When there is no water in the radiator, a car can overheat. Serious damage can occur like a blown head gasket an warping if one drives for a long time without placing water in the radiator.
the radiator will spew hot water on you
(Oil floats) mostly mixes with the water forming emulsion which clogs the radiator.
If you mean the cap for a car radiator the water will heat up and overflow. The radiator/cooling system requires a pressure of usually around 16 lbs which is what the radiator cap provides in stopping water from overflowing.
Circulating water will not be sufficient and the car will overheat.
It can break the water pump housing, engine block, radiator. Major damage.
there is supposed to be water in the radiator. your radiator is supposed to be evenly mix 50/50 with radiator fluid and water, preferably distilled water.
You replace the radiator
There is a water reservoir tank that goes to the radiator that you can add the water to.
As steam gives up energy inside the radiator, it undergoes condensation and transforms back into liquid water. This process releases heat energy that warms up the radiator.
Radiator coolant.
You can add water to a radiator any time it is low on water.
remove your radiator form your car then poor some hot water in radiator till water comes out nice and smooth.