Which agent can combine with copper in a drain to create a potentially explosive compound?

Sodium azide is the correct answer because it can react with copper to form copper azide, which is a highly sensitive and potentially explosive compound. This reaction can occur when sodium azide comes into contact with copper pipes or fittings, particularly in confined spaces like drains. Copper azide is noted for its sensitivity to heat and shock, making it hazardous in various environments.

Understanding the nature of sodium azide is essential for safety in laboratory and industrial settings, as improper disposal or accidental mixing with copper-containing materials can lead to dangerous situations. This highlights the importance of proper chemical handling and awareness of potential reactions between different substances.

In contrast, while picric acid is a well-known explosive, its specific reaction with copper does not yield a similar explosive compound in a drain setting. Ether, being a volatile organic solvent, does not have a significant reaction with copper that results in an explosive compound. Mercury, while toxic and hazardous, does not significantly react with copper to form an explosive compound in this context. Thus, sodium azide stands out as the correct choice due to its chemistry and the specific risks associated with its interaction with copper.