Question

Which one of the following statements about the zeolites is false ?

AIPMT 2004

• A. Some of the $Si{O}_4^{4-}$ units are replaced by $Al{O}_4^{5-}$ and $Al{O}_6^{9-}$ ions in zeolites
• B. They have open structure which enables them to take up small molecules
• C. Zeolites are aluminosilicates having three dimensional network
• D. They are used as cation exchangers

Answer 1

The correct option is A Some of the $Si{O}_4^{4-}$ units are replaced by $Al{O}_4^{5-}$ and $Al{O}_6^{9-}$ ions in zeolites

(A) Zeolites are natural ion exchangers. For example, permutit water softeners are sodium zeolites. They take up calcium ions from hard water and release sodium ions in water.
Thus, the statement A is correct.
(B) Zeolites are like molecular sieves. They trap small molecules such as water, ammonia and carbon dioxide in their interstices.
Thus, the option B is correct.
(C) zeolites are aluminosilicates having three dimensional network. They are termed aluminosilicates because, some of the the $S{i}^{4+}$ ions may be replaced by $A{l}^{3+}$.
Thus, the option C is correct.
(D) In zeolites, some of the the $S{i}^{4+}$ ions may be replaced by$A{l}^{3+}$. This results in unbalanced anionic charge. To maintain electrical neutrality, positive ions must be introduced. Thus, the statement D is incorrect.