The physical state of water at 0 degrees Celsius is solid, which is commonly known as ice. At this temperature, water molecules slow down significantly and arrange themselves into a crystalline structure, causing the transition from its liquid phase to solid phase. However, it's important to note that at standard atmospheric pressure, water can coexist as ice, liquid, and vapor simultaneously, depending on environmental conditions. This unique characteristic of water allows it to play essential roles in weather, climate, and biological processes, and its solid state affects ecosystems, particularly in colder climates.