The reaction between an acid and a base is known as neutralisation. Salt and water are produced in this process with the evolution of heat. Evolved heat is known as heat of neutralisation .

Where 57.1kJ energy is the heat of neutralisation for above reaction. This value remains same if both acid and base are strong. If one out of these is weak then amount of energy released will be lesser than 57.1 kJ 
    Aim : To observe the neutralization reaction.
    Procedure :
     •    Take a test tube and fill it one-fourth with dilute hydrochloric acid (HCl).
     •    With the help of a dropper, add 2–3 drops of phenolphthalein indicator (colourless) to it.
     •    Gently shake the test tube.
     •    Note down its colour.
     You will observe that the solution is colourless.                   
     •    Now put a drop of dilute sodium hydroxide with the help of a dropper and shake it gently.
     •    Continue adding dilute sodium hydroxide and shaking it till the pink colour just appears.
     •    At this point, the solution is just neutral.
     •    Add a drop of dilute hydrochloric acid to it
     •    What do you observe now ?
     You will notice that the pink colour disappears.
     •    Again add a drop of dilute sodium hydroxide.
     You will notice that the pink colour reappears.
     This happens because phenolphthalein is colourless in an acidic medium and pink in a basic medium.
     Inference : Drop by drop addition of dilute sodium hydroxide neutralizes dilute hydrochloric acid.