JEE Main · 2021 · 26 Feb 2021 · Shift 2 · Hydrogen

Calgon is sodium hexametaphosphate, $\mathrm{N}{\mathrm{a}}_2[\mathrm{N}{\mathrm{a}}_4(\mathrm{P}{\mathrm{O}}_3{)}_6]$, long known as Graham's salt, and it is a water soluble polymeric metaphosphate, so those two descriptions hold. It softens water by exchanging two of its sodium ions for calcium, which stays in solution as the complex anion $[\mathrm{N}{\mathrm{a}}_2\mathrm{Ca}(\mathrm{P}{\mathrm{O}}_3{)}_6{]}^{2-}$, so the calcium is sequestered rather than precipitated and that description holds as well. The second most abundant element by weight in the earth's crust is silicon, and Calgon contains sodium, phosphorus and oxygen but no silicon. The statement that is not true is that Calgon contains the second most abundant element by weight in the earth's crust.