Can a phase change or mixing create a new substance? Explain.

No.

Phases changes do not change chemical formulas. If you turned liquid water into ice, it's still water. Hasn't changed.

If you mixed something into a solution like calcium carbonate and water, the calcium carbonate just dissolves in the water and creates a concentration of CaCO3 and water. It's not something new because if you boiled the solution, all there would be left is the CaCO3 proving that the properties of water still exist, so there is no new chemical.