The ethanol put in gasoline is anhydrous ethanol, meaning it is dehydrated so that it has no water in it. Anhydrous ethanol will suck the moisture right out of the air, and it takes very little added moisture to make it heavier than gasoline. Once that happens, it sinks to the bottom of the tank/float bowl. Air gets to the gasoline through the float bowl and gas tank vents found in older equipment.
Here's an experiment to try. Put a half cup of fresh gasoline into a tuna fish can and let it set in the air for a few minutes. Almost immediately you will see the gasoline go cloudy and get really cold. A bit longer, and you will see little brownish globules of water/ethanol settle out into the bottom of the can.
That can be useful. If you want to remove the ethanol from modern gasoline, all you have to do is add water and shake. Within seconds it will all settle out to the bottom of the container.
-Chuck