Can a person really breathe liquid oxygen, as was demonstrated in the movie "The Abyss"? I'm not exactly sure what the liquid was called. They used this when diving really deep.

I've never heard of people breathing a liquid as a means of obtaining oxgyen for metabolism. Even if you could pack enough free oxygen into that liquid, is suspect that the effort required to breathe it in and out would be too exhausting to be practical. At great depths, divers do have to modify the mixture of molecules and atoms they breathe to avoid problems. The pressure in water increases rapidly with depth because water is very dense and supporting a few meters of water requires large amounts of pressure. As the pressure increases, various gases in air become more and more soluble in blood and tissues. At first, this dissolved gas is just a problem during the ascent to the surface, when it comes back out of solution as tiny bubbles and damages the tissues its in (the "bends). But eventually these dissolved gases become so concentrated that they become toxic. That's why very deep divers breathe gases containing lots of helium. Helium doesn't dissolve well in anything, even at very high pressures.

What is the buoyant force?

When you displace a volume of air and replace that volume with something else, the air around the volume still pushes on it as before. If that volume had remained air, then it would have just floated there, suspended by a force from the surrounding air. Now that the volume has been replaced by something else, it still experiences the same suspending force. That suspending force is the buoyant force. It's actually created by a slight imbalance in the pressures around the volume. The pressure at the bottom of the volume is slightly higher than on top, so the air exerts a net upward on the volume.

If you don't want your tires to blow from too much thermal energy, then why aren't tires white (to absorb less solar energy and thus receive less thermal energy)?

That's an interesting question. It's probably difficult to manufacture white tires and they'd probably look terrible after they'd been driven a while. The old white-wall tires where difficult to keep clean. Because the pressure in a tire varies with its temperature, your tires will probably go over their optimum pressure on a hot sunny day.

Why does the balloon collapse without air (in vacuum) but the jar did not?

A balloon's skin is too weak to support the air around it. If you don't put any gas inside the balloon, the atmosphere around it will push it inward and it will collapse. But the jar that I was using is made of an extremely strong plastic that easily supports the weight of the atmosphere on top of it. I could have jumped up and down on the jar and it wouldn't have broken.

How can a balloon be able to support the air around (pressure wise) and still rise? (I didn't understand the demonstration at all.)

If I could have filled a balloon with nothing at all, it would have floated very nicely because it would have had no weight and the only force on it would have been the buoyant force upward. But an empty balloon will be crushed by the surrounding air, which will push inward on its surface with enormous forces. To keep the balloon from crushing, I had to fill it with gas. I chose the lightest gases I could think of: hot air and helium. In the case of hot air, I was able to use a relatively small number of air molecules to create the pressure needed to keep the balloon inflated. With helium atoms, I had to use lots of helium atoms (the same number as I would have needed if I'd used air molecules instead) but helium atoms are very light. Although I had many helium atoms, their total weight was less than that of an equal volume of air. Thus the upward force on the helium filled balloon was more than its weight and up it went.

How do clouds exist? If oxygen molecules, which must weight less than water vapor are drawn toward earth, they why aren't the clouds?

First, water molecules are lighter than the average air molecule so a balloon filled with water vapor would actually float in air. But that isn't what you're asking. Clouds exist because when water condenses from vapor to liquid, it often forms extremely tiny water droplets. These droplets are so small that they experience lots of air resistance as they try to move about. The begin to fall but quickly reach terminal velocity at perhaps a millimeter per second. The water droplets drift downward so slowly that they hardly move. That's what's happening in a cloud. The water droplets are trying to fall, but air resistance is slowing their descents.

How does altitude affect the presence of molecules?

As you travel upward, the air around you has less and less pressure. That's because it's supporting less and less weight above it. As long as the temperature isn't changing much, this decrease in pressure is caused by a decrease in density: the air molecules are become less tightly packed together. The result is that at high altitude, each breath you take delivers less air molecules into your lungs. Actually, air usually gets colder at higher altitudes, a change which keeps the air's density from decreasing so rapidly. The higher you go, the colder the air gets and the more molecules you need in each liter of air to maintain its pressure.