Describe the reaction that occurs when aqueous copper(II) ions are treated with ammonia. What is the observation and the resulting complex ion?

When aqueous copper(II) ions (Cu^2+) are treated with ammonia (NH3), a deep blue solution forms due to the formation of the complex ion [Cu(NH3)4]^2+. The observation is a color change to deep blue. This answer is anchored to Aqueous ion tests (A-level only).

When aqueous copper(II) ions (Cu^2+) are treated with ammonia (NH3), a deep blue solution forms due to the formation of the complex ion [Cu(NH3)4]^2+. The observation is a color change to deep blue. is the best answer because it directly supports the AQA A-Level Chemistry objective to describe reactions of aqueous metal ions with ammonia. This reasoning is anchored to Aqueous ion tests (A-level only) in Reactions of ions in aqueous solution (A-level only), and it separates ammonia from similar A-Level ideas rather than relying on a vague recall statement. Other options are weaker if they use the wrong evidence, calculation, mechanism, observation, unit, or conclusion for this subtopic.