Porosity, << paw ROS uh tee, >> is the existence of many small holes or spaces in a material. In some porous materials, such as charcoal, earthenware pottery, and sponges, these holes connect. Gases and liquids can pass through the holes. In other porous materials, the small spaces inside are separated from one another by solid material. Bricks and certain types of foam rubber are examples of this kind of porous material. These materials are usually able to absorb liquids and gases.
Porosity is desirable in some materials and uses. For example, porous filters of charcoal can remove impurities from the air. However, engineers try to avoid using porous materials in other situations. Porosity in iron castings, for example, reduces the strength of the metal.
