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So, this is kinda a dumb question, but I'm curious. Both are citrus fruits and very related to each other and taste sour which corresponds to both being acidic. Why then are limes said to be alkaline, but lemons are acidic?

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  • $\begingroup$ Limes have calcium oxides which in water forms calcium hydroxide, which is basic due to the hydroxyl ions. Lemons have citric acid (component of vitamin C). I never tasted the former though. $\endgroup$ – Gaurang Tandon Feb 27 '18 at 15:34
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    $\begingroup$ lime: pH 2.40; and lemon: pH 2.30. source $\endgroup$ – Avyansh Katiyar Feb 27 '18 at 15:35
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    $\begingroup$ Doesn’t lime refer to limewater ie., $Ca(OH)_2$? $\endgroup$ – MollyCooL Feb 27 '18 at 16:12
  • $\begingroup$ @avatar just to clarify that page defines pH of the citrus fruit "lime", and not of its other popular usage, which is the inorganic oxides and carbonates of calcium $\endgroup$ – Gaurang Tandon Feb 27 '18 at 16:38
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The word "lime" has two meanings and they have essentially no relationship between them.

The "lime” that is inorganic oxide quicklime, CaO, reacts with water to give a relatively strong-base hydroxide::
$$ \ce{ CaO + H2O -> Ca(OH)2 }$$ $$ \ce{ Ca(OH)2 <=> Ca^2+ + 2OH-}$$
Due to the presence of these hydroxyl ions, the solution is alkaline.

"Limes" that are the green, hybrid citrus fruits are acidic with citric acid, just like lemons. Both have juice with a pH in the range of 2-3, for limes as in fruit a typical pH level is about 2.8. Hence the tart flavor for which both fruits are known. Like all citrus fruits, lemons and limes are a rich source of ascorbic acid, vitamin C.

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  • $\begingroup$ The OP was obviously talking about the fruit, so why the chemistry lesson on the mineral "lime"? $\endgroup$ – James Jan 6 '20 at 18:05
  • $\begingroup$ @James Because the OP was confused. Presumably he read online that "lime" is alkaline ("Why then are limes said to be alkaline?"). This isn't true for the fruit known as limes. But the alkalinity of the unrelated substance known as "lime" is very commonly talked about in gardening. It is incredibly common to see the alkalinity of "lime" discussed online. This answer is almost certainly the correct one the OP was looking for: Limes are as acidic as lemons, but the unrelated substance commonly referred to as 'lime' used in gardening is alkaline. $\endgroup$ – Jamin Grey Oct 15 '20 at 20:42

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