Tension between free states and slaves stated got very tight by 1819 when Missouri became a state. The Southerners thought it was unfair that just because that state was in the North territory it had to be free a state. Before this time there where 22 states so it had always been even but now since Missouri is the 23rd state it makes it uneven. In 1819 Missouri asked congress to be a slave state instead of a free state. To keep away from fights and debates congress decided to make a compromise, called the Missouri Compromise.
The Debate
The northerners were represented by Senator Rufus King and the southerners were represented by Senator William Pinckney. In the north they argued that congress had a choice to make it a slave or free state. In the south they argued that since the states are as free as the other states then they should be able to choose themselves if they want it to be a slave or free state. They could not stop fighting so before we started a civil war we decided to reach a compromise.
The Final Agreement
They ended up agreeing that Missouri would become a slave state but, Maine would have to be free. Also they added that except for Missouri slavery would be prohibited in the Louisiana Purchase land on the north side.
http://www.pbs.org/wgbh/americanexperience/features/general-article/lincolns-political-landscape/
This map shows the states that are in the south or the North. The north are the free slaves and the south are the slave states. When Missouri was added as a state it messed up the even amount of states in both the north and south so Congress came up with a compromise.