Silica gel contains $\left[\mathrm{Co}\left(\mathrm{Cl}_{4}\right)\right]^{2-}$ as an indicator. When activated, silica gel becomes dark blue while upon absorption of moisture, its colour changes to pale pink This is because,
(a)Co (II) changes its coordination from tetrahedral to octahedral.
(b)Co(II) changes its oxidation state to Co(III)
(c)Tetrahedral crystal field splitting is NOT equal to octahedral crystal field splitting
(d)Co(II) forms kinetically labile while Co(III) forms kinetically inert complexes.
Answer
Answer: A ✓ checked by 4AB · confidence high
Explanation
Dry: tetrahedral [CoCl₄]²⁻ (deep blue). With moisture the Cl⁻ ligands are replaced by water, giving octahedral [Co(H₂O)₆]²⁺ (pale pink): a change of coordination geometry, not oxidation state.