Final answer:
When water reaches its boiling point, adding heat does not increase its temperature; it instead facilitates the phase change from liquid to gas. The temperature remains constant at the boiling point (100°C) during this process, and turning up the heat only increases the rate of boiling, not the temperature.
Step-by-step explanation:
When water is boiling, adding heat does not increase the temperature. During the boiling process, water remains at a constant temperature, which is the boiling point of the liquid. Instead, the added heat is utilized to transform the liquid into its gaseous phase, which is known as vaporization or evaporation. The temperature of the water will stay at 100°C as it boils, no matter how much additional heat is supplied. Only after all of the liquid has evaporated will the temperature of the remaining vapor begin to rise if further heat is added.
It's important to recognize that during a phase change, such as boiling, the rate at which heat is added does not affect the temperature. The vigorousness of the boiling process merely indicates a more rapid conversion of water from the liquid to the gas phase due to the input of heat, but this does not equate to a rise in temperature. Hence, many cooks' belief that turning up the heat will cook food faster is mistaken; it merely causes the pot of water to boil away quicker, without affecting the actual cooking temperature.