Hello! Allow me to help!
Your question: If heat energy is continually being added from the hotplate why doesn't the temperature of the water increase at a steady rate?
My answer: Because at boiling point liquid getting converted into gaseous state, and now all given temperature is utilized in conversion of liquid to gaseous state, so no change in temperature occurs.
Why is my answer correct? Good question! Allow me to explain: Because adding heat energy usually results in a temperature rise, people often confuse heat and temperature. In common speech, the two terms mean the same: "I will heat it" means you will add heat; "I will warm it up" means you will increase the temperature. No one usually bothers to distinguish between these. Adding heat, however, does not always increase the temperature. For instance, when water is boiling, adding heat does not increase its temperature. This happens at the boiling temperature of every substance that can vaporize. At the boiling temperature, adding heat energy converts the liquid into a gas WITHOUT RAISING THE TEMPERATURE. Adding heat to a boiling liquid is an important exception to general rule that more heat makes a higher temperature. When energy is added to a liquid at the boiling temperature, its converts the liquid into a gas at the same temperature. In this case, the energy added to the liquid goes into breaking the bonds between the liquid molecules without causing the temperature to change. The same thing happens when a solid changes into liquid. For instance, ice and water can exist together at the melting temperature. Adding heat to an ice-water slush will convert some of the ice to water without changing the temperature. In general, whenever there is a change of state, such as the solid-liquid or the liquid-gas transition, heat energy can be added without a temperature change. The change of state requires energy, so added energy goes into that instead of increasing the temperature.
Hope this helps! UwU
-Maxwell