conclusion
Helium is an inert or noble gas that exhibits low activity in interaction with other substances. In the periodic system of chemical elements, it is in second place, giving way to hydrogen. In nature, the substance is in a gaseous state. Under certain conditions, it can pass into other aggregate states.
The main feature of liquid helium is its superfluidity and its inability to crystallize at normal pressure, even if the temperature reaches absolute zero. The properties of the isotopes of a substance are not the same. Their critical temperatures, the conditions of their boiling, and also the spin values of their particles differ.