A blue-green stain on the brass portion of the cylinder valve is evidence of contact with which substance?

• A. Oxygen
• B. Water
• C. Anhydrous ammonia
• D. Chlorine

Answer: (C)

Blue-green staining on brass surfaces points to ammonia exposure. Brass contains copper, and ammonia reacts with copper to form a blue copper-ammonia complex that deposits on the surface, giving a distinctive blue-green patina. This color is a practical indicator that ammonia has contacted the brass valve components. Other substances produce different patinas: oxygen tends to oxidize copper to brownish or reddish tarnish, water alone can cause dull green or white corrosion products, and chlorine forms copper chlorides with colors that aren’t the same blue-green hue. The characteristic blue-green stain is specifically associated with anhydrous ammonia, which is why that option is the best choice.