A solution of 35 to 40 per cent of water in formaldehyde or methanal is called formalin. Formaldehyde's formula is HCHO.
As formalin is a strong disinfectant and tissue hardener, it's used for preserving biological and anatomical specimens. It's also used as an antiseptic in sterilising surgical instruments.
We do not use formalin for preserving arthropods, only vertebrates. Specimens are not kept permanently in formalin since it is acidic and difficult to handle.
Although generally avoided for long-term storage, formalin may be used in instances where colour is important since alcohol dissolves most colours almost immediately.
