Call
Two molecules need to collide in order for a reaction to take place, hence the mobility of the molicules that leads the molicules to collide each other is one of the principal necessity.
Reactant molicules in gas phase or liquid phase readily collide with each other.
But mobility of molecules in the solid phase is ignorable. Collisions in solid phase doesn't happen at all.
Again, reactant molicules in solid are in contact with each other, they are not exposed enough to come across the molicule of each other as both the reactants present in solid phase.
This is why the solid lead nitrate cannot react with solid potassium iodide
N.B lead nitrate react with solid potassium iodide in solution.