When a wooden log burns it is essentially undergoing a combustion reaction. Explain why the ashes that form when a log burns have a lower mass than the log had before it started burning.

Answer 1

some of the mass burns off as smoke and floats away. also sparks might carry away some of the mass.

(also, a negligible amount of mass is emitted as heat)

Answer 2

Also some of the mass is gone as CO2, a gas.