Which of the following are the example of double salt?

A. $$\mathrm{FeSO}_{4} \cdot\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4} \cdot 6 \mathrm{H}_{2} \mathrm{O}$$

B. $\mathrm{CuSO}_{4}\cdot 4 \mathrm{NH}_{3} \cdot \mathrm{H}_{2} \mathrm{O}$

C. $$\mathrm{K}_{2} \mathrm{SO}_{4} \cdot \mathrm{Al}_{2}\left(\mathrm{SO}_{4}\right)_{3} \cdot 24 \mathrm{H}_{2} \mathrm{O}$$

D. $\mathrm{Fe}(\mathrm{CN})_{2}\cdot4 \mathrm{KCN}$

Choose the correct answer :

$A=\mathrm{FeSO}_{4} \cdot\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4} \cdot 6 \mathrm{H}_{2} \mathrm{O}$ - double salt <br/><br/>B. $\mathrm{CuSO}_{4} \cdot 4 \mathrm{NH}_{3} \cdot \mathrm{H}_{2} \mathrm{O}$ $=\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\right] \mathrm{SO}_{4} \cdot \mathrm{H}_{2} \mathrm{O} $ - complex salt <br/><br/>C. $\mathrm{K}_{2} \mathrm{SO}_{4} \cdot \mathrm{Al}_{2}\left(\mathrm{SO}_{4}\right)_{3} \cdot 24 \mathrm{H}_{2} \mathrm{O}$ - double salt <br/><br/>D. $\mathrm{Fe}(\mathrm{CN})_{2} \cdot 4 \mathrm{KCN}$ = $\mathrm{K}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]$ - complex salt <br/><br/>Option A, $\mathrm{FeSO}_{4} \cdot\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4} \cdot 6 \mathrm{H}_{2} \mathrm{O}$, is a double salt because it is formed by the combination of two different salts: iron(II) sulfate ($\mathrm{FeSO}_{4}$) and ammonium sulfate ($\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4}$). <br/><br/>Option B, $\mathrm{CuSO}_{4}\cdot 4 \mathrm{NH}_{3} \cdot \mathrm{H}_{2} \mathrm{O}$, is a complex salt because it contains a central copper ion coordinated to several ammonia molecules. <br/><br/>In this compound, the copper(II) ion (Cu²⁺) acts as the central metal ion, while the four ammonia molecules (NH₃) act as ligands, coordinating to the copper ion through their lone pairs of electrons. The sulfate ion (SO₄^2-) and water molecules (H₂O) are not involved in the coordination sphere and simply crystallize along with the complex. <br/><br/>Option C, $\mathrm{K}_{2} \mathrm{SO}_{4} \cdot \mathrm{Al}_{2}\left(\mathrm{SO}_{4}\right)_{3} \cdot 24 \mathrm{H}_{2} \mathrm{O}$, is also a double salt, formed by the combination of two different salts: potassium sulfate ($\mathrm{K}_{2} \mathrm{SO}_{4}$) and aluminum sulfate ($\mathrm{Al}_{2}\left(\mathrm{SO}_{4}\right)_{3}$). <br/><br/>The 24 water molecules in the formula are not directly involved in the salt formation but instead function to stabilize the crystal lattice by forming hydrogen bonds with the sulfate ions and other water molecules. <br/><br/>Option D, $\mathrm{Fe}(\mathrm{CN})_{2}\cdot4 \mathrm{KCN}$, is not an example of a double salt. It is a complex salt, formed by the coordination of iron(II) ions to cyanide ligands and potassium ions.