Final answer:
Formaldehyde is the most commonly used disinfectant and preservative chemical agent in hardening compounds. It works by methylating enzymes and nucleic acids, but it is toxic and carcinogenic.
Step-by-step explanation:
The most commonly used disinfectant and preservative chemical agent in hardening compounds is formaldehyde. Formaldehyde is an alkylating agent and works by methylating enzymes and nucleic acids, which inactivates them. It is commonly used as a gaseous disinfectant and biocide and is also used as a chemical fixative and as an embalming fluid. However, it is toxic and carcinogenic, so it is not used as an antiseptic.