Final answer:
A constant-pressure calorimeter is used for reactions in aqueous solutions, measuring ΔH. For combustion reactions, a constant-volume (bomb) calorimeter is utilized, measuring heat at constant volume.
Step-by-step explanation:
A constant-pressure calorimeter is most frequently used to measure ΔH for a reaction in aqueous solution, whereas a constant-volume calorimeter, often referred to as a bomb calorimeter, is usually used to measure the heat of a combustion reaction. While the former is suitable for reactions occurring at constant atmospheric pressure, the latter is ideal for reactions involving solid and gaseous reactants or products, especially exothermic ones such as combustion. The bomb calorimeter is a robust apparatus that measures the heat released by a reaction at constant volume, with the reactants placed in a sealed steel vessel filled with oxygen, which is necessary for combustion.