The highest temperature that water can reach under normal atmospheric pressure is 100 degrees Celsius (212 degrees Fahrenheit). At this temperature, water undergoes a phase change from a liquid to a gas, known as boiling. This is the boiling point of water at standard atmospheric pressure (1 atmosphere or 101.3 kilopascals).
It's important to note that the boiling point of water can vary depending on the surrounding pressure. For instance, at higher altitudes where atmospheric pressure is lower, the boiling point of water is lower than 100 degrees Celsius. Conversely, at higher pressures, such as in a pressurized vessel, the boiling point of water can be higher than 100 degrees Celsius.
Moreover, it's worth mentioning that under extreme conditions, such as at very high pressures or in specific experimental setups, water can exist in superheated states, where its temperature can exceed 100 degrees Celsius without boiling. However, superheated water is highly unstable and can rapidly boil or explode when disturbed, releasing the excess heat and returning to a lower temperature equilibrium state.
In summary, at standard atmospheric pressure, the highest temperature water can reach is 100 degrees Celsius, which is its boiling point.