Acid-base (Neutralization) reaction

Take about 2 ml of dilute NaOH solution in a test tube and add one drop of phenolphthalein indicator. Observe the colour of the solution.

- Add dilute HCI solution to the above solution drop by drop. Is there any change of colour of the solution?

• Why did the colour of the solution change after adding the HCl solution?

-Now add one or two drops of NaOH to the above mixture.

• Does the Pink colour reappear"
• Do you guess the reason for reappearance of pink colour?

In the above activity you observe that the pink colour disappears on adding HC/ because NaOH is completely reacted with HCI. The effect of base is nullified by an acid, Pink colour reappears on adding a drop of NaOH because the solution becomes basic once again. The reaction occuring between acid and base in the above activity can be written as:

NaOH_(aq)+HCl_(aq)→NaCl_(aq)+H₂O₍₁₎

The reaction of an acid with a base to give a salt and water is known as a neutralization reaction. In general, a neutralization reaction can be written as: Base + Acid → + Water

• Take a small amount of copper oxide (CuO) in a beaker and slowly add dilute hydrochloric acid while stirring. Observe the changes in the solution.
• Note the colour of the solution.