I have central heat and air in my house. In the hall behind were you change the filter is the drain pipe that runs into the slab for the unit. I have a leak I think that mayby the pvc pipe is stopped up or the drain pipe is. If so could I put some liquid fire in the drain pipe?

Not sure what's best, but what works is separate conduit at unit and blow through(May pull apart if someone planned a head).
Pour a little bleach down condensation pipe.
I see some AC Tech put a "T" where the 3/4" pipe exits the unit, then they put a "T' so 1 end points down for the drain, and the one that points up allows a 2" piece of pipe and end cap to cover the top when access is not required.