Nicotine is one of the few natural alkaloids. It is a colorless, volatile base (pKa = 8.5) that turns brown and acquires the odor of tobacco on exposure to air. Nicotine is hygroscopic, oily liquid that is miscible in water in its base form. As a nitrogenous base, it forms salt with acids that are usually solid and water soluble. Nicotine easily penetrates the skin. Nicotine burns at temperature below its boiling point and vapors will burn at 308k in air despite a low vapor pressure. Because of this, most of the nicotine is burned when a cigarette is smoked. However enough is inhaled to provide the desired effects.