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An ideal gas expands at constant temperature and constant pressure. Which of the following statements about its internal energy and entropy is correct?

1. Internal energy remains unchanged
2. Internal energy decreases
3. Internal energy increases
4. Entropy first increases and then decreases

Correct answer: Internal energy remains unchanged

Solution

The internal energy of an ideal gas is a function of temperature alone (U = nCvT). Since temperature is constant, delta U = 0, so internal energy remains unchanged. Entropy increases during any irreversible isothermal expansion (delta S = Q/T > 0), it does not first increase and then decrease. Option D is wrong. The answer is option A.

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