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I recently purchased some slaked lime online and suspect it is not good (not fully slaked?).

When added to water around 35 °C, it heats to about 80 °C. My understanding is that slaked lime should not have this reaction and I am therefore guessing there is still some quicklime remaining in it somehow. Is this correct, or is it normal for even slaked lime to heat so dramatically?

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You misunderstand what slaked lime is. Slaked lime is the common name for calcium hydroxide and the reaction to dissolve calcium hydroxide is very exothermic as you have noticed.

You also misunderstand about quicklime. Quicklime is Calcium oxide, $\ce{CaO}$. It is formed by heating calcium carbonate and driving off carbon dioxide. Quicklime is reacted with water to form calcium hydroxide so that the quicklime, which has now cooled, won't react with carbon dioxide in the atmosphere to form calcium carbonate again.

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    $\begingroup$ Thank you for that – I did completely misunderstand. I am using the calcium hydroxide in the preparation of a natural indigo vat and was under the impression that I should not let the starter vat get over 50c, which leaves me a bit puzzled as to how to do that when adding calcium hydroxide heats it so much (or maybe that referred to the initial water solution for dissolving the indigo?). Anyway, thank you for you explanation! Very helpful. $\endgroup$
    – Rob
    Sep 11 '16 at 1:52

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