Question

Which one of the following statements about the zeolite is false?
A. Zeolites are aluminosilicates having 3D network
B. Some of the $Si{O_4}^{4 - }$ units are replaced by $Al{O_4}^{5 - }$ and $Al{O_6}^{9 - }$ ions.
C. They are used as cation exchangers.
D. They have an open structure which enables them to take up small molecules.

Answer 1

Zeolites are basically microporous three dimensional (3D) network silicates in which some of the silicon atoms are supposed to be replaced by the aluminium atoms. They are considered to be shape selective catalysts.

Complete answer:
Zeolites are actually crystalline solid structures made up of silicon, aluminium and oxygen atoms that lead to the formation of a framework consisting of cavities or channels inside which cations, water or small molecules are able to reside. They are sometimes known as molecular sieves. Let us discuss a few properties of zeolites mentioned in the options one by one.
Option A: Zeolites are aluminosilicates having 3D networks. These are known as aluminosilicates because some of the $S{i^{4 + }}$ ions can even be replaced by $A{l^{3 + }}$. Hence, Option A is correct.
Option B: In zeolites, some of the $S{i^{4 + }}$ ions can even be replaced by $A{l^{3 + }}$. This results in the unbalanced anionic charge. And in order to maintain electrical neutrality, it becomes necessary to introduce the positive ions. Hence, the statement in Option B is incorrect.
Option C: Zeolites are considered to be natural ion exchangers. For instance, permutit water softeners are also sodium zeolites. They are supposed to take up calcium ions from the hard water and then release sodium ions in the water. Hence, the statement in Option C is correct.
Option D: Zeolites behave like molecular sieves. They are supposed to trap the small molecules like water, carbon dioxide, and ammonia in their interstices. Hence, the statement in Option D is correct.

**As a result, Option B is false

Note:**
The most important characteristic of zeolites is considered to be the removal of ammonia ($N{H_3}$) as well as ammonium ($N{H_4}^ +$) from water. Zeolites are supposed to remove the ammonium ions through ion-exchange mechanism and, at higher concentration it uses the mechanism of adsorption. The ammonium ions that are present in wastewater are actually exchanged for sodium ions.