Soap burns your eyes because it is at a different pH level than the membranes in your eye. Soap is a lot more basic than your eye is, meaning on the pH scale, it is usually above seven with seven being neutral, above seven being basic and below seven being acidic. Because soap is a base, it burns when it comes in contact with your eyes. In response, your eyes tear up in order to restore a normal pH balance. Also, in my opinion, another reason soap burns when it comes in contact with your eye is that pretty much anything that wasn't made to be in your eye will burn when it gets there.