The opposite of an acid, an alkali substance creates a bitter taste and slippery feel when dissolved in water and will turn red litmus paper blue. An alkali has a pH greater than seven; when used for bathing, highly alkaline waters cause drying of the skin. Alkalis may include the soluble hydroxide, carbonate and bicarbonate salts of calcium, magnesium, potassium and sodium. A hydroxide alkali may also be called a base.