If the physical states of reactants and products are not given, an uninformed person
might try to bring about the reaction by mixing solid KBr with solid AgNO 3 . These
solids would react very slowly or not at all. Imagining the process on the microscopic
level, we can understand that for a product like silver bromide to form, the Ag 1 and
Br2 ions would have to come in contact with each other. However, these ions are
locked in place in their solid compounds and have little mobility. (Here is an example
of how we explain a phenomenon by thinking about what happens at the molecular
level, as discussed in Section 1.2.)