DIFFUSION OF LIQUIDS

Imagine a jar on board a space ship in which the environment is weightless (there is no acceleration force). Suppose that the jar is filled with liquid, and then another liquid that does not react chemically with the first liquid is introduced into the jar. Gradually, the two liquids blend together until the mixture is uniform throughout the jar. This blending process is called diffusion. In a liquid, diffusion takes place rather slowly; some liquids undergo the process faster than others. Alcohol diffuses into water at room temperature much more quickly than heavy motor oil into light motor oil. Eventually, however, when any two liquids are mixed (as long as they don’t react chemically, as do an acid and a base), the mixture will become uniform throughout any container of finite size. This happens without the need for shaking the container because the molecules of a liquid are always in motion, and this motion literally causes them to push and jostle each other until they become uniformly mixed.
If the same experiment is conducted in a bucket on Earth where there is acceleration force produced by gravity, diffusion will occur, but “heavier” liquids will sink toward the bottom and “lighter” liquids will rise toward the surface.
Alcohol, for example, will float on water. However, the “surface” between the alcohol and water will not be sharply defined, as is the surface between the water and the air. The motion of the molecules constantly tries to mix the two liquids. However, gravitation prevents the mixture from becoming uniform throughout the bucket unless the two liquids are of exactly the same density. We’ll talk about the meaning of density for liquids shortly.