Final answer:
The physical processes that cause formation include ion-pair formation held together by electrostatic forces, phase changes like melting or boiling, and the accretion of planetesimals in planetary formation driven by gravity.
Step-by-step explanation:
The question posed is about the physical process that causes the formation of certain phases or structures in materials or solutions. There are various physical processes described, including ion-pair formation, phase changes like melting or boiling, and the aggregation of planetesimals in planetary formation.
Ion-pair formation is a process where a cation and an anion come into close contact in a solution, held together by electrostatic forces. This pair migrates as a unit with a net charge that is the sum of the individual ions' charges. It is described as an intermediate state between an ionic solid and fully dissociated ions in a solution.
In the context of phase changes, the process of melting involves going from a solid to a liquid state, and boiling implies transitioning from a liquid to a gas state. These are physical changes that do not involve a change in chemical composition, merely in the physical state of the substance.
Lastly, the formation of planets through the accumulation of planetesimals is another physical process. This action is guided by mutual gravitational forces where small bodies collide and aggregate to form larger planetary bodies, which then differentiate into layers based on density because of intense heat from impacts and radioactive materials.