I just tried preparing calcium formate by adding $\ce{Ca(OH)2}$ to ammonium formate, but I observed no bubble formation and no smell of ammonia. Why doesn't calcium hydroxide, being a strong base, displace ammonia?

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    $\begingroup$ There is a multitude of possibilities. For one thing, Ca(OH)2 isn't that strong a base. For another, maybe it does displace ammonia, but the solution is too dilute to detect the smell, let alone the bubbles. NH3 never forms bubbles. $\endgroup$ – Ivan Neretin Dec 23 '19 at 15:21
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    $\begingroup$ Did you check the solubilities of calcium formate,16.1 g/100 g (0 °C), and calcium hydroxide, 0.173 g/ 100 ml (20 °C)? $\endgroup$ – MaxW Dec 23 '19 at 17:48

You probably did get your anticipated reaction, but unlike carbon dioxide or hydrogen ammonia is highly soluble in water. Unless your reactants are more highly concentrated than is possible with the limited solubility of calcium hydroxide, the ammonia remains in solution instead of giving off bubbles or a smell.

There are various ways to render this reaction visible:

  1. Heat the solution, driving out any dissolved ammonia. If you generate a smell of ammonia, or better if the solution emits a gas that turns moist red litmus paper blue, you found the ammonia. You should compare with a blank where you just have the unreacted ammonium formate.

  2. If you used solid calcium hydroxide, double-check how much you dissolved. Calcium hydroxide has only a solubility of 1-2 grams per liter, so if you dissolved more than that it must have reacted. If you were using magnesium hydroxide the solubility difference is even more evident, for magnesium hydroxide is also strongly basic enough to displace ammonia but its solubility without the reaction is very low.

  3. Check the pH. Ammonia is a weaker base than the calcium hydroxide that displaces it, so you get a lower pH than you would have gotten just adding the same amount of calcium hydroxide to plain water (which in this case you would do for comparison).

Chemistry is an experimental science. Therefore experimental design is essential for chemistry.


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