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When I look at pronunciation guides for opacity and opaque I see the following:

opaque: oh-peyk (a hard A)

opacity: oh-pas-i-tee (a soft A)

Since their root seems to be the Latin opācus, why do they have different pronunciations?

Aside, I've been pronouncing opacity as oh-pey-si-tee (hard A) for most of my life, so I'm a little embarrassed.

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You could have a look at this question here which is on a similar theme. – Brian Hooper Aug 30 '11 at 17:27
up vote 2 down vote accepted

The root word and its derivatives don't share the same stress pattern, unless the latter is -ful, -ness, etc.

unique, boutique, technique, baroque, oblique, etc--all these words have a specific pattern.

V+que pattern:

  • o+que > OH
  • i+que > EE
  • a+que > AY

VC+que pattern: you don't hear the above set of vowels in this case. Instead, you hear CAT, BOT, BET, BUT, BIT vowels, because the stressed syllable is closed.

In -ty pattern, the stress falls on the antipenult (third vowel from the right).

In 'opacity', the stress falls on the third from the right, which is 'a'. Since it is stressed, it attracts a coda consonant, making it a closed syllable.

o-PAC-i-ty: CAT Vowel, instead of GATE vowel in opaque.

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Argh... another rule in English learned by route rather than being explicitly stated. I did a review of some of the other words ending in "[aeiou]city", and agree they all soften the vowel. – BIBD Nov 17 '11 at 15:58
I'm a native English speaker (more or less - I also grew up speaking some Finnish), and I had also always thought both were with a soft A (o-pack). It can be frustrating! – Hannele Apr 10 '13 at 15:08

Neither of the above answers is really correct. The correct answer is that the variation in "opaque" ~ "opacity" (and many similar words, e.g. "divine" ~ "divinity", "serene" ~ "serenity", "profound" ~ "profundity"; also "wild" ~ "wilderness", etc.) is due to a sound change known as Trisyllabic laxing. This is well-explained in the Wikipedia article I just linked to. It also caused other variations like in "south" vs. "southern" -- at the time that this change happened, the latter word was actually "southerne", with three syllables.

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The answer is related to a linguistic process called Apophony, but I can't remember the exact term for this. However, the process is motivated by the syllables. Take a word like atom as an example. As we add morphemes to it, it changes the vowel quality: atom /ˈædəm/ -> atomic /əˈtɑmɪk/ -> atomicity /ætəmˈɪsɪti/.

edit: I should also add that the shift normally occurs around primary stress, which is why the O in opacity isn't changing.

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This is not the same process as apophony. Apophony is where a phonemic alternation indicates a grammatical difference, as in "sing" vs. "sang" vs. "sung". The changes in the words you indicated are secondary changes that stem from the fact that (a) certain morphemes move the stress onto them or onto a preceding syllable; (b) English vowels are reduced in unstressed syllables. Similar changes occur in Russian and (to some extent) Egyptian Arabic, among other languages. – Urban Vagabond Dec 31 '12 at 1:06

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