This observation is often due to the formation of gaseous products during the reaction. For example, when burning fuels or carbonates react with acids. Gaseous carbon dioxide is often evolved leading to decreased mass after the reaction. Some other reasons can be evaporation of volatile organic compounds, formation of insoluble products, dehydration to produce water vapor or sublimation in specific reactions involving high heat or vacuum like decarboxylation reactions.

Mass is generally conserved under normal circumstances in these chemical reactions unless specifically involving nuclear reactions.