Formaldehyde is a gas which when added to water is used as a preservative of biological tissue samples and as an antiseptic. The most common solution is a mix of 40% formaldehyde and 60% water, known as Formalin. Although it is in widespread use, it is both toxic and carcinogenic. As little as 1 ounce of Formalin can be fatal to an adult.
As an antiseptic, formaldehyde is also poisonous to bacteria.
Formaldehyde prevents tissue from decomposing by linking amino acids to other nitrogen atoms, making the atomic bond much stronger and preventing other biological processes from breaking those bonds. It is the main preservative used in embalming solutions .