Diffusion of Liquids:

Visualize a jar on board a space ship in which the atmosphere is weightless (that is, there is no acceleration force). Assume that the jar is filled with liquid, and then another liquid which does not react chemically with the first liquid is introduced into the jar. Slowly, the two liquids blend altogether until the mixture is uniform throughout the jar. This blending procedure is termed as diffusion.

In liquid, diffusion occurs instead slowly; some liquids undergo the procedure faster than others. The alcohol diffuses into water at room temperature much more rapidly than heavy motor oil into light motor oil. Ultimately, though, whenever any two liquids are mixed (as long as they do not react chemically, as do an acid and a base), the mixture will become consistent throughout any container of finite size. This occurs without the requirement for shaking the container since the molecules of a liquid are always in motion, and this motion literally causes them to push and jostle each other until they become consistently mixed.

When the same experiment is conducted in a bucket on Earth where there is acceleration force produced by gravity, diffusion will take place, but "heavier" liquids will sink toward the bottom and "lighter" liquids will mount toward the surface. Alcohol, for illustration, will float on water. Though, the "surface" between the alcohol and water will not be harshly defined, as is the surface among the water and the air. The motion of the molecules continuously tries to mix the two liquids. Though, gravitation prevents the mixture from becoming uniform throughout the bucket unless the two liquids are of accurately similar density.