Question

Assertion :Air quickly leaking out of a balloon becomes cooler. Reason: The leaking air undergoes adiabatic expansion.

• A. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.
• B. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion.
• C. Assertion is correct but Reason is incorrect.
• D. Both Assertion and Reason are incorrect.

Answer 1

The correct option is A Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.

Air cools down due to adiabatic expansion as air has to do work against external pressure at the cost of its internal energy.

There is no energy transfers into or out of the system by heat, but the air molecules escaping from the balloon do work on the surrounding air molecules as they push them out of the way. Thus $W$ is negative and $\Delta E_{int}$ is negative. The decrease in internal energy is evidenced by the fact that the escaping air becomes cooler.