Question

A balloon filled with a gas shrinks when cooled. When it is done shrinking, which of the following is the same as it was originally?
A.The rms velocity of gas particles
B.The density of gas
C.The pressure in the balloon
D.The kinetic energy of gas particles

Answer 1

Hint: When put into a freezer, the balloon shrinks as the temperature decreases between inside and outside of the balloon. After the temperature difference becomes zero between inside and outside of the balloon, pressure and volume decreases according to PV=nRT.

Complete answer:
As the balloon is cooled by any means, there remains a temperature difference between inside and outside the balloon, thus to make the temperature difference zero, the temperature inside the balloon decreases.
Therefore, pressure and volume will also change in order to cope up with the change in temperature. Also, the gas molecules get closer together and thus the density also increases. Therefore, option B and C are not the correct answer.
Now, as the temperature inside the balloon decreases, the heat is rejected from inside the balloon to outside and thus the kinetic energy inside the balloon also decreases. Thus, option D is also not the correct answer.
Rms velocity or Root mean square velocity is calculated individually for each molecule and thus it does not depend on the temperature or pressure or volume. Thus, rms velocity depends on the number of particles.
As the number of particles in the balloon remains same as earlier, the rms velocity also remains the same, thus option A is the correct answer.

Note:
When the balloon is subjected to liquid nitrogen, the balloon shrinks easily and loses its volume. But, after it is taken out it swells up again to make the temperature difference zero between inside and outside the balloon. Thus, the balloon becomes just like it was before.