Why Does the Water in a Car Radiator Often Boil Explosively When the Radiator Cap Is Removed?

A careful guideline about checking coolant on automobiles is to never remove the radiator cap when it is hot due to its explosive nature. Caution should be used as antifreeze coolant is also a dangerous chemical that can cause injury to your skin, face and eyes.

Temperature of Coolant

Radiator coolant, which is mixed with water, is pressurized and becomes heated when a car is running. As a car's engine cools, the coolant will take a while to cool down. While it is still hot, it can reach temperatures up to 220 degrees Fahrenheit, which can cause serious burns in less than one second.

Physics

Due to the pressurized nature of the cooling system, the coolant's boiling point is raised by 3-degrees Farhenheit for each PSI unit above the atmospheric pressure. The coolant expands while under pressure. When the radiator cap is removed, the expanded coolant has an outlet, spraying explosively through the opening.

Safety Considerations

When checking the coolant, if the engine is hot, check the coolant overflow reservoir. The reservoir should have a hot minimum and maximum line. Adding coolant or water to the reservoir tank can help ensure the car is running safely while still hot. If the car engine is cold, use a rag to cover the radiator cap while turning it. Replacing the cap is relatively inexpensive; however, be sure to check that the pressure rating of the new cap matches that of the old one.