At 100°C and 1 atm pressure, water is in the liquid phase. This temperature and pressure are commonly known as the boiling point of water at sea level.
When water reaches its boiling point, it can transition from liquid to gas (steam) when heat is applied. However, at exactly 100°C and 1 atm, water is at the threshold of changing phases, but it remains in the liquid state until it begins to boil and convert into vapor.
To elaborate, the physical state of water is influenced by both temperature and pressure. Under standard atmospheric pressure (1 atm), water boils at 100°C. Therefore, at this specific temperature and pressure, any water that is not actively boiling remains in its liquid state.