It is now generally accepted that the stressed Si-O-Si bonds at the crack tip react with water, and other polar molecules, to form two Si-OH bonds according to the following chemical reaction:
Si-O-Si-H2O=2Si-OH
In this process, illustrated in Fig. 12.10 and referred to as dissociative chemisorption, a water molecule is assumed to diffuse and chemisorb to the crack tip (Fig.12.10a) and rotate so as to align the lone pairs of its oxygen molecules with the unoccupied electron orbitals of the Si atoms (Fig.12.10b) Simultaneously, the hydrogen of the water molecule is attracted to a bridging oxygen.