NEVER NEVER NEVER REMOVE A COOLANT CAP WITH A HOT ENGINE
The coolant can run upto temperatures 110Deg C
That will burn and scold simple as
Remember the cap allows pressure to build to about 1BAR, that 1 bar allows the boiling point to go over 100deg C
Air in the system can a ""boiling"" of the coolant
Boiling coolant because lack of flow can cause possible up-roars
Overfilled coolant
over-pressured, such as combustion gases entering the coolant system, which ""forces"" the water out
Ziggy