When you open a bottle of rum, the alcohol will start to evaporate. This process will continue until the bottle is completely empty. When alcohol evaporates, it forms a gas called ethanol. Ethanol is poisonous and can cause blindness, loss of consciousness, and even death if consumed in large quantities. If you notice that your rum has gone bad, there are few ways to tell for sure. The most reliable way to determine if your rum is no longer safe to consume is to smell it and see if it has a unpleasant odor. If it does, you should dispose of it immediately.