What are some characteristics of acid ?

Some characteristics of the acids are:
1) pH < 7: Acids have a pH value of less than 7. 
2) Increases proton concentration of the solution: acids, when dissolved in a solution, tend to donate protons and this increases the proton concentration of the resulting solution. This is the reason why the pH of the solutions decreases when an acid is added to them.
3) Turn the litmus paper red from blue: If we conduct the litmus paper test on an acid, we find that a strip of blue litmus paper will turn red when dipped in the acid.
4) Sour taste: acids generally have a sour taste. 
5) React with metals: acids react with metals and produce a salt and hydrogen gas as a product. For example, calcium reacts with nitric acid to produce calcium nitrate and hydrogen. 
6) Chemical formula starts with an "H": the chemical formula of acids starts with a hydrogen atom or "H". Common examples of acids are hydrochloric acid (HCl), sulfuric acid (H2SO4), nitric acid (HNO3), etc.