Question

1.What is going on at the atomic level if a precipitate forms in a double replacement reaction?
2. Has a single or double replacement reaction occured if water or a gas is formed? What would you perceive as evidence that a reaction has occured if either water or a gas is formed?

Answer 1

1. Usually electrons are moving from one atom to another that results in closed shells (or at least more stable shells) and the resultant compound is insoluble in the solvent.
2. For a gas, you will see bubbles forming in the solution. right?
It's tougher to know if water is formed, especially in aqueous solutions, but the heat of the reaction from H^+(aq) + OH^(aq)==> H2O(l) is exothermic. For very dilute solutions you probably would not notice the heat but a temperature probe should be able to measure it.