Question: why do most chemical reactions never reach 100 completion

title: Yield (chemistry)
text: Purification steps always lower the yield , through losses incurred during the transfer of material between reaction vessels and purification apparatus or imperfect separation of the product from impurities , which may necessitate the discarding of fractions deemed insufficiently pure . The yield of the product measured after purification ( typically to > 95 % spectroscopic purity , or to sufficient purity to pass combustion analysis ) is called the isolated yield of the reaction . Yields can also be calculated by measuring the amount of product formed ( typically in the crude , unpurified product ) relative to a known amount of an added internal standard , using techniques like gas / liquid chromatography , or NMR spectroscopy . A yield determined using this approach is known as an internal standard yield . Yields are typically obtained in this manner to accurately determine the quantity of product produced by a reaction , irrespective of potential isolation problems . Additionally , they can be useful when isolation of the product is challenging or tedious , or when the rapid determination of an approximate yield is desired . Unless otherwise indicated , yields reported in the synthetic organic and inorganic chemistry literature refer to isolated yields , which better reflect the amount of pure product one is likely to obtain under the reported conditions , upon repeating the experimental procedure .