Final answer:
The heat capacity of an ideal gas is greatest when the gas is diatomic and volume is constant.
Step-by-step explanation:
The heat capacity of an ideal gas is greatest when the gas is diatomic and volume is constant. The heat capacity of an ideal gas depends on the degrees of freedom of each molecule in the system. For a diatomic gas, each molecule has more degrees of freedom compared to a monatomic gas. This means that a diatomic gas can store more energy in the form of internal vibrations and rotations, leading to a higher heat capacity. When the volume is constant, the gas cannot do any work on its surroundings, allowing all the heat supplied to the gas to be used in increasing its internal energy, resulting in a larger heat capacity.