Maine entered the Union as a free state.
Missouri entered the Union as a slave state.
Step-by-step explanation:
The Missouri compromise happened in the year 1820 and was passed by the congress. This compromise when passed made Maine a part of the union of the United States.
Missouri also became a part of the union but the difference between the two becoming parts of the union was that Maine joined as a free state and Missouri joined as a slave state to the union.
The focus of these was to maintain a balance in the power of regions of north and south in the United States.