1. Formaldehyde (CHOH) is a gas.

2. Formalin 40 or 100% formalin is 37% (w/w) formaldehyde or 40% (w/v) formaldehyde.

3. Paraformaldehyde is a polymerized formaldehyde.

4. Methanol or some form of stabilizer is used in formalin to prevent formaldehyde polymerization.

5. When methanol is not desirable for fixing, formaldehyde solution is made directly from paraformaldehyde by heating the solution, thus the paraformaldehyde fixing solution.