Specific surface area measures the surface of a solid object compared to its mass, important for materials with defects. There are three methods to measure it: adsorption, particle distribution, and gas resistance. Results vary depending on the material used.
The specific surface area is a measure of a solid object. It compares the surface of the object to its mass and is particularly relevant in materials such as soil or with materials that are theoretically a smooth surface but actually have defects such as scratches. There are different measurement techniques, which can produce varying results, with each method being suitable for particular types of material.
From a mathematical point of view, this is a very simple concept. For example, a 4-inch (10 cm) cube has a total surface area of 6 x 4 inches x 4 inches, which equals 96 square inches (660 square centimeters). If the cube has a mass of 7 ounces (about 200 g), the specific surface area is 13.7 square inches per ounce (about 3.3 square centimeters per gram).
This formula can also be used to measure deficiencies and inconsistencies. For example, a set of dice will have a slightly different surface area and mass than a pure cube due to the dimples that indicate the numbers. Whether this leads to a larger or smaller specific surface area depends on the size and depth of the dimples. In theory all dice will have the same specific surface as each other, but there may be variations if they are not made consistently. This concept can be applied on a much finer scale, such as in scratches on an otherwise smooth piece of metal.
There are three main ways to measure specific surface area. The first is through adsorption, which is where particles of a dissolved gas, liquid or solid attach to the material being measured; a common example of the process is when moisture is “absorbed” by silica gel. A complex formula known as the BET equation can calculate surface area using data observed during this process. However, the result varies depending on the material used in the adsorption.
A simpler measure, more appropriate for materials such as soil, is particle distribution. This involves using a variety of methods to sort individual particles of the material by size. This can be done simply using a range of sizes, or in a complicated way such as using laser beams.
The third method is used for powder materials. It involves forcing a gas such as air through a bed of powder and measuring the resistance caused by the particles. A common use of this method is in assessing the quality of powdered cement, the idea being that the specific surface area will affect the speed of setting.
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