Which one of the following statements about the zeolite is false?
- They are used as cation exchangers.
- They have open structure which enables them to take up small molecules.
- Zeolites are aluminosilicates having three dimensional network.
- Some of the SiO44- units are replaced by AlO45- and AlO69- ions in zeolites.
Answer
(4) Some of the SiO44- units are replaced by AlO45- and AlO69- ions in zeolites.
Solution
As per NCERT, if aluminium atoms replace a few silicon atoms in the three-dimensional network of silicon dioxide, the overall structure is an aluminosilicate carrying a negative charge, balanced by cations such as Na+, K+ or Ca2+; zeolites are such aluminosilicates.
(1) is true: the loosely held cations can be exchanged, so zeolites (permutit) are used as cation exchangers in softening hard water.
(2) is true: the open, porous network takes up small molecules, which is why zeolites act as molecular sieves (e.g. ZSM-5 converts alcohols directly into gasoline).
(3) is true, as they have a three-dimensional aluminosilicate network.
(4) is false, because Al replaces Si only at the tetrahedral positions (as AlO4 units); octahedral AlO69- units are not part of the zeolite framework.
Answer: (4)