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Spontaneous balloon popping - Page 2

Now that you’ve looked at the simulation, let's see if you came up with the right answer. There are two important pieces of information necessary in order to understand why the balloon pops. First, balloons are porous. They have millions of tiny holes called pores that allow molecules to pass from the inside of the balloon to the outside as well as from the outside to the inside. In the simulation, the spaces between the dotted lines represent the pores. Second, we have to consider not only the sulfur hexafluoride but also the molecules that make up the air. It turns out that the balloon pops because of the difference in how fast SF6 and air molecules can diffuse through the pores in the balloon.

Air consists of about 78% nitrogen molecules (N₂), which are smaller and lighter than the sulfur hexafluoride. Their lesser mass means that they move faster, on average, than sulfur hexafluoride molecules at room temperature and their smaller size means they can pass through smaller holes. These two properties taken together mean that they can diffuse through the balloon’s pores much more quickly than can the SF₆. The surprising truth is that these nitrogen molecules diffuse INTO the balloon and ultimately cause it to pop because they increase the pressure inside the balloon.

But why would N₂ diffuse INTO the balloon? This happens because the partial pressure of N₂ is not the same on the inside and the outside of the balloon. If you're not familiar with the term partial pressure, this equation can help to make it clear:

        P_Air = P_N2 + P_O2 + P_Ar

What this equation means is that the total pressure of the air consists of the sum of the pressures exerted individually by each chemical species in the air. These individual pressures are known as partial pressures. Worded differently, the pressure of a mixture of gases is the sum of the pressures that each component in the mixture (like N₂, O₂ and Ar) would exert if it were in the container by itself. Air is about 78% N₂, 21% O₂, and 1% Ar. This means that if P_Air were 1 atmosphere, then P_N2 = 0.78 atm, P_O2 = 0.21 atm, and P_Ar = 0.01 atm.

Ok, so now we’ve discussed partial pressure. Because the partial pressure of N₂ is not the same on the inside and outside of the balloon, there is a natural tendency for N₂ molecules to flow into the balloon in order to try to make the partial pressures of N₂ the same on the inside and outside of the balloon. A similar tendency exists for SF₆, except that because there are no SF₆ molecules outside the balloon initially, SF₆ has a tendency to flow out of the balloon. However, because it is a large and slow molecule, it diffuses out of the balloon at a much lower rate than the rate at which N₂ diffuses into the balloon. Did you observe this when you were watching the simulation earlier?