Why was California's application for statehood controversial?

California's application for statehood in 1850 was controversial primarily due to the issue of slavery. The rapid population growth from the Gold Rush led to its request for admission as a free state, which threatened to upset the delicate balance between free and slave states in the Senate. This conflict intensified existing sectional tensions, contributing to the debates that would eventually lead to the Compromise of 1850, which aimed to address these contentious issues. Additionally, the application raised questions about the status of territories acquired from Mexico and the future of slavery in those regions.