The most likely solid residue that turns from green to black when heated is copper(II) carbonate (CuCO₃).

Here's why:

* Decomposition: When heated, copper(II) carbonate decomposes into copper(II) oxide (CuO) and carbon dioxide (CO₂) gas.

* Color Change: Copper(II) carbonate is green, while copper(II) oxide is black. This color change is a clear indication of the decomposition reaction.

Reaction Equation:

CuCO₃ (s) → CuO (s) + CO₂ (g)

Other possibilities:

While less common, there are other compounds that might exhibit a similar color change upon heating:

* Nickel(II) carbonate (NiCO₃): This compound is also green and decomposes to black nickel(II) oxide (NiO) upon heating.

* Some green pigments: Certain pigments containing copper or iron might also undergo a similar color change.

Note: It's crucial to consider the specific context and other properties of the unknown substance to identify it definitively.