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Helium: the inert gas
Helium is used to fill party balloons and weather balloons.
Helium is about seven times less dense than air, so a balloon filled with helium will rise if the balloon itself is light enough. Hydrogen can do the same job but unlike helium it is flammable.
Helium is used in breathing gas mixtures to prevent deep sea divers suffering from 'the bends'.
The pressure increases as a diver descends, so breathing gases have to be supplied under pressure. Nitrogen in air causes 'nitrogen narcosis' below 30 m, a dangerous condition similar to being drunk. The high partial pressure of nitrogen underwater allows excess nitrogen to dissolve in the blood. If the diver returns to the surface too quickly, nitrogen bubbles form and block blood vessels. Helium is much less soluble than nitrogen: 'heliox', a mixture of 80% helium and 20% oxygen, reduces the chance of these potentially fatal problems happening.
Magnetic resonance imaging (MRI) scanners help diagnose conditions such as tumours and cysts.
MRI and nuclear magnetic resonance (NMR) spectroscopy need very powerful magnetic fields, provided by superconducting magnets. Metals only become superconductors with zero resistance at very low temperatures. Liquid helium is the coldest liquid on Earth and boils at just -268.9 °C, so it is used to cool the coils of the magnets. Proton NMR is widely used in chemistry to analyse the structure of organic compounds.
Welding and metal refining
Oxygen is a reactive gas in the air that interferes with welding and metal refining.
Helium, or argon, is used to form an inert shield during welding. This prevents the hot metal oxidising and weakening the weld. Argon is the most abundant in the air, making it the cheapest noble gas. Argon is used to stir the molten metal during the manufacture of steel, and it is also used in one of the stages of titanium extraction.
Helium atoms are very small compared to other atoms, making it the ideal gas for detecting leaks in pressurised systems. Helium atoms can pass through the smallest gaps and do not react with the component being tested. A mass spectrometer is used to analyse the air around the component to discover any escaped helium.
Gas lasers are sources of intense laser light. Lasers used in supermarket bar-code readers are filled with neon or a mixture of neon and helium, while krypton lasers are widely used in eye surgery. Xenon lasers produce ultraviolet light in high energy pulses, and are used to cut extremely hard materials.
Neon is the starter gas that warms up street lamps when they are first switched on - notice that they glow red before becoming producing their characteristic orange light. The most familiar use of neon is in brightly coloured 'neon lights'. Ordinary filament lamps contain argon or nitrogen and argon. High-energy fluorescent lighting tubes and airport runway lights contain krypton or argon and krypton. Lighthouse lamps contain xenon or argon and krypton.