[Test for Ca2+] On adding aq NaOH, a white ppt is formed. The white ppt is Ca(OH)2.
Isn’t Ca(OH)2 an alkali/ soluble in water? Isn’t Ca(OH)2 limewater? So why would there be a precipitate?
Ca(OH)2 is sparingly/ slightly soluble in water (~1.7 g in 1 dm3 of water). Compare with NaOH (~1100 g in 1 dm3 of water) which is very soluble.
Under the conditions of the laboratory test, the amount of Ca(OH)2 produced is greater than the water can dissolve, hence it precipitates out as solid Ca(OH)2.
[Test for Ca2+] On adding aq NH3, no ppt is formed. Since NH3 can ionise in water to produce OH–, shouldn’t Ca(OH)2 (white ppt) be formed too?
For a precipitate to form, there must be sufficient amounts of the constituent ions in solution. Since aq NH3 is a weak alkali, the amount of OH– is very little and insufficient for solid Ca(OH)2 to form.
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