I have heard that it is not chemical, but have lived with people who needed the alcohol every night. A person can be psychologically dependent without being physically dependent, but a person can't be physically dependent without being psychologically dependent. A person who is addicted to drinking develops an increased physical tolerance to the booze, and needs to gradually drink more in order to achieve the same amount of drunkenness.
Chronic alcoholism occurs when there are both physical and psychological addictions.
Alcoholism is treatable and controllable, but not curable. And, it's much easier to treat the physical dependency than to treat the psychological dependency.