Final answer:
In an isothermal expansion of an ideal gas, the internal energy does not change and the pressure of the gas decreases. Heat is added to the gas, enabling it to do work on its surroundings.
Step-by-step explanation:
For an ideal gas that undergoes an isothermal expansion, several facts must be true according to the first law of thermodynamics and the characteristics of ideal gases:
- The internal energy of the gas does not change (D) the internal energy of the gas does not change) because the internal energy of an ideal gas is solely a function of its temperature, and in an isothermal process, the temperature remains constant.
- Work is done by the gas (B) work is done on the gas is incorrect, as the gas itself is doing work on the surroundings by expanding), as gas expansion against an external pressure requires energy.
- The pressure of the gas decreases (C) the pressure of the gas decreases) because, for the gas to maintain a constant temperature while its volume increases, its pressure must decrease according to the ideal gas law (PV=nRT).
- Heat must enter the gas from a heat reservoir to allow for expansion at constant temperature, compensating for the work done by the gas (E) heat is removed from the gas is not correct, as heat is actually being added).
Therefore, the correct statements are C and D.