The water is boiling in radiator because it's temperature(radiator's) reaches the boiling point of water when the engine works for a long time.
In the radiator over flow bottle.
It changes the freezing and boiling point.
The boiling point of the mixture is higher than the boiling point of pure water; this slows down the evaporation of the coolant.
The boiling point of water is 212 degrees, most vehicles now days operate ftom 205 to 215. A 50% mix of distilled water and antifreeze will extend the the boiling point to 265 degrees at sea level. If you go up in elevation the boiling point will decrease, so make sure you have a known good or new radiator cap to prevent premature water boiling in higher elevations
if its out of the top you might need a new cap they are pressurized and sometimes the spring is bad
Boiling water can only get up to 100 degrees C.
If the engine has been running and is hot, shutting off the ignition allows the water in the water jacket to continue to be heated. Therefore, until the engine cools down below boiling temperature, it will continue to heat the water. Boiling is prevented by a couple of things; anti-freeze (which raises the boiling temperature) and the radiator cap (because water boils at a higher temperature when under pressure). If your car is overheating, you should not remove the radiator cap (until the vehicle is cool), you can be severely burned as the pressure drops rapidly, boiling the liquid and forcing it out of the radiator!
Water under pressure has a higher boiling point, if the water is already hot enough to boil it will suddenly erupt when the cap is removed and pressure relieved.
Water boils at 100 degrees C. Using a radiator cap, the system holds pressure, raising the boiling point (Just like your grandmother's pressure cooker). Using the correct coolant mix of water and antfreeze also raises the boiling point.
check your radiator cap chances are it isn't any good and you need to replace it because it isn't holding pressure.