Can you use only water in a automobile radiator?

Yes you can use only water but only if you are absolutely 100% sure that the car will not be parked anywhere that the temperature drops below (freezing) 32 degrees F. or Zero Celsius.

If you do park the car in a place where the temp drops below freezing and the water freezes you will crack your radiator and also this could crack the block of your engine too.

So as a rule, if you live where it could freeze then you want to put Antifreeze in with the water; usually 50/50 (read the bottle). The mixture varies depending on how cold you anticipate the temp will get where the car is parked.

If you havent got any antifreeze you can put just water in , and this could last you all summer and into the fall, but just remember to drain some out and get the right amount of antifreeze in there before winter comes.

I learned the hard way as one summer I filled my car with just water in the radiator, and as winter approached I kept telling myself that I better put in some antifreeze. Well, I kept putting it off and then sure enough; one morning I noticed some rain puddles in the driveway had frozen over, and so to was my radiator. The ice destroyed my radiator as water expands when it turns to ice, Luckily it didnt destroy the whole motor as it was not that cold say perhaps 31 deg F. also I noticed it early. If I had left it much longer the motor would have been destroyed too.

By the way, you can even use just water in the coldest of winter, as long as you never turn off the car long enough for the water to freeze.

So yes, in a jam you can use just water, but if you park the car and it freezes you'll be sorry. Now you know why they call it anti freeze! Anti freeze performs the same function as the water does (heat transfer); however, it is designed (chemically) to have a much lower freezing point (than water).