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Do bubbles implode or explode, and wich one has more density: dry ice or regular ice?
Question Date: 2006-09-19
Answer 1:

Great questions!
Lets talk about bubbles first in particular lets talk about the bubbles commonly made with soapy water. When a bubble pops, most of the soapy water collapses to a small droplet (or multiple small droplets). The way in which this collapse happens, however, is a little different from what I think of when I hear the word implosion. If a bubble were to implode, I would think of the surface of the bubble being quickly sucked in, making the bubble smaller and smaller until there was no air left inside. This is quite different from what actually happens when a bubble pops the bubble doesnt actually shrink in the form of a bubble. Instead, it tears and collapses.

When a bubble is floating along before it pops, there are two main forces acting on the surface of the bubble, an outward force due to pressure and an inward force due to surface tension. The pressure inside the bubble is greater than the air pressure outside the bubble, because you blew extra air into it. This results in a force pushing outward on the bubble, but the bubble does not explode because there is an inward force to balance this outward one. The inward force is the result of surface tension. All of the water molecules in the bubble are attracted to each other because they can make weak bonds with each other called hydrogen bonds. The result is that each region of the water pulls on the neighboring regions, kind of like a stretched rubber band. If you consider a small region of water on the surface of the bubble being pulled equally from all sides onthe surface, the result is a total force pointing toward the center of the bubble.

So what makes a bubble collapse when it pops? If you poke it with a pin, or for any other reason a small hole develops in the surface of the bubble, the forces will no longer be balanced. In particular, the force due to surface tension will no longer point straight to the center of the bubble. Instead, water near the hole in the bubble will be pulled away from that hole, towards the rest of the water. The pressure inside the bubble changes quickly too, as air inside the bubble escapes through the hole. What starts out as a tiny hole will very quickly become a large tear, and pretty soon all of the water will collapse into the smallest possible droplet because of the attraction between water molecules.

Regarding your second question, dry ice has a greater mass density than water. Dry ice is the solid form of carbon dioxide. It has a density of ~1.5g/cm3 while water is ~0.9g/cm3. It is interesting to note that although dry ice has a greater mass density (mass per unit volume), regular ice has more molecules per unit volume. The molecules are packed together a little more closely in regular ice, but dry ice has a greater mass density because each carbon dioxide molecule has more than twice as much mass as a water molecule.

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