Final answer:
Eli Whitney's invention that significantly changed the South and expanded slavery was the cotton gin. It made cotton farming profitable, leading to increased demand for slave labor. Whitney's attempts to enforce his patent were unsuccessful in the South.
Step-by-step explanation:
The invention by Eli Whitney that dramatically changed Southern life, made cotton king, and significantly expanded slavery was the cotton gin. Patented in 1794, the cotton gin removed seeds from cotton fiber much faster than manual labor. Initially, Whitney hoped the invention would render slavery obsolete, but the opposite occurred; it made large-scale cotton farming profitable. As cotton production soared, so did the demand for slave labor to plant, harvest, and process the cotton. Despite Whitney's patent, Southern planters created similar seed-separating devices, ignoring his legal rights. Eli Whitney's cotton gin played a pivotal role in the economic growth of the South, built largely on the backs of enslaved people.