Note: Solvent viscosity affects k through the 'cage effect,' where reactants in a liquid get trapped in a cage of solvent molecules. They "rattle around" in such a cage at tremendous rates, making hundreds of collisions before squeezing between solvent molecules and into a new solvent cage. If another reactant also gets trapped in the cage, many of the collisions are between the reactants and a reaction is likely to occur. If there is not another reactant in the solvent cage, the molecule cannot react until it escapes the cage.