scribes chemical substances of sufficient purity for use in chemical analysis, chemical reac-
tions or physical testing. Purity standards for reagents are set by organizations such as
ASTM International. For instance, reagent-quality water must have very low levels of impu-
rities like sodium and chloride ions, silica, and bacteria, as well as a very high electrical re-
sistivity.
                                          Adhesive
      Adhesive is substance capable of sticking to surfaces of other substances and bonding
them to one another. The term adhesive cement is sometimes used in place of adhesive, espe-
cially when referring to a synthetic adhesive. Animal glue, a gelatin made from hides,
hooves, or bones, was probably known in prehistoric times; it remained the leading adhesive
until the 20th cent. It is now used especially in cabinetmaking. Animal glue is sold both as a
solid (either ground or in sheets, to be melted in a water-jacketed glue pot and applied while
hot) and as liquid glue (an acidic solution). Adhesives from vegetable sources are also impor-
tant; they include natural gums and resins,  mucilage,  and starch and starch deriva-
tives. They are commonly used for sizing paper and textiles and for labeling, sealing, and
manufacturing paper goods. Other adhesives derived from animal and vegetable sources in-
clude blood  glue,  casein  glue,  fish  glue,  rubber  adhesives,  and  cellulose  deriva-
tives. Adhesives having special properties are prepared from synthetic resins. Some synthetic
adhesives, such as the epoxy resins, are strong enough to be used in construction in place of
welding or riveting. Adhesive tapes have a coating of pressure-sensitive adhesive.
                                           Catalyst
      Catalyst is substance that can cause a change in the rate of a chemical reaction without
itself being consumed in the reaction. Substances that increase the rate of reaction are called
positive catalysts or, simply, catalysts, while substances that decrease the rate of reaction
are called negative catalysts or inhibitors.
      Enzymes are the commonest and most efficient of the catalysts found in nature. Most of
 the chemical reactions that occur in the human body and in other living things are high-ener-
gy reactions that would occur slowly, if at all, without the catalysis provided by en-
zymes. For example, in the absence of catalysis, it takes several weeks for starch to hydro-
 lyze to glucose; a trace of the enzyme ptyalin, found in human saliva, accelerates the reaction
 so that starches can be digested. Some enzymes increase reaction rates by a factor of one bil-
 lion or more. Enzymes are generally specific catalysts; that is, they catalyze only one reaction
 of one particular reactant (called its substrate). Usually the enzyme and its substrate have
 complementary structures and can bond together to form a complex that is more reactive due
 to the presence of functional groups in the enzyme, which stabilize the transition state of the
 reaction or lower the activation energy. The toxicity of certain substances (e. g. , carbon
 monoxide and the nerve gases) is due to their inhibition of life-sustaining catalytic reactions
 in the body.

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