The temperature of water remains constant at 100 degrees Celsius (or 212 degrees Fahrenheit) under normal atmospheric pressure because it undergoes a phase change known as boiling. At this temperature, the water molecules gain sufficient energy to overcome the attractive forces holding them together in the liquid state, and they transition into a gaseous state, forming water vapor.
When water reaches its boiling point, the energy input from the heat source is primarily used to break the intermolecular bonds between the water molecules rather than increasing the temperature further. This phase change requires a significant amount of energy, known as the latent heat of vaporization. As a result, the temperature remains constant until all the water has been converted into vapor.
The constant temperature during boiling is known as the boiling point, and it is a characteristic property of the substance under specific conditions of pressure. For water at sea level, the atmospheric pressure is around 1 atmosphere (approximately 101.3 kilopascals), and at this pressure, the boiling point of water is 100 degrees Celsius. However, it's important to note that the boiling point of water can change with variations in pressure. For example, at higher altitudes where the atmospheric pressure is lower, the boiling point of water is lower than 100 degrees Celsius.