Diffusion is defined as the spreading out from a concentrated source which results in an increase in the entropy (degree of disorder) of the substance. Diffusion occurs because of the random movement of molecules of the substance which allows them to separate from one another. The greater the space between these molecules the greater the ability for the molecular particles to spread out from one another. The more packed the molecules are in the substance the less space to maneuver ,and therefore, the more difficult for diffusion to occur. For example, solid substances are composed of particles (molecular or ionic) that are more tightly packed together. Because of this tight packing, the particles are not capable of diffusing very rapidly so the speed of diffusion occurs very slowly. If you placed a Lead brick and a gold brick in contact with one another and wrapped them together, it would take several years before the two bricks were "glued" together. This adhesion of the two bricks to one another is due to the fact that the Lead and Gold atoms are moving very slowly. The atoms are able to diffuse into each others solid matrix thus causing them to adhere to one another but only after several years being in contact with each other's surface.
On the other hand, liquid substances are composed of particles that are less packed and indeed can move more freely. Having more space between the liquid particles makes diffusion more likely to occurs at a faster rate. For example, if one placed a drop of liquid colored ink into a glass of liquid water the ink drop would slowly but noticeably diffuse out throughout the water. Since the ink particles are farther away from each other, and the water molecules are spread out, the molecules of ink can diffuse fairly rapidly so that the water is colored homogeneously throughout with the color of the ink in a matter of minutes.
Gaseous substances are in a league all to themselves. The molecular particles of gas are much more distant from one another than either liquid or solid particles are to each other. That is why gases are transparent. Light can pass through the relatively large spaces between the molecules of gas without any distortion. Liquids distort light more so because the particles are closer ,and the light passing through a liquid can be deflected more easily. If a gas possessing an odor such as Ammonia were allowed to escape an open bottle, it would not take but a few seconds for the detectable odor of Ammonia to be detected throughout the room. Ammonia gas molecules having a great deal of space between the particles of Ammonia and air could travel at a faster rate.