Acids and bases (alkaline) have different pH levels. If a substance is an acid it will have a pH of less then 7. Acids are also sour to the taste. Some examples of acids are vinegar and lemon juice. If a substance is a base then it will have a pH above 7. Some examples of bases are baking soda and ammonia. Pure water has a pH of 7 and is considered neutral. Hydrogen ions determines the pH level.
Acids and bases have some physical properties as well. Acids, like citric acid, can sting mucous membranes and tastes very sour. Bases are usually bitter to the taste and have a slimy or soapy texture.