An acid is a chemical that releases hydrogen ions when it is dissolved in water. When placed in water, the aqueous solution that results will react with bases and some metals to form salts. Acids have a pH level of less than 7, have a sour taste, and will turn blue litmus paper red.
Safeopedia Explains Acid
Acids exist in two main types, strong and weak. Strong acids have the property of fully dissolving in a solution, while weak acids will only partially dissolve in a solution. When exposed to water, all acids produce hydrogen ions. Strong acids tend to have a higher concentration of hydrogen ions than weak acids.