My teacher made me write that that the formation of calcium hydroxide (slacked lime) from calcium oxide (qucklime) is described as

$\ce{CaO + H2O -> Ca(OH)2}$

and that the process

  1. forms bubbles
  2. is an exothermic reactions
  3. makes a hissing sound

Why does it make a hissing sound and form bubbles?

There is no gas evolved in the reaction, then why does formation of bubbls take place?

And what is the reason of the hissing sound being produced?

  • $\begingroup$ Because your teacher is wrong? The observations sound like the observations of $\ce{Ca + 2H2O -> Ca(OH)2 + H2}$. $\endgroup$
    – DHMO
    Oct 1, 2016 at 9:24
  • 3
    $\begingroup$ Because the reaction is so exothermic that the water starts boiling? $\endgroup$ Oct 1, 2016 at 9:26
  • $\begingroup$ No $\ce{H2}$ gas evolves. $\endgroup$
    – MaxW
    Oct 1, 2016 at 12:06
  • $\begingroup$ ?? everyones confused $\endgroup$ Oct 1, 2016 at 15:24
  • $\begingroup$ This reaction is exothermic enough to be used for self-heating food packaging. That's going to cause some sizzling in the water. $\endgroup$
    – Zhe
    Oct 4, 2016 at 1:57

1 Answer 1


Good eye for the inconsistency. Cultivate that, it will serve you well. But this is an apparent, rather than a real inconsistency. The reaction is VERY exothermic. Check out how they make quicklime to find out how much heat it takes.

Where the water touches the quicklime, enough heat is evolved to boil a little bit of the water. Hence the bubbles and hiss.

  • $\begingroup$ well..i don't really get why the hiss is formed though...like the science behind the production of a sound $\endgroup$ Oct 1, 2016 at 17:21

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