In cooking, a syrup (from Arabic ???? sharab, beverage, via Latin siropus) is a thick, viscous liquid, containing a large amount of dissolved sugars, but showing little tendency to deposit crystals. The viscosity arises from the multiple hydrogen bonds between the dissolved sugar, which has many hydroxyl (OH) groups, and the water. Technically and scientifically, the term syrup is also employed to denote viscous, generally residual, liquids, containing substances other than sugars in solution. Artificial maple syrup is made with water and an extremely large amount of dissolved sugar. The solution is heated so more sugar can be put in than normally possible. The solution becomes super-saturated. Refining (also called affining) is the process of purification of a substance. The term is usually used of a natural resource that is almost in a usable form, but which is more useful in its pure form. For instance, most types of natural petroleum will burn straight from the ground, but it will burn poorly and quickly clog an engine with residues and by-products. The term is broad, and may include more drastic transformations, such as the reduction of ore to metal (for which see Refining (metallurgy)). The refining of liquids is often accomplished by distillation or fractionation. Gases can be refined in this way as well, by being cooled and/or compressed until they liquefy. Gases and liquids can also be refined by extraction with a selective solvent that dissolves away either the substance of interest, or the unwanted impurities. Many solids can be refined by growing crystals in a solution of the impure material; the regular structure of the crystal tends to favor the desired material and exclude other kinds of particles. Chemical reactions are often used to remove impurities of particular types. The use of silicon and other semiconductors in electronics is highly dependent on precise control of impurities, and a number of special techniques have been developed, such as zone refining.