Four balloons of equal size are each filled with a gas under one of the following conditions: A) Hydrogen gas at room temperature, B) Hydrogen gas.. at 0 deg. C, C) Clean air at room temperature, D) Clean air at 0 deg. C. Which of these balloons will shrink the fastest?

Expert Answers

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In answering this question, I am going to assume that as each of the balloons is filled, the starting temperature will remain constant as the balloons are then observed over a period of time.

If the temperature is constant, then the only way the balloons will shrink is if the molecules of gas inside the balloon escape through the tiny pores that are actually present in the walls of each of the balloons.  You may have observed this with helium balloons that shrink over time.

Because these pores are very small, t he smaller the size of the gas molecule inside the balloon the greater the probability that it will hit the pore in the right way to allow it to escape.  This means that all else being equal the hydrogen molecules will escape through the balloon faster than the balloons filled with air.

The next consideration is the difference in temperature between the hydrogen at room temperature and the hydrogen at 0 degrees celsius.  As the temperature increases the average velocity of the gas molecules also increases.  This means that the faster moving molecules will have more kinetic energy.  Because the faster moving hydrogen molecules at room temperature will hit the inside walls of the balloon more often than the slower moving hydrogen molecules at 0 degrees, they will escape more often and so the balloon filled with hydrogen at room temperature should shrink the fastest.

The other complication in this is that to fill the two balloons to the same size will require more hydrogen gas at 0 degrees than at room temperature because gases become more dense as the temperature decreases and the pressure also decreases.  This would further shift the shrinking to the hydrogen filled balloon at room temperature.

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