An acid is an ionic compound that produces positive hydrogen ions (H+) in an aqueous solution. It has a sour taste and its pH value is less than 7. 

An acid reacts with certain acid-base indicators changing their color. Blue litmus turns red in color in the presence of acid but red litmus does not undergo any color change in the presence of acid. 

Strong acids are corrosive and can burn skin and destroy fabric and other materials.