A water ionizer is a small household appliance that transforms the pH (measure of acidity or basicity/alkalinity of a liquid) level of regular tap water in order to make the water more alkaline. Liquids, in this case water, with a pH level less than 7 indicates the water is acidic. Water with a pH level higher than 7 is more alkaline. Pure water has a pH of 7, making it neither acidic nor alkaline, thus neutral.