Which observation would most strongly indicate that a displacement reaction has occurred in a metal solution?

• A. Temperature change only
• B. A gas is produced
• C. A color change in the solution only
• D. Colour change, metal coating, or formation of a solid

Answer: (D)

In a metal displacement reaction, a more reactive metal pushes out a less reactive one from its aqueous salt solution. The clearest evidence comes from observable changes that show new metal and new ions appearing.

A color change in the solution happens because the metal ions in the solution are being replaced by the more reactive metal, altering the solution’s color if the ions are colored. For example, when a reactive metal displaces copper from copper sulfate, the blue color (from Cu2+ ions) fades as copper ions are removed from solution.

Seeing the metal itself change on the solid piece is another strong sign: the displaced metal can deposit onto the more reactive metal, forming a visible coating of metal on the surface. That metal coating directly shows a new metal is being deposited from the solution.

Formation of a solid, such as a precipitate or the solid displaced metal, is also a strong indicator. If the product of the reaction is insoluble, it will appear as a solid in the solution; alternatively, the displaced metal may accumulate as a solid coating on the electrode.

Temperature change or gas production alone is less reliable for this specific type of reaction, since these can occur in many different processes and aren’t definitive on their own. The combination of color change, metal coating, or solid formation provides the most convincing evidence that a displacement reaction has occurred.