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Matter clings to the sides of measuring containers during transfers
Sample is spilled during pouring and stirring
Tiny amounts of sample escape due to bubbling and fizzing
The product has not been dried out completely
The sample is filtered and some stays on the filter paper
The dry product is not pure
The reaction naturally does not go to completion
Product is collected before the reaction is finished
The mass of the product is calculated while it is hot
A metal product has oxidized due to air exposure (eg. Cu becomes CuO)
The product is volatile, or evaporates easily
Factors that make the percent yield lower than it should be
Factors that make the percent yield higher than it should be