Which one of the following statements about zeolite is not true

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Zeolites are aluminosilicates having three dimensional network

They have structure which enables them to take up small molecules

They are used as cation exchangers

Some of the ${SiO}_{4}^{4 -}$ units are replaced by ${AlO}_{4}^{5 -}$ and ${AlO}_{6}^{9 -}$ ions in zeolites

answer is D.

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Detailed Solution

(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 Si4+ ions may be replaced by Al3+.
Thus, the option C is correct.
(D) In zeolites, some of the the Si4+ ions may be replaced by Al3+. This results in unbalanced anionic charge. To maintain electrical neutrality, positive ions must be introduced. Thus, the statement D is incorrect.