|MadSci Network: Chemistry|
You are correct that matter (stuff) can be described as solid, liquid, gas (or another state called "plasma", which is a separate discussion altogether). However, if you mix matter, you can end up with a system containing more than one type of matter. Imagine, for example, the following systems:
a balloon whipped crean (air within a liquid) hair mousse (see whipped cream) salad dressing (solid herbs in a liquid) etcAll of the above are mixtures.
The same is true with a bubble. It can be described as a pocket of air "trapped" inside a liquid. Bubbles can be pretty cool. Take a close look at a freshly poured soda, a freshly poured beer, and any other carbonated beverages you can find. Note how the bubbles behave. Wild. Many different phenomena are happening simultaneously. Surface tension (on the walls of the glass), dissolution, compression, molecular transport. The reason a bubble stays a bubble, is that the weight of the atmosphere (aka atmospheric pressure) pushes down on the liquid. The air that is trapped is compressed (like squeezing a balloon) from all directions, resulting in a spherical shape. If the air can dissolve in the water fast enough, the bubble would go away. However, this process is limited by the solubility of air in the liquid, and how fast air can be transported into the liquid.
All in all, this covers lots of neat chemistry and physics. Email me if you'd like to discuss further.
Try the links in the MadSci Library for more information on Chemistry.