When you add more heat to already boiled water, the temperature of the water does not rise significantly. Instead, the additional heat causes the water to undergo a phase change from liquid to gas, resulting in an increase in the rate of evaporation.
When water reaches its boiling point (100 degrees Celsius or 212 degrees Fahrenheit at sea level), it undergoes a phase transition from a liquid to a gas, forming water vapor. At this point, the water is said to be in equilibrium with its vapor phase. The heat energy added to the system is utilized to break the intermolecular bonds between water molecules and convert them into the gaseous state.
As long as there is liquid water remaining in the container, the temperature of the mixture will stay constant at the boiling point. This is because the added heat energy is primarily used to convert the liquid water into water vapor rather than increasing the temperature of the liquid. The temperature remains constant until all the liquid water is converted into vapor.
Once all the liquid water has evaporated, further addition of heat will start increasing the temperature of the remaining water vapor. At this point, the system is no longer at the boiling point but is now in the gaseous phase, and the temperature will rise with additional heat input.
It's important to note that the temperature at which water boils can vary with changes in atmospheric pressure. At higher altitudes where atmospheric pressure is lower, water boils at lower temperatures.