Question

Which of the following pairs would be expected to form precipitate when solutions are mixed?
(A) \[{K^ + },\,S{O_4}^{2 - }\]
(B) \[N{a^ + },\,{S^{2 - }}\]
(C) \[A{g^ + },\,N{O_3}^ - \]
(D) \[A{l^{3 + }},\,O{H^ - }\]

Answer 1

Hint: We should first know what we mean by precipitation to give answer to this question. When cations and anions in an aqueous solution react to generate a precipitate, an insoluble ionic solid, precipitation processes take place. The solubility principles for typical ionic solids can be used to determine whether or not such a reaction takes place. Thus, using solubility rules we can say that Al³⁺, OH^- precipitate when mixed. The precipitate formed is Al(OH)₃.

Complete Step by Step Solution:
The precipitation reaction is a chemical process that occurs in an aqueous solution when two ionic bonds join and an insoluble salt precipitates out. Precipitates are the insoluble salts that result from precipitation processes. An illustration of a double displacement reaction is precipitation.

There are various outcomes that can happen when a material is combined with a solvent. The substance's solubility, which is defined as the solute's highest achievable concentration, is what determines the outcome. We determine which chemicals are soluble and to what extent using the solubility laws.

The solubility rules are:
The salts containing \[C{l^ - },\,B{r^ - },\,{I^ - }\,\] are also soluble. $A{g^ + },\,P{g^{2 + }},\,{\left( {H{g_2}} \right)^{2 + }}$ halide salts are significant exceptions to this norm. $PbB{r_2},\,H{g_2}C{l_2},\,AgCl$ are hence insoluble.
The element salts of Group 1 $\left( {L{i^ + },\,N{a^ + },\,{K^ + },\,C{s^ + },\,R{b^ + }} \right)$ are soluble.

Silver salts are generally insoluble. Most silver salts are insoluble, with the exception of $AgN{O_3},\,Ag\left( {{C_2}{H_3}{O_2}} \right)$ .
The majority of hydroxide salts are barely soluble.
The transition metal and $A{l^{3 + }}$ hydrate salts are insoluble therefore, $Fe{\left( {OH} \right)_3},\,Al{\left( {OH} \right)_3},\,Co{\left( {OH} \right)_2}$ are so insoluble.
There are many more solubility rules but for now from the above rules we can say that $Al$ is a cation in the ${3^{rd}}$ group and precipitates as hydroxide $Al{\left( {OH} \right)_3}$ .
Hence, D. Is the correct option.

Note: Kindly note that there are more solubility rules other than the ones mentioned above in the solution above. Also the exceptions of solubility rules are very important to arrive at the correct answer. Many such questions can be made for various cations and anions.