Short answers, pulled from the story.
A free state banned slavery and the domestic slave trade, while a slave state allowed both to remain legal. Even in free states, the Fugitive Slave Clause and the Fugitive Slave Acts of 1793 and 1850 required that an enslaved person crossing into free territory be returned to their owner rather than freed.
When the Civil War began, the United States held 34 states in total, 15 of which were slave states. Eleven of those slave states seceded to form the Confederate States of America, while Delaware, Maryland, Kentucky, and Missouri remained in the Union as border states.
Because every state sent two senators to Congress regardless of population, slave-state politicians treated an even number of slave states and free states as essential to keeping the Senate balanced. Between 1812 and the 1850s, states such as Missouri and Maine, or Arkansas and Michigan, were admitted in matched slave-free pairs for this reason.
West Virginia became the 35th state on the 20th of June 1863 and was the last slave state ever admitted to the Union. It entered under the Willey Amendment, a gradual emancipation plan that freed only children born after the law took effect.
Yes. Utah Territory legalized slavery under an 1852 territorial law even though it did not become a state until 1896, and slavery remained legal in Washington D.C. until 1862. California's constitution banned slavery, but an 1850 law still allowed the forced indenture of thousands of Native Americans.
Slavery legally ended throughout the United States on the 18th of December 1865, when the Thirteenth Amendment to the Constitution was ratified. It abolished slavery and involuntary servitude nationwide, except as punishment for a crime.