Complex sentence: "Copper carbonate" was the first compound to be broken down into several compounds: in 1794 by the French chemist Joseph Louis Proust (1754--1826) thermally decomposed it to CO 2 and CuO, cupric oxide, a black solid.
Split into simpler sentences: Copper carbonate was the first compound to be broken down into several, separate elements (copper, carbon, and oxygen). When heated, it thermally decomposes to form CO 2 and CuO, cupric oxide, a black solid.