Step-by-step explanation:
The ion that produces a dark blue color when exposed to NH₃ (ammonia) is Cu²⁺ (copper(II) ion). The chemical equation for the reaction is:
Cu²⁺(aq) + 4NH₃(aq) → [Cu(NH₃)₄]²⁺(aq)
This reaction forms a complex ion called tetraamminecopper(II) ion, which is dark blue in color.