Diffusion simply describes what things do in nature, when a material spreads out and or disperses. Sir Isaac Newton, a GREAT English scientist, helped us define diffusion when he wrote his "Second Law of Thermodynamics". His law said that things will become more disorganized as time went on. Diffusion works like that. If you take a drop of food coloring and drop it into water, it will spread out and cover the whole glass, but it won't be as dark as it was before. What happened was that it became less concentrated. When it wasn't in the water, it was just a drop of food coloring. When it got dropped into the water, it was mixed with the water and became: food coloring AND water!

So if there is oxygen in the air and in the water, but it is more concentrated in the air, then it will spread out until it is equally concentrated!