A little distilled water is taken in a beaker.

A. What is the pH value of distilled water?

B. What happens to the pH value when the following substances are added to the water in the beaker? Justify your answer.

a) Caustic soda

b) Vinegar

A. pH value of distilled water is 7.

The pH scale is the method to express the acidity/alkalinity of a solution. It is based on the amount of H⁺ ions present in a given solution.

Lesser the value of pH, more will be the acidity. For a neutral solution the pH value is 7. Distilled water is neutral in nature because equal number of H⁺ and OH^- are present in it. Thus distilled water has a pH value of 7.

B. The pH value will change when caustic soda or vinegar is added to the distilled water as follows:

a. pH value will increase and become much greater than 7 on adding caustic soda to water.

Caustic soda is general name for NaOH which is a strong base. So when we add caustic soda to water, it will make the water basic in nature. A basic solution has a pH greater than 7. So we can say that adding caustic soda will make the pH of water greater than 7.

b. pH value will decrease and become slightly less than 7 on adding vinegar to water.

Vinegar contains acetic acid which is a weak acid. So when we add vinegar to water, it will make the water acidic in nature. An acidic solution has a pH less than 7. So we can say that adding vinegar will make the pH of water less than 7.