An ion is an atom that carries an electrical charge and is therefore reactive with its environment. An acidic substance releases hydrogen ions (H+) in water. Acids are known as proton donors because the H+ ion has one extra positively charged proton trying to stabilize itself by combining with a negatively charged ion. A basic substance releases a hydroxide ion (OH-) in water. Bases are called proton acceptors because the hydroxide ion will accept a proton to stabilize itself. Interestingly enough, when acids and bases are combined, the result is a neutral salt.
For example, a strong acid like hydrochloric acid combined with sodium hydroxide (a strong base) results in a naturalization reaction with the byproducts sodium chloride (table salt) and water. PH is an important biological indicator because most life forms have a very small range of pH in which they can survive. For example, the acid-base ratio in the human body is a delicate balance. Even a slight change in the blood’s pH in either direction can result in death. Plants are also susceptible to minute pH changes in the soil. That is why soil that is too acidic for a plant is neutralized with calcium carbonate fertilizer, a base.
CONCLUSION If we put the red and litmus paper to the materials with acid it would change color. Blue litmus is an acid-base indicator whose color in acid is different to that in a base. The presence of an acid, (pH less than 7) turns blue litmus red, therefore this compound is acidic. When litmus paper touches and acid, the paper changes to red. When it touches a base, the paper changes to blue.