Science Questions

Wed, 19th Mar 2003

Part of the show Cambridge Science Festival - Is there Life in Outer Space


Cordelia asked:

Why can you smell some gases, but not others ?


Actually, methane doesn't smell either, but you probably thought it did because it is often found with hydrogen sulphide which is very stinky and smells just like rotten eggs. Gases get into our noses to excite our olfactory (or smelling nerves) which are connected to our brains to allow us to smell. How a gas smells depends on it's size and shape, and how it can fit into our smell receptors in the nose. It also depends of course on how easily a gas evaporates. Gases which evaporate more easily smell stronger because more vapour manages to get into the nose. Gases which smell tend to be the ones which are heavier, and more than 15 times the weight of hydrogen which is the smallest atom, so methane and carbon monoxide (CO) don't smell but hydrogen sulphide (H2S), and chlorine (Cl2) do. Why does it smell when we break wind? This is because the your gut contains lots of bacteria and enzymes to digest your food and in this process gases like hydrogen sulphide (H2S) are produced. Interesting fact - we break wind on average 14 times a day. However a word of warning, if the concentration of hydrogen sulphide increases too much, your olfactory, or smelling, nerves are paralysed and you lose the ability to smell it. Other gases which don't smell are Oxygen, Nitrogen, and steam. Gases which do smell include chlorine, bromine, iodine and sulphur dioxide.


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