An acid is a chemical substance that has a sour taste and is corrosive if concentrated. It has a pH value less than 7. It turns blue litmus paper (pH indicator) red. Citric acid is a common acid found in lemon.
Alkali and base are the same thing. An Alkali is a chemical substance that has a soapy feeling and a bitter taste. It is corrosive if concentrated. It has a pH value greater than 7. It turns red litmus paper blue. Alkalis are commonly found in shampoos. The burning in eyes when such products get into eyes is caused by alkalis. Sodium hydroxide is a common alkali.
