Final answer:
The property that remains constant when using a bomb calorimeter to measure the heat of reaction is volume. This is because the bomb calorimeter operates at constant volume, allowing for the determination of the change in internal energy rather than enthalpy.
Step-by-step explanation:
When a bomb calorimeter is used to determine the heat of reaction, the property of the system that remains constant is the volume. A bomb calorimeter is specifically designed to measure energy changes in chemical processes, particularly for reactions like combustion that produce gaseous products, at constant volume. The steel vessel, known as the "bomb", has a fixed volume and seals in reactants and excess oxygen.
During the combustion reaction, the temperature of the water bath may increase, but the volume of the system inside the calorimeter does not change. Thus, the heat released by the reaction is equivalent to the change in internal energy (ΔU), not enthalpy (ΔH), because the pressure is not constant.