Alkalis are all Arrhenius bases, ones which form hydroxide ions (OH) when dissolved in water. Common properties of alkaline aqueous solutions include:
* Moderately concentrated solutions (over 10 M) have a pH of 10 or greater. This means that they will turn phenolphthalein from colorless to pink.
* Concentrated solutions are caustic (causing chemical burns).
* Alkaline solutions are slippery or soapy to the touch, due to the saponification of the fatty substances on the surface of the skin.
* Alkalis are normally water-soluble, although some like barium carbonate are only soluble when reacting with an acidic aqueous solution.