Answer:
After the Louisiana Purchase, new states in the north became free states, for example: Ohio and Michigan, while southern states continue to employ slavery, for example, Alabama and Mississippi.
Controversy arose when Missouri was about to become a slavery state. The reason is that Missouri, while considered a southern state to this day, it's objectively somewhat northernly, and some congressmen from the North thought it should be a free state. (Others obviously thought that slavery was immoral and should be abolished in all states, but no one until Lincoln would do that).
An agreement, known as the Missouri compromise was achieved. Missouri was allowed to become a slavery state and enter the union, at the same time that Maine became a free state and entered the union. This kept the power balance between North and South.
Besides, all new territories south of the 36° 30' parallel were allowed to have slavery, while territories north of the line were to be free states.
