Temperature too warm or too cold (most reactions proceed faster at w]higher temperatures); use of wrong chemical (sodium nitrite instead of sodium nitrate); incorrect measurement (adding .5 grams instead of .05 grams); improperly cleaned glass wear that had a film of some chemical on its surface; incorrect timing (letting a mixture react for 10 minutes when 5 minutes was called for); use of an outdated or contaminated chemical; many others.
