希硫酸, 稀硫酸. My dictionary says they are both “dilute sulfuric acid”. What is the difference in usage here? Is there just one optional radical?
English has “sulfuric” and “sulphuric”, and that’s an American/Commonwealth regional difference.
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The matter is in the permitted kanji. The kanji 稀, 'rare,' is the proper choice for the word (cf. Chinese xīliúsuān 稀硫酸 same). It, however, is not included in the Jōyō list (though it is permitted for names), so, say, chemistry school textbooks cannot use it to teach. The need to write words with rare kanji frequently leads to the phenomenon called kakikae, when graphically similar (and preferrably identically read) characters are substituted instead. 希, however, is Jōyō (and even 4th grade), so using it as a replacement seems a valid idea, though its meaning is 'hope,' and so it is a worse choice. Similar case is replacing 綺麗 by 奇麗 to avoid the usage of non-Jōyō 綺, though, strictly saying, 綺麗 just means 'beatiful,' while 奇麗 should mean 'astoundingly, rare-to-find beatiful.'
Note. Strictly saying, 希 as a replacement for 稀 is neither recent nor Japanese-only; it is amply known from Chinese, and the second round of simplifications in the 1970s even tried to abolish 稀 altogether, replaced by 希.