Chemical formula HCOOH or CH₂O₂. It is an acid with a similar vapor pressure curve to water with a freezing point of 8°C and an evaporation temperature of 108°C. The first formic acid was extracted from wood ants (latin formica rufa), which is how it got its name.

HCOOH is used in >98% concentration as a reducing agent for flux-free soldering. Strictly speaking, the acid acts as a flux, but only the surfaces are reduced and the acid can be removed residue-free in a subsequent vacuum step.