I have an answer below, but would it kill people to type more than a sentence fragment?
If you want to turn styrofoam INTO liquid, just pour a little gasoline on it (or many other solvents).
If you want to make foam FROM a liquid then try this goop:
Two-Part Pour Foam Kit Go to: www.westmarine.com and do a search for model # 314080
This isn't "styrofoam" brand foam, but is what is normally used in boat hulls and igloo coolers. Actual styrofoam is comprised of small, crumbly nodules and is created through a complex, proprietary process.