eli5 – why gold with 24 karat is only 99.9 pure?
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Normally karat say how many 24th of the stuff you have is real gold.
12 karat means half of it is gold.
16 karat means 2/3 of it is gold.
23 karat means 23/24 of it is gold
24 karat should mean it is all pure gold.
In practice 100% pure is not really a thing you can do easily (or at all), so we say 24 karat and expect that is pure gold, but also accept that it is only as close as possible to being pure.
We accept that 99.9 is close enough to 100% pure to be acceptable for most purposes.
There are some higher values that are expressed in nine. 99.9% pure is three nines. 99.99% is four nines. 99.999% pure is five nines.
Wikipedia says the purest anyone has ever gone was six nines, by the Australian Mint over half a century ago.
24 karat is usually good enough though.
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