Final answer:
The initial pressure of water that is a saturated liquid at 100 Celsius and is heated to become a saturated vapor is 1.00 atmosphere (atm), which is the standard atmospheric pressure at sea level.
Step-by-step explanation:
The student is asking about the initial pressure of water when it changes from a saturated liquid to a saturated vapor state at 100 Celsius in an isobaric (constant pressure) process. Since we are given the temperature at which the phase change occurs and understand that we're dealing with water at its boiling point, the pressure in question is the equilibrium vapor pressure of water at its boiling point. Water boils at 100 °C under a standard atmospheric pressure of 1.00 atm. From this, we conclude that the initial pressure of the water before heating is 1.00 atm.